LTE Backhaul

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LTE Backhaul

Course Code: LT1202 Duration: 1 day Technical Level: 2

Other areas of expertise include:

_{IP Networks and Protocols} Service Enablers WiMAX _LTE UMTS GSM and GPRS _TETRA _CDMA

Transport and Signalling Telecoms Industry Dynamics

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First published 2012

WRAY CASTLE LIMITED, BRIDGE MILLS, STRAMONGATE, KENDAL, LA9 4UB, UK

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Section 1 Section 2 Section 3 Section 4 Section 5

Mobile Backhaul Requirements Layer 1 Options

Layer 2 Options Layer 3 Options

Typical Backhaul Scenarios

LTE BACKHAUL

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MOBILE BACKHAUL

REQUIREMENTS

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Mobile Backhaul Requirements

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Defining Backhaul

The backhaul service provided to a network’s remote access nodes can be generically divided into several basic areas: The access node itself will generally be a cellular base station (BTS, Node B, eNode B), the backhaul link supplied to the access node is termed the ‘first’ or ‘last’ mile connection and forms part of the Access Transport Network.

Unless a backhaul link operates in pure point-to-point mode, in which case it will connect directly to one of the operator’s core network sites, the access link will connect to an ‘aggregation’ point. These nodes are referred to differently by different operators but they are commonly known as ‘transmission high sites’, Point of Concentration (PoC) ‘aggregation sites’ or even ‘first aggregation sites’. In networks that employ microwave access links the first aggregation node is often equipped with a tall tower or is located on a hill or tall building (hence the term ‘high site’), this allows it to act as a hub for microwave links emanating from local base station sites. First aggregation points generally aggregate traffic from multiple low capacity access links onto a smaller number of high capacity microwave or fibre connections that lead further back into the operators network. Some network designs incorporate further levels of aggregation in the access network, leading to ‘second aggregation points’ and ‘second mile’ connections.

In legacy 2G and 3G radio access networks backhaul links often connected to remote BSC (Base Station Controller) or RNC (Radio Network Controller) sites, which served as the radio resource management nodes for an area of the access network. In addition to signalling and management functions, these sites provided a further aggregation point for access traffic as all access connections for nodes in a given area would be routed to the controller site. In more modern network designs the access network controller has either been moved backwards to a core network site or doesn’t exist at all but many networks have kept the remote sites operational to continue to act as traffic aggregation points, with high capacity fibre connections to the core network.

The high capacity connections established between the first/second aggregation points and the network controller site is often termed the ‘metro transport network’. Connections between remote network controller sites and the core network can also be carried by the metro network.

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Access Network Backhaul

Backhaul can be provided in many ways using many different technologies. Some of the key distinctions between backhaul models are outlined in the diagram.

One basic aspect is the ownership of backhaul resources, which can either be owned by the network operator or provided by a partner, supplier or outsourcing provider. Operator-owned backhaul solutions were popular with some operators in the early days of GSM (Global System for Mobile Communications), when many companies built extensive private microwave networks to serve their backhaul requirements. These same operators are now faced with huge challenges and, potentially, the huge costs associated with upgrading or replacing those networks to support the requirements of evolved 3G and new 4G access networks.

Another basic differentiator of backhaul solutions revolves around the technologies employed to carry links: options include radio (microwave, point-to-multipoint radio and satellite systems), copper (leased lines and xDSL broadband links) or optical fibre.

A final differentiator is provided by the choice of Layer 1 networking technologies selected, choices including PDH, SDH, WDM and OTN and Layer 2 data link technologies including Ethernet and MPLS. Networking, layer 3, choices are limited to a decision of whether to use IP (Internet Protocol) in the access network or not; HSPA (High Speed Packet Access) and LTE (Long Term Evolution)networks will always be based on IP transport and will generally employ some form of IP-based RAN (Radio Access Network).

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Traditional Backhaul Requirements

The basic requirements that define the type of backhaul to be employed and the configuration of access network connections can be summarised as shown in the diagram.

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Radio Network Evolution and IP Convergence

Two of the main drivers for network operators to revisit and refresh their backhaul environments are the evolution of radio access networks and the continuing convergence of network technologies towards an all-IP architecture.

Legacy radio access networks, such as those employed by GSM and R99 UMTS, were based on a variety of technologies such as E1 TDM (in GSM), E1 TDM/Frame Relay (in GPRS) and E1/ATM (in UMTS). As network operators refresh their 2G and 3G access networks and deploy evolved HSPA and LTE systems, many are taking the opportunity to replace the legacy technologies employed in their backhaul environments with Ethernet/IP-based systems.

Converged and combined backhaul solutions that serve the needs of 2G, 3G and 4G access networks can be based on a range of Layer 1 and Layer 2 technologies with a host of IP-related Layer 3 technologies operating over the top. One further driver for the evolution of mobile backhaul networks is the increased use of site sharing as a means of reducing operating costs. Whereas in the past each network operator generally maintained its own estate of transmitter sites it is increasingly common to find operators pooling resources at shared sites, with an associated pooling of site leasing, power and transmission costs. Site backhaul links in these cases may be required to carry the traffic generated by multiple base stations belonging to several different operators.

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Evolved Network Backhaul Requirements

The enhanced requirements imposed on backhaul solutions by evolved access network technologies and architectures are outlined in the diagram.

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Motivation for Packet Transport

With packet-based traffic likely to become dominant in the years to come, CAPEX considerations for transport systems are becoming critical. Continuing down the traditional route of building backhaul solutions based on an NxE1 TDM model is too costly. As the graph shows, there is a direct link between traffic growth and transport costs in the TDM model. If revenue grew in step with traffic, this might not be a problem. However, in a data-dominant world, where flat-rate charges are the norm, then Average Revenue Per User (ARPU) will not increase in line with traffic growth. It is imperative that operators deploy new transport technologies in order to reduce the cost per bit and break the linear relationship between traffic growth and transport costs.

Ethernet is a technology that can bend the cost curve downwards. Ethernet connectivity is near enough ubiquitous; port costs are relatively cheap; it is well known and understood, and is the main connection of choice for most customers; it is naturally suited to carry packet-based technologies, and the physical interface scales well in terms of capacity. Implementing carrier Ethernet services will mean added investment now, but the incremental costs to meet future capacity demands will be relatively small. Installing a 100 Mbit/s interface at a cell site now is likely to meet demands for years to come, assuming appropriate access to the site such as fibre or Ethernet microwave radio.

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4G LTE Protocol Stacks

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4G RAN Example

The higher bandwidth requirements of the 4G LTE RAN limit the options available to an operator in respect of backhaul alternatives.

A typical LTE site will require several tens of megabits per second of backhaul capacity. This, coupled with the fact that LTE backhaul traffic is always carried by IP, generally limits an operator’s choices to either a packet microwave link or a direct optical fibre connection.

