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GPON in Mobile Backhaul
Ram Krishna, DDG (FLA), Mrs. Laxmi Director (FLA), Naveen Kumar ADG(FLA-II), N.P.Vadher, ADG(FLA-I) and Divya Sharma ADET (FLA)
Telecommunication Engineering Centre, Department of Telecommunications,
Govt. of India, New Delhi
Abstract
In a hierarchical telecommunications network the backhaul portion of the network comprises the intermediate links between the core network, or backbone network and the small sub networks at the "edge" of the entire hierarchical network. Backhaul is the obligation to carry packets to and from that global network. Backhaul generally refers side of the network that communicateswith the global Internet or an Ethernet Exchange or other core network access location.
This document describes how GPON Technology addresses the need for increased bandwidth, scalability and reliability in mobile backhaul networks while providing lower capital and operation expenses, backward compatibility and an easy migration path to an all– IP network.
1. Introduction
As Mobile Service Providers deploy 3G networks, subscribers are now using their smart phones for true broadband data services, such as Internet access and streaming video. High-bandwidth consumption is no longer limited to the home -- and driven by the deployment of 3G and future 4G systems, fast growth is predicted for mobile broadband adoption.
In their market forecasts, industry analysts anticipate that data volumes will grow at an accelerating rate based on the broader range of broadband data services. At the same time, revenue per megabyte continues to fall, driven down by increasing competition and the introduction of flat rate pricing. To remain competitive, mobile operators must be prepared to provide a network that can not only meet bandwidth demand, but also deliver a healthy profit margin. To achieve this goal, new technology is required to translate this significant rise in traffic into revenue opportunities.
2 In the mobile network, cell site base stations are connected to their network controllers by the mobile backhaul (MBH) network. Traditionally, this is a TDM network optimized for voice but this type of network does not cost-effectively scale to support the growth of mobile data traffic. Consequently, it is critical that operators adopt an affordable backhaul solution that does scale and provide end-to-end packet-based transport.
Looking specifically at the access portion of the Mobile Backhaul Network, about half of all cell sites today are backhauled by microwave, with the other half by wireline systems. In the case of wireline access, E1/T1 leased lines are typically used but these are expensive and their costs scale linearly with bandwidth, making them particularly unsuited to meet the bandwidth demands of mobile broadband. One strategy for a packet-based wireline access network is to leverage GPON residential broadband deployments for mobile backhaul.
Figure 1 Cost challenges of 3G and 4G multimedia services mandate backhaul optimization Source: 2012, Infonetics Research
2. GPON Mobile Backhaul System Architecture
GPON, standardized in the ITU-T G.9 84 series of recommendations, is one of the dominant point-to-multipoint (P2MP) fiber technologies being deployed today. For fixed line access, PON technology already provides proven cost and performance advantages over competing access technologies. The massive worldwide deployment of GPON networks, providing service to millions of users, makes the economics of GPON particularly attractive to mobile carriers.
3 Figure 2 GPON Mobile Backhaul System Architecture
Figure 2 show a typical GPON-based backhaul deployment which supports both legacy TDM and IP networks. The L2 bridge aggregates all traffic flows with no need for routing in the RAN.
2.1 Converged Packet Transport: A Platform Supporting Both
E1/T1 and Packet
The GPON element at the cell site that interfaces to the base station is the cell site gateway. Basic cell site gateway functionality includes termination of the GPON link on the network side and presenting physical interfaces to the base station. At this time, the majority of cell site interfaces are E1/T1, with the availability of native Ethernet interfaces continuing to grow over time. Therefore, to realize the CapEx and OpEx savings that can come from the deployment and operation of a single converged MBH network, the cell site gateway must present to the base station:
• E1/T1 interfaces with a circuit emulation services (CES) interworking function for packet transport.
• Native Ethernet interfaces.
With the next generation of radio networks such as high-speed packet access (HSPA) and long-term evolution (LTE) the capacity to/from the base station will increase significantly compared with today’s requirement of a couple of E1/T1s. In some cases the backhaul capacity for a three sector site will be in the order of 100-400 Mbps. This clearly requires more backhauling capacity. Since cell-size is decreasing and fiber is driven deeper into the
4 network an attractive option is then to use GPON as a backhauling technology to/from base stations.
2.2 Redundancy and Protection
The Mobile Service Provider's service availability targets may lead, in some cases, to the addition of redundancy at key points in the network. In a GPON network the failure cause is fiber which can occur on the network side as well as towards ODN. Network side standards protections techniques are implemented which provides protection on packet network for example RPR, Spanning protocol etc. On the other side two main types protection techniques are defined these are Type “B” and Type “C”. "Type B" redundancy scheme provides a second diversely-routed feeder fiber connects the optical splitter to a second GPON port at the OLT . On the other hand "Type B" redundancy scheme provides a second diversely-routed feeder fiber connects the PON port at ONT toward second GPON port at the OLT. The latter one is more suitable for the purposes and ensures better QoS for the mobile users.