Packet microwave systems employ a range of techniques to transport IP traffic, including native Ethernet radio systems and those that carry IP or IP/Ethernet over legacy TDM services – an example of this would be IP/Ethernet over SDH radio. Packet microwave is an option often chosen by network operators who wish to build and maintain their own backhaul infrastructure. Packet microwave systems are offered in a range of transmission bandwidths including STM-1 and STM-4 for TDM-based systems and 10, 100 or 1000 Mbit/s (also known as Gigabit Ethernet or GigE)

Optical fibre-based services are usually provided to operators by third-party transmission providers using technologies generically known as FTTP (Fibre to the Premises). Although legacy SDH systems are still available, many newly commissioned fibre services are based on more modern MPLS and Carrier Ethernet techniques, which offer typical line-speed data rates of 100, 1000 or 10,000 Mbit/s (known as 10 GigE).

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Small Cell Backhaul Examples

A key component of any 4G operator’s rollout plans will be the deployment of HeNB (Home eNB) femtocell devices alongside more traditional pico and nano cell elements. Collectively, the market for femto, pico and nano scale cellular connectivity is known as the ‘small cells’ market.

Femtocells are often owned by the subscriber, they provide a ‘bubble’ of 4G coverage around an HeNB and are usually deployed in the customer’s home or small business site. Backhaul connectivity for femtocells is typically routed via the customer’s own broadband service and across the Internet to the operator’s core network. More traditional pico and nano cell deployments are generally owned by a network operator and are used to provide ‘donor cell’ in-building or campus coverage for SMEs (Small/Medium sized Enterprises) and corporate customers. Backhaul for these types of small cell is generally supplied and paid for by the network operator.

HeNB femtocells can be deployed in one of three modes – closed, where the resources of the cell are available only to registered users of the associated CSG (Closed Subscriber Group); open, where the cell is open to any network user, or hybrid, where a subset of cell resources are openly available but others are reserved for CSG users only.

Studies have suggested (see http://www.techweekeurope.co.uk/news/london-needs-70000-cells-for-4g-broadband-40779, for example) that for operators to meet their coverage and peak data rate objectives in dense urban areas, they will be required to deploy an order of magnitude of more cells than was required to support 2G or 3G coverage. No operator has the resources to pay for the backhaul for such large numbers of sites and so it is assumed that a large proportion of the additional sites will be femtocells with backhaul costs met by the subscriber.

Operators have suggested that they could institute some form of discount or profit-sharing scheme to entice subscribers to deploy open or hybrid access HeNBs. Some commentators have suggested a micro-payments method similar to that employed by power companies who make payments to customers who generate their own electricity (from wind turbines, for example, or from combined heat and power generators) for the power fed back into the grid. Whatever economic model is followed, the data rate available from an HeNB femtocell will be limited to the rate available over the customer’s broadband connection, which may not be capable of providing macro cellular data rates.

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3GPP Transport Network Definitions

3GPP provide separate RNL (Radio Network Layer) and TNL (Transport Network Layer) definitions for the access network portions of the network types that they specify.

GSM networks were originally designed to employ TDM (Time Division Multiplexing) transport technologies on the A/A-bis/Ater access network interfaces. The supported options were E1 (2 Mbit/s) or T1/JT1 (1.5 Mbit/s).

GPRS/EDGE services shared the A-bis interface with GSM between the BTS and the BSC but then travelled over the Frame Relay-based Gb interface to the SGSN (Serving GPRS Support Node).

Release 99 UMTS networks were designed to use ATM at the TNL, but after R5 could also employ IP-based interfaces to support UMTS, HSPA and HSPA+ services.

LTE was designed as an all-IP environment, so the S1 and X2 access network connections between eNBs and the EPC (Evolved Packet Core) use only that TNL technology.

Network operators who wish to consolidate different generations of access network traffic onto a combined backhaul service are able to use packet based technologies to carry or replace the TNL options specified for legacy network types. This means that GSM A-bis interface traffic can be encapsulated (using a process known as CEoPSN or Circuit Emulation over Packet Switched Networks). GPRS Frame Relay traffic can be encapsulated in IP to travel over an IP based Gb interface and traffic from UMTS Node Bs can either be generated as ATM traffic and then encapsulated by IP or can be created as native IP traffic.

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Transport Network Layered Architecture

One way of visualising the configuration of the Transport Network Layer is as shown in the diagram.

In this view the access node (base station) is shown on the left and the access node controller, for 2G or 3G networks, or core network nodes, for 4G networks, is shown on the right. The Radio Network Layer the top of the backhaul protocol stack on both sides for 2G and 3G networks but only on the left side in 4G (as all radio or access stratum functions are performed by the eNB in LTE networks).

Below the RNL lies the TNL, which has been divided into four sublayers.

Sublayer A represents the specified TNL technology for a given generation of cellular system, e.g. TDM for 2G networks or ATM for UMTS.

Sublayer A traffic may be encapsulated by protocols operating at Sublayer B, which includes options such as MPLS, PWE3 (Pseudo Wire Edge-to-Edge Emulation) ‘pseudo wires’ and Ethernet VLANs (Virtual Local Area Networks). This layer provides for the logical encapsulation of upper layer traffic in order for it be carried reliably by the chosen data link technology at Sublayer C.

In most modern backhaul environments Sublayer C consists of Carrier Ethernet, which provides a relatively simple and cost-effective method of carrying widely diverse traffic types, and offers traffic encapsulation, traffic discrimination and connection capabilities. Other options at this layer include parts of SDH and MPLS.

Finally, at Sublayer D, traffic is passed to the underlying transmission technology that will physically carry it over a backhaul connection. Physical layer options in modern backhaul environments include packet-based microwave, copper-based xDSL and various form’s of optical fibre transmission.

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HSPA/LTE Cell Throughput Expectations

The starting point for any dimensioning exercise is to establish the traffic levels that can be expected to be generated by a typical cell site.

Typical peak and average throughput figures for various radio access technologies are shown in the diagram. These figures are based on extrapolations of vendor published figures and on real life experience of testing and using the types of network mentioned. These figures should not be used as the basis for any real world dimensioning or capacity calculations.

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Industry Initiatives and Forums

The development of evolved network backhaul technologies, architectures and standards has been facilitated by a number of industry-led initiatives and forums as well as by the established standards organisations and vendors.

The NGMN (Next Generation Mobile Network) Alliance is formed from operators, vendors and other interested parties and provides a forum to discuss and agree strategies related to the adoption and deployment of new network technologies. In particular, the NGMN has initiated projects to examine the backhaul requirements and options for evolved mobile access networks. Many of their reports are freely available for download at their website.

The MEF (Metro Ethernet Forum) has been responsible for defining and developing standards to ensure the interoperability of Carrier Ethernet. Its membership is drawn from telecoms operators, transport network providers, vendors and other interested parties. MEF technical specifications are freely available from their website.

The BBF (Broadband Forum) develops technical specifications to promote interoperability between vendors and operators of broadband transport systems. Originally formed as the ADSL Forum (and the DSL Forum) the BBF in its current form was created from the merger with the IP/MPLS Forum. BBF technical standards covering DSL, broadband and MPLS are available for download from their website.

Traditional standards bodies such as ETSI (European Telecoms Standards Institute) and 3GPP (3rd Generation Partnership Project) have also provided input to the debate surrounding next generation backhaul requirements.

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LAYER 1 OPTIONS

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Layer 1 Options

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Evolving Backhaul Transmission Options

The backhaul transmission solutions employed in early mobile networks generally used systems based on PDH (Plesiochronous Digital Hierarchy) data rates. E1 (2 Mbit/s) was the primary rate PDH interface type in Europe and many other regions, whilst T1/JT1 (1.5 Mbit/s) was the base standard employed in the US, Japan and several other countries. Higher-order transmission was deployed as multiples of the primary rate, either as direct multiples (2xE1, 4xE1, 8xE1, etc) or in the steps dictated by the PDH standards in use. In Europe these would have been E1 (2 Mbit/s), E2 (8 Mbit/s), E3 (34 Mbit/s) and E4 (140 Mbit/s) and in the US they would have been T1 (1.5 Mbit/s), T2 (6 Mbit/s), T3 (45 Mbit/s), T4 (275 Mbit/s).

Legacy TDM microwave and copper-based transmission solutions were typically scaled to fit in with the strictures of the PDH standards, although more modern systems were designed to use the higher data rates, more efficient multiplexing services and improved inter-working capabilities of SDH (Synchronous Digital Hierarchy), which is known as SONET (Synchronous Optical Network) in the USA. SDH offers transmission capacities calculated in units known as STMs (Synchronous Transport Modules); STM-1 operated at 155 Mbit/s and was capable of multiplexing up to 63 E1 tributaries or a similar quantity of data structured in some other format. Higher order multiplexing options were STM-4 (622 Mbit/s), STM-16 (2.5 Gbit/s) STM-64 (10 Gbit/s) and STM-256 (40 Gbit/s). The highest order multiplexing versions of SDH were only available over fibre connections. SDH microwave systems were available that offered STM-1 or STM-4 and copper-based SDH systems generally topped out at STM-1.

Fibre optical transmission systems are available that operate in a variety of frequency/wavelength bands but generally conform to one of two physical fibre types: Multi-mode fibres which have a comparatively large cross-sectional area of around 50–60µm and are applicable to short distance (of up to a few hundred metres) communication using relatively inexpensive equipment; single-mode fibres which are thinner, around 10µm, but are able to operate over much longer distances using more expensive transmission equipment. Data rates of 100 Gbit/s or more are possible with optical fibre transmission, especially when WDM (Wavelength Division Multiplexing) techniques are employed.

The graph in the diagram provides an indication of the popularity of the three basic backhaul physical layer options prior to the commencement of large-scale LTE rollouts.

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Backhaul Architectures

Backhaul transmission links may be configured in many ways. Some methods offer lower cost of deployment but little in the way of redundancy, whilst others sacrifice capacity at the expense of resilience. The choice of which method to use is generally dictated by the type of network being built, by the importance of the individual sites being connected and by operator policy.

A selection of the most common backhaul architecture types is shown in the diagram, as is an indication of the trade-off between capacity, resilience and cost inherent in each option.

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Layer 1 Options

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Broadband-based Backhaul

The exponential growth of data traffic in mobile networks has led operators and vendors to examine innovative methods of configuring their backhaul, including the possibility of providing different forms of backhaul for traffic with differing time sensitivity. For example, some operators have configurations for HSPA sites which employ traditional backhaul methods for real time and signalling traffic – using TDM-based microwave or leased line connections – but which also use less expensive methods for transporting non-real time packet data traffic.

Most of these less expensive methods rely on commercial broadband services to carry traffic, using transmission technologies from either the xDSL family of wireline broadband systems or on FTTP (Fibre to the Premises). The most commonly deployed forms of xDSL are ADSL and VDSL.

ADSL (Asymmetric Digital Subscriber Line) is the most widely-deployed form of DSL and offers typical maximum data rates of 24 Mbit/s downstream and 3 Mbit/s upstream. It is asymmetrical in data transport terms due to the asymmetric nature of most Internet use patterns, people tend to download more than they upload. VDSL (Very-High Speed DSL) is seen as next-generation version of ADSL, offering potential data rates of up to 100 Mbit/s in both directions.

Both versions of xDSL operate over standard copper local loops using OFDM (Orthogonal Frequency Division Multiplexing) principles which create multiple parallel channels on a transmission line each operating at a different frequency and each carrying a low bit rate subset of the line’s overall data throughput. ADSL lines operate using dozens of frequency ‘subcarriers’, VDSL lines use hundreds. The carrying capacity of an xDSL line is determined by the bandwidth of frequencies available to reliably bear subcarriers, which in turn is governed by the quality of the line and the distance the signals are required to travel.

Higher frequency subcarrier channels are only reliable over short distances and quickly become unusable, meaning that full rate services are only available to subscribers located close to the DSLAM (DSL Access Multiplexer) equipment, which is generally located either in street cabinets or at the local telephone exchange. ADSL can generally be expected to offer a full 24 Mbit/s service over distances of a few hundred metres and the data rate drops off rapidly with distance; VDSL operates at full speed over distances of a few tens of metres.

FTTP (Fibre To The Premises) provides, as the name suggests, a broadband service in which a fibre optic cable is routed (or ‘blown’) all the way to the subscriber’s premises. Data rates of up to 1 Gbit/s are possible, with higher data

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Optical Fibre

The overriding limiting factor in the use of wire pairs is the resistivity of the conducting material (usually either copper or aluminium). The resistance and thus the loss per unit length of a wire pair can be decreased by increasing the cross-sectional dimensions of the pair, but this is limited by cost and practical factors. In addition, as the frequency of the transmitted signal increases, it has a tendency to travel in a very thin skin at the surface of the conductors, which greatly reduces the cross-sectional area through which it travels, thus increasing the resistance. Furthermore, at frequencies of the order of GHz, there is a significant extra loss in the insulating material, which separates the conductors.

Optical fibres use a fundamentally different form of transmission to that in wire pairs, i.e. the information is sent as a fluctuating beam of light along a glass tube. It is now possible to manufacture pure glass, which causes very little loss in the transmitted light. As light frequencies are very high it follows that fibre can be used for signals with much greater bandwidths (i.e. hundreds of megabits per second). Further advantages of optical fibres are immunity from EM interference, which makes it suitable for use even at relatively low bit rates in electrically noisy environments, and that it does not create a spark hazard and is therefore suitable for use in areas subject to fire risk. It also generates negligible interference and is therefore extremely difficult to tap.