3. TECHNICAL CHALLENGES
3.1 Bandwidth
Fig 3 Backhaul Capacity Requirement at the cell sites Source: Heavy Reading, October 2009
5 The introduction of new mobile data services over 3G and evolving to LTE will lead to maximum bandwidths of 100 to 200 Mb/s per base station. A wholesale backhaul provider may need to backhaul traffic from multiple operators sharing the same site.
These bandwidths can be managed easily in deployed GPON networks that support 2.5 Gb/s downstream and 1.25 Gb/s upstream. Even assuming a high video service take-rate and a worst-case scenario where every residential subscriber is simultaneously streaming multiple and unique high-definition video channels at about 10 Mb/s each, a rough calculation shows that about half (or more) of the downstream bandwidth is still available for multiple LTE base stations on the same PON. In the case where GPON is being considered exclusively for MBH, it is clear that 10 to 30 LTE base stations could be accommodated on a single PON.
In the unlikely event that GPON bandwidth becomes scarce, the well-known PON engineering technique of reducing the fiber split can decrease sharing and free up bandwidth for high-bandwidth users and base stations
3.2 Quality of Service
Mobile backhaul infrastructures are subject to a number of key performance indicators that must be stringently met. These include limitations on packet loss, delay and jitter to ensure that voice services meet the quality criteria for ease of conversation, and practical response times for mobile data services.
To meet the required QoS levels and simultaneously maintain cost-efficiency, sophisticated traffic management capabilities have to be implemented.
Figure 4 shows an example implementation of upstream data flow in a GPON system that is configured to carry all types of services. All service flows are multiplexed through a bridge and are classified according to VLAN and/or priority tagging (it is possible to use a dedicated physical port for each service, but since most GPON ONT devices have a single or dual UNI configuration, separation of services using VLANs is typically required).
6 Each service flow has a dedicated queue, and traffic flows from the same service originated by different users are mapped to the same queue. The classifier maps frames to queues according to the frame’s service type, which is typically identified by VLAN and/or priority. Each queue is then mapped to a dedicated T-CONT, which enables the OLT to identify and provide the corresponding QoS per service.
Figure 4 Upstream Data Flow
Circuit Emulation Service traffic is mapped into a Type 1 T-CONT which is given a fixed (Constant bit rate (CBR)) upstream bandwidth allocation. The OLT grants one or more upstream allocation every N GTC frames (N ≥1). By controlling N, it is possible to control the maximum upstream latency experienced by the CES traffic.
The VoIP T-CONT is given a dynamic bandwidth allocation. By using a Type 2 (assured bandwidth) T-CONT, required latency and jitter characteristics can be met without using fixed bandwidth allocation. Bandwidth is provided only as needed, without over-provisioning. In Figure 4, VoIP traffic is further classified to media and signaling (Session Initiation Protocol, Media gateway control protocol) flows which are carried in separate queues. A scheduler provides strict priority to the media packets. This implementation prevents signaling bursts from causing latency for media packets.
Premium data is mapped to a status-reporting type 3 T-CONT. This type of T-CONT enables implementation of an SLA that allows operators to allocate both assured and maximum bandwidth.
7 BE data is mapped into Type 4 T-CONT, which includes only a best effort component.
3.3 Synchronization
Clock synchronization is a key requirement for reliable backhaul of mobile services. Cellular networks need accurate clock synchronization to prevent call drops during handoffs from cell to cell. GSM and UMTS base stations must hold a carrier frequency accuracy of 16ppb over the service life of the equipment. Drift beyond this limit decreases call hand-off performance, increases call-drop rates and lowers quality of service.
Cell sites must therefore use a clock source that is synchronized with the overall network clock. Traditionally, GSM base-stations recovered the clock from the T1/E1 backhaul line, while some CDMA networks have always relied on GPS receivers at the base-stations as the source for synchronization. GPS is a costly option, and native TDM over GEM is not supported in most GPON systems (in any case, T1/E1 cannot be used in 3G/4G deployments). Therefore, an alternative solution must be found for accurate clock distribution over the PON.
This document presents two implementation options for clock recovery over the GPON network:
• Using PON clock differential timing to distribute accurate clock from the OLT to ONTs • Using IEE1588 v2 transparent clock over PON
3.3.1 Using PON Clock Differential Timing to Distribute
Reference Clock from OLT to ONTs
This clock recovery method uses the fact that a GPON is synchronous – all ONTs operate synchronously to a master clock signal (PON clock) transmitted from the OLT. In addition, the propagation delay (known as RTD – Round Trip Delay) of the optical path for each ONT is known with very high accuracy (typically <10ns). This means that if an accurate reference- clock source (for example, a GPS-based clock) is available at the GPON OLT, it is possible to recover an accurate clock at the ONTs by tracking the frequency and phase relation between the reference clock and the PON clock. This method could be implemented in existing GPON systems using simple, low-cost glue logic in an Field Programmable Gate Array(FPGA) or Complex Programmable Logic Device(CPLD) at the OLT and ONT (see Figure 5).