The fibre itself consists of glass core, a glass cladding of lower reflective index than the core and one or more protective coatings. The light is contained within the core of refraction; the precise way it travels down the fibre depends on the dimensions and the refractive profile of the core.

The transmitter is a Light Emitting Diode (LED) or a high-performance laser (Light Amplification by Simulated Emission of Radiation); the receiver is a light-sensitive photo-diode or photo-transistor. Most optical fibre systems are used for digital transmission with the information sent as a sequence of pulses and spaces.

Typical wavelengths used are between 800 and 1,600 nm, although in practice three wavelengths are commonly used, i.e. 850 nm, 1,300 nm (1.3 µm) and 1,500 nm (1.55 µm), each having a bandwidth of 100 GHz.

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Layer 1 Options

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Types of Optical Fibre

Each type of optical fibre cable is classified by one of the methods of rating the index mode. In practice there are three commonly used types of optical fibre cable. These are multimode stepped index, multimode graded index and monomode.

Multimode Stepped Index

In a multimode stepped index fibre the cladding and the core have a different but uniform index (the index steps up between the cladding and the core). Some of the light emitted by the core strikes the core/cladding interface at an angle greater than the critical angle (critical angle is the angle at which total internal reflection first occurs) and is refracted into the cladding where it is absorbed; the rest of the light is propagated along the core by means of total internal reflections at this interface. Depending on the angle at which it strikes the interface, the propagated light takes a variable amount of time to reach the receiver. This stretching out, or dispersion as it is commonly referred to, tends to cause the pulses and spaces to merge. The effect restricts the bit rate and/or transmission distance.

Graded Index

Dispersion can be reduced by using a core material that has a graded refractive index, i.e. it declines in a particular way from a maximum at the core axis down to the value of the cladding’s index at the interface. The light is refracted by an increasing amount as it moves away from the axis. The average refractive index on the longer paths is less than that on the shorter paths because the former go closer to the core/clad interface. The average velocity on the longer paths is therefore greater than that on the shorter ones with the result that the propagation time along the different paths is almost constant. The received pulse widths in graded index fibre are therefore less than those in stepped index fibre, allowing an increase in the bit rate and/or transmission distance.

Monomode

A multi-mode fibre is so called because it allows light to travel down the fibre in a number of different modes, each of which is associated with a particular propagation angle, i.e. angle relative to the core axis. As the diameter of the core is decreased there is a reduction in the number of modes and when the diameter becomes less than approximately 10 µm only one mode is possible. There is very little dispersion with this type of fibre and used with a high laser diode, a very high performance can be obtained, i.e. bit rates in the order of hundreds of megabits per second combined with transmission of the order of tens of kilometres. Monomode is also known as ‘single mode fibre’.

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Capacities and Distance Limitations

Optical fibre cabling is available in two main types: multi mode and single mode.

Multi mode fibres have a larger diameter, of between 50 and 100 µm, which provides multiple paths for signals to travel along. Optical signals propagate along multi mode fibres by reflecting off the inside face of the outer layer of the fibre and the dispersal related to each reflection can cause the signal to scatter over distance. Multi mode fibres are therefore comparatively cheap to produce but only work effectively over short distances.

Single mode fibres are thinner than multi mode, with diameters of between 8 and 10 µm. This makes them more expensive to produce but allows them to carry signals that stay coherent over longer distances. This is due to the internal construction of the fibres, which are radially graduated in density ensuring that signals refract back towards the centre of the fibre before they have an opportunity to reflect off the outer layer. No reflections means less dispersal, allowing signals to travel further before regeneration is required.

One of the methods employed to determine the capacity and distance rating for an optical fibre is the calculated Bandwidth-Distance product, which is presented as MHz x km. The bandwidth of a signal describes the range of frequencies that can be carried, and the distance component estimates how far the suggested bandwidth can be coherently transported. A product of 500 MHz x km, for instance describes a signal covering 500 MHz that can be coherently carried for 1 km or 1000 MHz signal carried for 0.5 km and so on.

Given that the most commonly deployed Layer 2 option for next generation network backhaul is Ethernet. The diagram demonstrates some of the available physical layer standards that have been developed to transport Ethernet over both multi mode and single mode fibre connections.

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Layer 1 Options

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Microwave

Most modern-day microwave point-to-point radio links are a subsystem within a much larger network such as a mobile phone network. Such a link could be required, for example, between a local BTS and a BSC, especially in areas where other links such as copper or fibre are not practicable.

When considering the installation of a microwave link, several factors need to be taken into consideration:

■

■ the transmitter power output ■

■ the type and size of transmit and receive antenna, both of which provide gain and directivity ■

■ transmission line losses between equipment and antennas ■

■ free space path loss ■

■ anomalous propagation effects ■

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Types of Modulation

Radio links generally offer a harsh environment in which to transmit data, many forms of interference exist and multiple sources of fading can affect a signal. Microwave links have an advantage over other forms of radio communication in that they operate over tightly focused, line-of-site connections allowing each link to be individually optimized to match the conditions encountered.

Cellular and point-to-multipoint radio transmission systems are able to make use of relatively low order digital modulation schemes such as QPSK (Quadrature Phase Shift Keying) and 16 QAM (Quadrature Amplitude Modulation), with very short or high quality links sometimes capable of using 64 QAM. The more controllable medium offered by a microwave link is, conversely, able to employ much higher order modulation techniques.

Traditionally, the kinds of microwave service employed to backhaul cellular base station traffic has operated in licensed frequency bands over medium length distances of 10–20 km. These systems were ale to employ modulation schemes such as 128 QAM or even, for more modern systems, 256 QAM to achieve data rates in the order of a few hundred megabits per second, although there is a difference between the gross data rate that a link is able to carry and the actual throughput that can be achieved by users across those links.

512 QAM and 1024 QAM systems were introduced in the late 2000s and offered user throughput data rates of over 300 Mbit/s, making them capable of carrying two SDH STM-1 signals or two or three Fast Ethernet channels.

From 2012 onwards microwave systems that were able to support even higher order modulation schemes become available, starting with 2048 QAM and progressing to 4096 QAM.

Microwave equipment manufacturers have predicted that links will eventually be capable of handling 10 Gbit/s and will therefore allow microwave to continue to be employed as a backhaul bearer even as the level of capacity demanded by HSPA+ and LTE sites continues to increase.

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Layer 1 Options

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Dual Ethernet/TDM Radio

Cellular operators who wish to transport a mixture of legacy and next generation traffic often opt for dual Ethernet/TDM microwave systems. Unlike Ethernet encapsulation systems, which carry next generation traffic wrapped up in a TDM bearer, dual protocol systems transport both types of traffic in their native format in a combined frame.

A typical dual Ethernet/TDM node will support separate Ethernet (10/100/1000 Mbit/s) and TDM (E1/T1 or STM-1, for example) interfaces and will connect to both legacy and next generation base station types.

Traffic will be handled by separate Ethernet and TDM switches, which will connect to a common multiplexing element. The multiplexer would combine Ethernet and TDM frames and place them into a microwave transport frame, which will itself contain a header carrying identification and synchronization information.

A 256 QAM microwave link might therefore be capable of carrying a 100 Mbit/s Fast Ethernet service and SDH STM-1 or a multi-E1 service simultaneously.

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Gigabit Ethernet Microwave

Microwave links capable of carrying Gigabit Ethernet services exist and employ a form of inverse multiplexing to achieve the required physical layer data rate.

A typical GigE microwave link consists of four standard 256 QAM links combined along a signal path. The GigE microwave transmission node would consist of a controller unit, hosting a GigE interface and a multiplexing/inverse multiplexing element, coupled with four 256 QAM IDU (Indoor Unit) elements.

The IDUs would connect to a set of ODUs (Outdoor Units) that would feed signals to the same parabolic antenna. The separate 256 QAM signals are transmitted using a combination of techniques such as RTA (Radio Traffic Aggregation), frequency division multiplexing and XPIC (Cross Polarised Interference Cancellation) to allow them to share a transmission path without interfering with each other. Other techniques, such as AMC (Adaptive Modulation and Coding), allow link configuration to be dynamically adjusted in response to changes in air interface conditions. Links that employed FDD (Frequency Division Duplex) would use separate sets of transmit and receive frequencies to provide a bi-directional transmission path between nodes

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Layer 1 Options

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NLOS Wireless Backhaul

Traditionally, cellular network operators have employed a variety of E1 or T1-based transmission technologies to backhaul traffic from base station sites. The most popular methods have been cable-based leased-line connections or microwave links.

Microwave links are generally owned and operated by the cellular operator directly. This imposes a different set of costs. The costs associated with planning the LOS requirements of microwave links and the licenses that need to be acquired can also be expensive.

WiMAX is seen by some cellular operators as a viable alternative backhaul method. WiMAX links can be directly owned and managed by the cellular provider, without having to pay leasing costs to a potential competitor. Unlike traditional microwave systems, some versions of the WiMAX air interface are designed to work at the lower end of the microwave range at frequencies that can take advantage of multipath effects. Such systems may not require a direct line of sight between transmitter and receiver in order to maintain a radio connection.

Several manufacturers have solutions that are designed to carry traditional E1/T1 services over WiMAX. Many of them employ an E1 over IP over WiMAX protocol stack, where IP applications are used to maintain the strict real-time synchronization required by E1/T1 links.

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LAYER 2 OPTIONS

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Layer 2 Options

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Ethernet Line Services – EVPL . . . 3.10 Ethernet Label Switching (ELS) . . . 3.11 Carrier Ethernet Summary . . . 3.12 The MPLS Architecture. . . 3.13 MPLS Shim Header . . . 3.14 MPLS Packet Forwarding. . . 3.15 Architecture of MPLS-based IP-VPNs . . . 3.16 VPLS (Virtual Private LAN Service) . . . 3.17 VPLS Services . . . 3.18

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Generic Layer 2 Options

A wide range of different Layer 2 technologies have been employed to provide Data Link layer functionality for cellular backhaul links.

Legacy 2G systems employed primary rate E1/T1 PDH connections running over 2/1.5 Mbit/s physical layer connections.

3G UMTS networks were originally designed to employ ATM as their Layer 2 backhaul technology and links were often configured to run over E1 bearers of the initial part of their journey. As 3G sites became busier networks were able to enhance the backhaul data rates available by employing IMA (Inverse Multiplexing for ATM) to bind multiple E1/T1 links together to support higher rates of ATM traffic. Sites that required greater amounts of capacity were often provided with SDH connectivity to transport their ATM-based backhaul traffic.

Later 3GPP Releases allowed operators to evolve their 3G sites onto native IP connectivity, which could continue to be routed over legacy backhaul transmission services or could be diverted to run over new Ethernet or MPLS-based connectivity.

4G LTE systems operate in an all-IP environment and the most commonly deployed Layer 2 technologies selected to support these next generation networks are Carrier Ethernet and MPLS.

Carrier Ethernet and MPLS can also be used as the basis for multi-RAT backhaul services that carry traffic for combined 2G, 3G and 4G access networks.

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Ethernet (1)

The first version of Ethernet was developed at the Xerox Corporation’s Palo Alto Research Centre in the early 1970s by a team led by Bob Metcalfe and ran over coaxial copper cables at a data rate of 2.94 Mbit/s. Work on defining a standards-based version of Ethernet was started by the IEEE (Institute of Electrical and Electronics Engineers) in 1980. The date of the meeting that kick-started this effort, in February 1980, led to it being known as the 802 project. A more commercially-orientated version of Ethernet, known as Ethernet II, was jointly described by a team of engineers drawn from Digital, Intel and Xerox in 1982. Modern Ethernet systems operate using a mixture of 802.x and Ethernet II derived protocols and standards.

Ethernet operates at Layers 1 and 2 of the OSI (Open Services Interconnect) protocol model and is designed to provide LAN (Local Area Network) facilities for upper layer protocols.

The IEEE 802.2 standard defined the LLC (Logical Link Control) layer for Ethernet which was mainly used, in association with the related SNAP (Sub Network Access Protocol), as a means of identifying the upper layer protocol for which a given Ethernet packet was carrying traffic and also for facilitating the bridging of traffic between different LAN types. LLC/SNAP are not in general use.

The main functionality of standards-based Ethernet is described in the 802.3 family of specifications. 802.3 describes the frame structure to be employed by Ethernet, which is slightly different from that specified for Ethernet II. It also describes the operation of the MAC (Medium Access Control) protocol at Layer 2, which controls the way in which network adaptors gain access to the shared medium of the LAN.

Layer 1 functions are split between the PCS (Physical Coding Sublayer), which operates the CSMA/CD (Carrier Sense Multiple Access/Collision Detection) service that informs the operation of the MAC layer; the PMA (Physical Medium Attachment) sublayer, which manages the mapping of electrical signals onto physical bearers and therefore also defines the layout of physical connectors. The PMA is also responsible for handling physical transmission functions such as signal scrambling and synchronization.

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Ethernet (2)

The PMD (Physical Medium Dependent) sublayer provides definitions of the various physical media (copper cables, optical fibres) that can be used to transport Ethernet traffic.