8 Figure 5 Block Diagram of Clock Distribution Using PON Clock Differential Timing
In Figure 4, FPGA1 acts as a frequency/phase comparator between the reference clock and the GPON clock, constantly monitoring and updating their frequency and phase relationship. This information is sampled by the OLT CPU on a periodic basis. The OLT CPU sends periodic frequency/phase updates to the ONT’s CPU via the PON management channel. The frequency/phase information is therefore known at the ONT, which enables FPGA2 to recover the reference clock from the GPON clock.
This method provides significant cost benefits, as it saves the carrier from deploying costly GPS receivers at each base station. It also enables installation of base stations in areas with limited GPS reception.
3.3.2 IEEE1588 Transparent Clock over PON
The following method may be used if an IEEE1588 slave clock is already installed at the Node B sites. In that case, accurate implementation of IEEE1588v2 requires that each network element measure the packet “time of fly” and report it to the next element. In order to implement this, we may treat the GPON system as a single network element and measure the “time of flight” from OLT CNI to ONT UNI. This can be implemented using two FPGAs at the CNI and UNI boundaries.
9 Figure 6: 1588 Transparent Clock Block Diagram
The transparent clock procedure works as follows:
1. FPGA1 snoops the IEEE 1588 packets on the PAS5211CNI interface (XAUI). 2. The IEEE 1588 packet arrival is timed with a clock aligned to the GPON clock. 3. The timestamp is sent to the PAS52 11 CPU.
4. FPGA2 captures IEEE 1588 packets on the PAS7401 UNI interface (GMII). 5. The IEEE 1588 packet departure is timed with a clock aligned to the GPON clock. 6. The timestamp is sent to the PAS7401 CPU through the management channel.
7. The CPU calculates packet “time of flight” according to arrival time and departure time. 8. IEEE 1588 “follow up” packet is created and sent to the UNI.
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4. Advantages of GPON in Mobile Backhaul Networks
Many advantages that have made GPON successful as a fixed line access technology are relevant to radio access networks:
• Minimized central office assets. Due to GPON’s point-to-multipoint architecture, one OLT port can typically control up to 64 ONTs, thus minimizing power and floor space utilization in the central office sites.
• High reliability and low OPEX. The passive optical distribution network provides higher reliability and minimizes truck rolls and OPEX.
• Efficient topology. The PON tree topology requires less fiber deployment than point-to- point or ring topologies.
• Scalability and future proofing. A fiber distribution network provides unlimited bandwidth and does not need to be replaced when the next generation comes along.
• Synergy with fixed line access. Operators who are deploying both fixed and mobile access can utilize the same infrastructure.
• QoS and SLA for multiplexed services. GPON T-CONT (upstream transmission containers) architecture enables multiplexing of different service flows (for example TDM and IP) on the PON while providing differentiated Quality of Service and SLA per service type.
• Redundancy and Automatic Protection Switching: GPON provides standards-based mechanisms for implementing redundancy and automatic protection switching.
5. Limitation of GPON in Mobile Backhaul Networks
• Complex layering model Ethernet/GEM/GTC encapsulation means complex management
• Not justified for use where fiber plant is not already deployed • Per-customer upgrade is possible, once if infrastructure is available • Not supportive of multiple synch streams per connection
• Upstream BW provisioning is possible only up to 1.0 Gbps • Less flexibility in native transport of TDM and Ethernet
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6. Conclusion
Implementing GPON technology in the mobile backhaul network allows mobile operators to increase bandwidth capacity and lower costs while maintaining high reliability and quality of service. GPON reduces costs through its efficient architecture and topology, which means reduction in central office assets and fiber through its high reliability, it reduces maintenance OPEX. GPON is an extremely scalable technology that offers forward compatibility, future proofing the mobile operator’s investment. QoS and the sophisticated mechanisms to enable SLAs allow mobile operators to offer guaranteed performance to customers. Standards-based redundancy and Automatic Protection Switching makes GPON highly reliable.
Selecting the right GPON solution is critical to the success of any implementation in the mobile backhaul network. The technology must be backwards compatible to support TDM over GPON, with accurate clock synchronization and QoS for mobile applications.
Today many technology providers are having end to end GPON Solution with an additional cost on redundancy and automatic protection switching which addresses the needs of mobile operators to ensure excellent network performance and very high reliability.
7. References
[1] Gal Adler, Using GPON in Mobile Backhaul Networks, White Paper, Issue 2: November 2010, PMC-Sierra, Inc.
[2] Ed Harstead, Reinforcing Mobile Backhaul With GPON, OSP Magazine [3] Whitepaper on Meeting the Mobile Backhaul challenges, Transmode