Collectively, the functionality specified to support the use of Ethernet over different physical media are described using ‘XBaseY’ notation for standards names applied to different version.

The ‘X’ part of the name refers to the maximum line rate of the physical medium type; 10 for 10 Mbit/s, 100 for 100 Mbit/s, 1000 for 1G bit/s, with 10G, 40G and 100G used for even higher order line rates.

The ‘Base’ part of the name refers to the transmission mode employed at the physical layer; originally two options were defined – ‘Base’ for ‘baseband’ transmission and ‘Broad’ for ‘broadband’ transmission. Typically, only ‘baseband’ transmission is employed in modern Ethernet systems (where the term ‘baseband’ refers to a transmitted signal that occupies the entire available bandwidth of the physical medium) and the ‘Broad’ option has vanished.

The ‘Y’ part of the name refers to the physical medium employed by the variant, with ‘T’ used originally to denote ‘twisted pair’ copper cables and ‘F’ used to identify a ‘fibre’ medium. As the set of physical media employed by variants of Ethernet has multiplied, the ‘Y’ component of standards names has become more complex.

Each variant of IEEE Ethernet is described in its own 802.3 document. The IEEE append an identifying letter to the end of the standards name for each new variant they approve, so the original 802.3 specification was followed by 802.3a, then 802.3b and so on. Recent additions to the 802.3 family include 802.3bf and 802.3bg, which serves as an indication of the number of variants that have been developed.

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Ethernet Physical Layer Family

Ethernet supports a variety of physical medium variations of three main cable types; twisted pair copper cables, single-mode fibre cables and multi-single-mode fibre cables.

Copper cable implementation of Ethernet almost all employ UTP (Unshielded Twisted Pair) cables and are usually suffixed with a ‘T’, as in ‘100BaseT’. The quality, noise immunity and data carrying capacity of UTP cables are classified in Categories; a Category 6 (or Cat6) compliant cable uses better quality copper and has more twists per metre than a Cat5 cable, allowing it to be used to carry higher data rates. 100BaseT Fast Ethernet is able to operate over up to 100 m of Cat5/6 UTP, whereas the higher data rates of GigE, 10 GigE and upwards are available over shorter and shorter distances. 40 or 100 GBaseCR Ethernet, for example, only operates across a maximum of seven metres.

Fibre optic physical layer options are generally suffixed with ‘SX’ for single mode fibre or ‘LX’ for multi-mode fibre but a variety of different suffixes also exist for versions with particular characteristics.

Not shown in the diagram, and not generally used for backhaul purposes, there are also suffix ranges of ‘P’ to be used with PON (Passive Optical Network) technologies and ‘K’ to be used in system backplane implementations.

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Ethernet Frame Structure

Upper layer data is carried across an Ethernet-based connection or network encapsulated in an Ethernet Frame. The structure of an Ethernet Frame is outlined in the diagram. There are small differences between the structures used by a standard 802.3 frame and by an Ethernet II frame. The 802.3 frame format is commonly employed by consumer and SoHo (Small Office, Home Office) systems, whereas Ethernet II is more often found in corporate and service provider networks.

Both frame types start with a preamble, which is a repeating pattern of ‘101010101’. Ethernet employs decentralised asynchronous transmission, meaning that there is no central device providing a permanent synchronization signal in a network. Instead each receiving device must synchronize itself with the clock rate employed by a transmitting device and the preamble allows a receiver to sync with a signal before attempting to decode the frame that it carries. The first difference between 802.3 and Ethernet II is that the preamble for the former ends in the eighth byte with ‘11’, whereas the preamble for the latter is seven bytes long and is followed by an SFD (Start of Frame Delimiter) field, which also ends with ‘11’.

Both types of frame then carry the destination and source addresses for the frame in the form of 48 bit (6 byte) MAC Addresses. 802.3 frames then carry a 2 byte Length indicator field that specifies the length of the payload carried by the frame, whereas Ethernet II frames use these 2 bytes to carry an EtherType field that identifies the upper layer protocol being carried by the frame. 802.3 frames were designed to be used in association with LLC/SNAP headers that would have provided the same functionality as the EtherType field. The developers of Ethernet II decided that the use of an EtherType field was more efficient than the insertion of an LLC/SNAP header.

Both types of frame then insert a payload area that encapsulates an upper layer data packet, in the vast majority of cases this will be an IP packet. The data field must be a minimum of 46 bytes long to allow the CSMA/CD function to operate correctly, so frames that encapsulate packets that are smaller than this length must also contain a variable amount of padding. The maximum size of upper layer packet that can be carried by a standard Ethernet frame is 1500 bytes, although larger packets can be created for variants such as Gigabit Ethernet.

Both types of frame end with an FCS (Frame Check Sequence) field that contains as 32 bit CRC (Cyclic Redundancy Check) value calculated from the substantive fields (e.g. not the preamble, SFD or FCS) of the frame. A minimum inter-frame gap of 12 bytes is specified as a means of ensuring that false collisions aren’t detected based on echoes of the

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VLANs and 802.1q

Ethernet was originally designed to provide local area networking services to nodes connected to the same local physical medium or to devices connected to the same hub or switch. This came to be seen as a limitation to the way in which organisations created workgroups and to their ability to network between associated devices that were physically remote. Later iterations of Ethernet therefore added the ability to create ‘virtual’ LANs or VLANs.

A VLAN operates in the same way as a physical LAN; it provides an all-informed broadcast medium linking a set of devices together with the added benefit that the members of a VLAN do not need to be connected to the same physical medium. VLAN functionality is managed by Ethernet switches. When a node is to be included in a VLAN the Ethernet switch that serves it is configured with the appropriate VLAN ID on the port that connects to the node. Any frames transmitted by the node are automatically copied by the switch to ports configured with the same VLAN ID. When frames transmitted by a VLAN member are forwarded from the originating switch to other switches a VLAN ‘tag’ is added to the frame to indicate the VLAN to which it belongs. Receiving switches will then forward the received frame through any local ports configured as members of that VLAN.

The ability to add and remove (or ‘push’ and ‘pop’) VLAN tags plus the signalling intelligence that allows switches to advertise details of the VLANs that their local connected nodes belong to are provided by VLAN trunking protocols. There are two main trunking protocols in common use; VTP (VLAN Trunking Protocol) is a proprietary Cisco protocol and 802.1q (commonly known as ‘dot 1q’) is an open-standards IEEE protocol.

802.1q adds tags to VLAN frames as they travel between switches. Dot 1q tags are inserted between the Source MAC Address and EtherType fields and are removed before the frames are forwarded to end-user devices. A dot 1q tag is 4 bytes long and consists of a TPID (Tag Protocol ID), which usually defaults to a value of 8100 in hexadecimal. Two further fields, PCI (Priority Code Indication) and CFI (Canonical Format Indicator), are generally unused and are set to default values, leaving the VLAN ID field as the only substantive part of the tag. The VLAN ID Field is 12 bits long, allowing a network (or a region of a network) to define up to 4096 separate VLANs.

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Q-in-Q VLAN Stacking

The Dot 1q concept was extended to provide support for service provider networks with a facility that become known as ‘VLAN Stacking’ or ‘Q-in-Q’, which allows additional Dot 1q tags to be inserted (or ‘pushed’) into a frame to create a hierarchy of VLAN connectivity. Q-in-Q was standardised in the IEEE 802.1ad specification, although it was largely based on developments made by a number of proprietary systems.

In its basic, most commonly deployed model, Q-in-Q allows service providers to push a second VLAN tag into an Ethernet frame. This second tag is known as an ‘S-Tag’ and is generally used as a means to identify traffic belonging to a particular customer network. The original tag would have been created by the service provider’s customer to identify the internal VLAN to which the traffic belonged and is therefore known as a ‘C-Tag’. Other implementations of VLAN Stacking employ multiple layers of tagging.

The S-Tag is ‘pushed’ into a frame when it enters the service provider’s network (it is positioned between the Source MAC Address and the existing C-Tag) and is ‘popped’ out again when it exits that environment to return to the customer’s network. Q-in-Q therefore allows service providers to offer connectivity between customer sites over a shared network.

Switching of Q-in-Q traffic within the service provider is generally based on a combination of destination MAC Address and S-Tag, meaning that service provider switches need to support an enhanced set of functions beyond those supported by ‘normal’ Ethernet switches.

An S-Tag is constructed in the same way as any other Dot 1q tag and service providers have access to the same pool of 4096 VLAN IDs as any other implementation of the standard. To overcome the obvious limitations this would impose on service scaling, many providers partition their networks into separate ‘regions’, each of which is able to support the full set of VLANs. Regions are interconnected using 802.1 Provider Bridges or Provider Backbone Bridges (PBBs). VLAN Stacking is one of the fundamental functions employed by Carrier Ethernet and an example of a Carrier Ethernet frame, including S-Tag and C-Tag, is shown in the diagram.

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VLAN Stacking Implementations

In this example a mobile network operator has contracted with a Carrier Ethernet provider to receive backhaul aggregation services. The provider’s Ethernet network offers connectivity between the operator’s core network sites and their remote aggregation sites. Onward connectivity from the aggregation site to each base station site is carried by operator-owned Ethernet-based backhaul transmission such as GigE Microwave.

The base station in this example has been configured to belong to the operator’s VLAN 3, so traffic destined for that site will be tagged by core network Ethernet switches with Dot 1q VLAN ID 3.

When outbound Ethernet frames pass through the Carrier Ethernet service provider’s gateway they are provided with an additional Dot 1q tag. The existing tag now becomes the C-Tag and the new tag pushed on at the gateway becomes the S-Tag. The service provider has assigned VLAN ID 1027 to this customer virtual connection, which is the value carried in the S-Tag.

The S-Tag allows the frame to be switched through the provider’s network to the appropriate cell site or aggregation site gateway, where the S-Tag is popped from the frame and the original, single-tagged frame is available to be forwarded to the destination cell site.

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VLANs for Radio Access Networks

VLANs can and are employed to support packet-based backhaul deployments. In a generic Ethernet backhaul environment, VLAN management can be handled by Ethernet switches deployed at various points. In the example shown in the diagram the interface between the operator’s core network and the access/backhaul network is managed by an IP router acting as a CNG (Core Network Gateway). Base station sites in each region are connected, via 1st/2nd Mile backhaul, to aggregation sites at which Access Network Gateway Ethernet switches provide local VLAN forwarding. Connectivity between the CNG and each ANG, in this example, is provided by a CEth (Carrier Ethernet) provider’s Q-in-Q VLAN stacking service.

Sets of base stations are configured into VLANs. Each VLAN might be defined to support the set of sites in a particular geographic region or ‘cluster’. Alternatively, different VLANs might be defined to serve the base stations belonging to a particular RAT (GERAN, UTRAN or EUTRAN) in a cluster.

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Ethernet Line Services – EVPL

The EVPL (Ethernet Virtual Private Line) service employs service multiplexing at the SP’s Edge device to enable multiple services (EVCs) to be delivered over a single physical connection (UNI) to a customer premises. In the example above, a single physical UNI carries multiple service flows, e.g. traffic belonging to multiple VLANs, where each service flow is mapped to a different EVC. The EVCs may terminate at different end points on the Carrier Ethernet Network.

The EVPL may be used to replace Frame Relay or ATM L2 VPN services to deliver higher bandwidth, end-to-end services, and it supports ‘hub and spoke’ connectivity via the Service Multiplexed UNI at the hub site. This is similar to Frame Relay or Private Line hub-and-spoke arrangements.

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Ethernet Label Switching (ELS)

The concept of ELS, aka Q-in-Q tunnelling mode, is to use the same 802.1ad Ethernet frame format but to forward based on the ingress port number VLAN-Id but not the MAC address. Tag stacking may still be used and there is potential for basing switching decisions on double tags as well as single tags. MAC learning and flooding are replaced with VLAN switch configuration, which is used to preconfigure a point-to-point Ethernet Virtual Connection across the network.

Note that tags may be added, swapped or removed by the Ethernet ‘cross connect’ switch. Additionally, bridged connections can be made from a single port to multiple ports.

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Carrier Ethernet Summary

This section has discussed the evolution of transport networks from being voice centric to becoming data centric and the importance, going forward, of building transport networks that are optimized for packet transfer. Ethernet connectivity is becoming, if it is not already, the connectivity option of choice. Transporting Ethernet on an end-to-end basis is vitally important to customers and service providers alike.

As Ethernet services become business critical it becomes equally critical that the technologies used to transport it are reliable. The term ‘carrier Ethernet’ is often thought to describe a technology, but in practice it is a reference to providing the reliable end-to-end transfer of Ethernet service flows.

The MEF has provided an industry-accepted definition of carrier Ethernet: ‘A ubiquitous, standardized, carrier class Service and Network defined by five attributes that distinguish it from familiar LAN based Ethernet’. Additionally, the MEF have defined a standard set of Ethernet services, traffic shaping parameters and CoS marking scheme so that QoS can be applied in the transport network.

Transport attributes have been identified together with a number transport technologies that provide these attributes. Carrier Ethernet Transport is not a single technology but a multilayer network architecture that combines a flexible packet layer with an efficient optical layer, SDH/NG-SDH/OTN, allowing end-to-end QoS, resilience and deterministic performance.

The main forwarding technologies emerging for packet transport solutions are MPLS-TP, PBB-TE and Ethernet Label switching.

One of the major requirements of transport networks is OAM, which includes fault management and performance management.

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The MPLS Architecture

MPLS introduces some new terminology for Layer 2 switching.

An LSR (Label Switched Router) is an MPLS Layer 2 switch, which also has the appropriate control plane functions to participate in setting up and tearing down LSPs.

An LSP (Label Switched Path) is an end-to-end MPLS virtual connection between a pair of edge-LSRs, and across a network of LSRs. This is similar to ATM Virtual Circuits and Virtual Paths.

An edge-LSR is a special LSR that originates or terminates LSPs, and can classify IP traffic for forwarding across the most appropriate LSP by placing the packet in a FEC (Forwarding Equivalence Class).

An FEC is a collection of different packets that are treated identically by the MPLS network. So, for example, in a best-endeavour Internet, all packets to the same aggregate network prefix would typically be within the same FEC.

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MPLS Shim Header

MPLS adds a shim header to the packet being switched through the network. It uses information in the header to switch packets through the network and for the purposes of managing per hop behaviour, i.e. QoS.

The shim header contains the following fields:

label – 20 bits

Experimental (EXP) bits – 3 bits

Stack (S) bit – 1 bit

Time To Live (TTL) bits – 8 bits

The 20-bit label is used to identify a packet for label forwarding. The EXP bits may be used for QoS.

The S bit indicates if a label stack is in place, or that this is the last label in the stack (i.e. S=1), if this is set to a 0 then there are further labels in a label stack down to the label in a header with the S bit set to 1.

The TTL is effectively a hop count and can be used in conjunction with the IP packet TTL at the edge routers of an MPLS network.

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MPLS Packet Forwarding

LSPs would have been established usually as a result of a suitable Link State Interior Gateway Protocol such as OSPF (Open Shortest Path First) providing network topology and reachability information in the shape of a routing table. These LSPs would be created as entries in Label Forwarding Tables as a result of receiving suitable labels from their downstream neighbours to reach each of the networks identified in the routing table.

At the ingress edge-LSR, inbound packets are mapped to their most appropriate FEC to the appropriate initial label value.

A FEC would treat all packets in a similar fashion; therefore any packets wishing to cross the MPLS’s network and reach a particular destination could follow the same FEC.

The forwarding operation is straightforward; the NHLFE (Next Hop Label Forwarding Entry) within an LSR Label Forwarding Table (LFT) maps an inbound label to the appropriate outbound label for the path, and writes the new label into the packet.

At the egress LSR, the MPLS label is removed from the packet and conventional layer 3 routing forwards it towards its final destination.

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Architecture of MPLS-based IP-VPNs

MPLS VPNs operate from a service perspective at the IP layer and classify routers in the customer and service provider network into one of four types:

■ ■ C (Customer Router) ■ ■ CE (Customer Edge) ■ ■ PE (Provider Edge) ■ ■ P (Provider Router)

The C (Customer) routers are conventional enterprise routers, with no special relationship to the VPN service and would not expect to have any knowledge of the MPLS VPN service. The CE device is just a conventional router with support for normal routing protocols. The PE device peers with the customer connections to their CE devices in the customer facing side and onto Label Switched Paths through the MPLS network on the other. The P (Provider) routers in the service provider core are also ignorant of the VPN address and routing information, and simply provide transport across the core network using LSPs.

The PE device uses a Virtual Routing and Forwarding (VRF) instance to tie together the connections to a particular customer’s CE device and the VPN connection across the MPLS network.

Decisions about how to switch the traffic are made at the originating PE router, which understands the customer VPN locations and the LSPs in place across the core. Therefore it can apply a pair of labels (label stack) as traffic enters the network from customer sites.

The inner label is a VPN label allowing the traffic to be routed to the correct VRF at the destination PE router. This label is not examined or changed by the core routers.

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VPLS (Virtual Private LAN Service)

VPLS (Virtual Private LAN Service) is a Layer 2 VPN architecture built for multipoint connectivity and which has broadcast capability and so supports services such as Ethernet over MPLS.

The architecture defined in the current VPLS builds upon the approach taken to implementing IP-VPNs using MPLS. PE routers connect customers to the service provider network and label stacking tunnels customer traffic with VPLS identifiers across an MPLS core LSP network, which fully meshes the PE routers; as a result the CE devices are able to communicate across the MPLS network to other CE devices as though they were connected to a common LAN. Where this solution differs from the IP-VPN service is in the implementation of a learning bridge between the PE routers, rather than the propagation of customer routes.

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VPLS Services

VPLS may be offered in two ways, either TLS (Transport LAN Service) or EVCS (Ethernet Virtual Connection Service). For TLS the VPLS network performs unqualified learning in that all the customer MAC addresses are populated into the same Forwarding Table, thus for a given customer all the MAC addresses would need to be unique across the TLS. The EVCS uses an outer VLAN tag such as that provided by IEEE802.1q to differentiate between each of the customer’s VLANs. This way a separate forwarding table and broadcast domain can be managed for each VLAN. When a customer is attached to a physical circuit, either the port is associated directly with their VPN service and its identifier, or (more usually) an 802.1q VPN ID tag representing the customer VLAN is mapped internally to a separate instance of a FIB (Forwarding Information Base). This is the bridging equivalent of the VRF (Virtual Routing and Forwarding) used in IP-VPNs and is known as a VFI (Virtual Forwarding Instance) or VSI (Virtual Switching Instance). Configuration of an instance of the VPLS service only requires that the physical port is mapped to the customer VPLS identifier in the PE router, or that the 802.1q tag is mapped to the VPLS identifier in the PE router.

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LAYER 3 OPTIONS

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S1 Interface . . . 4.2 X2 Interface . . . 4.3 X2 Deployment and Routing. . . 4.4 Switching vs Routing . . . 4.5 LTE S1/X2 Interface Options . . . 4.6 IP RAN Backhaul Requirements . . . 4.7 Synchronisation Options. . . 4.8 Legacy TDM Synchronisation. . . 4.9 Packet Network-based Synchronisation. . . 4.10 NTPv4. . . 4.11 IEEE 1588v2/PTP. . . 4.12

Sync-E . . . 4.13 Redundancy and Protection . . . 4.14 RSTP/MSTP. . . 4.15 G.8031/G.8032 Protection . . . 4.16 RAN QoS Requirements . . . 4.17 DiffServ. . . 4.18 Carrier Ethernet CoS . . . 4.19 RAN Security Requirements. . . 4.20 IPsec Modes and Services. . . 4.21 IPsec Access Architecture . . . 4.22 SeGW (Security Gateway) . . . 4.23

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LTE BACKHAUL

MOBILE BACKHAUL

REQUIREMENTS

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LAYER 1 OPTIONS

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CONTENTS