Circuit Emulation Pseudo-Wire (CE-PW)
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Executive Summary
This paper describes a new family of products that can be used by service providers to dramatically reduce the costs associated with delivering one of today’s most demanding and ubiquitous services: private-line or leased-line service.
Developed by Axerra Networks and commercialized by global carriers throughout the world, the AXN Pseudo-Wire Gateway™ enables these carriers to provide Pseudo-Wire services, offering both “legacy” and emerging services over converged IP/MPLS, Carrier Ethernet, and hybrid fiber/coax (HFC) infrastructures. Legacy services comprise those communications services that enterprises and business users rely on today, including leased-line, Frame Relay, ATM, and full-featured TDM voice. This paper focuses on the Circuit Emulation Pseudo-Wires (CE-PWs) that provide leased-line services for entire T1/E1, T3/E3, as well as Fractional T1/E1 services. Adopters of Pseudo-Wire technology are afforded the opportunity to reduce total operating costs while at the same time:
• Preserving complete transparency to applications and protocols, even proprietary protocols
• Requiring no change in customer premise equipment or its configuration • Maintaining compatibility with both voice and data applications by
preserving time slot alignment
• Retaining the security that their customers’ businesses depend on • Preserving reliability and quality of service (QoS) through ongoing
performance monitoring and visibility into traffic statistics
• Utilizing a service that provides a seamless migration path to emerging IP services
The following sections describe the CE-PW service and the AXN Pseudo-Wire Gateway that enables it, with particular attention to the key benefits from the business customer’s perspective.
Circuit Emulation: A Most Challenging Service
Thanks to their predictability, reliability, and flexibility, leased-line services remain the most ubiquitous and widely used communications services for corporate networking worldwide. Leased-line services are used to support corporate voice applications in tie-trunk
applications, as well as a multitude of data applications. Leased-line services are not only the most widely used, but also, in many ways, the most demanding. Leased-line traffic is
traditionally carried over a connection in a circuit-switched network. This connection is made by a dedicated physical link or a dedicated time slot between two endpoints. Maintaining the reliability and quality furnished by these circuit-switched connections is critical, as the very operation of many businesses depends on the processes and applications that run over leased-line services. Because leased-line traffic may consist of voice, data, or a mixture of the two, the Quality of Service (QoS) parameters to support all of these traffic types are the most rigorous of any service.
Leased-line services are transparent by nature, seamlessly transporting any bit stream between two customer locations. The transparent nature of leased-line services means that numerous proprietary protocols are in use today on leased-line networks. In fact, many different types of Customer Premise Equipment (CPE) have been developed and are
currently in use that depend on the transparent nature of leased-line services. In addition, the CPE and protocols used in conjunction with leased-line networks often depend on the robust clocking and synchronization that are inherent in circuit-switched connections. Finally, the applications running over leased-line implementations are often sensitive to delay, requiring consistent and predictable delays or the result is troublesome timeouts at the application or session layer.
As IP, MPLS, HFC, and Ethernet are essentially packet-based rather than circuit-oriented transmission technologies, the CE-PW service must emulate the characteristics of circuit-switched connections while utilizing the packet-based infrastructure. CE-PW must emulate the transparent nature of circuit-switched connections, both for data and timing, in order to provide compatibility with today’s applications, both voice and data, and the CPE deployed to support them. Doing so requires that a number of challenges be met.
In particular the following criteria must be addressed: • Minimize end-to-end latency and ensure QoS • Optimize bandwidth utilization
• Manage jitter and accommodate Packet Delay Variation • Maintain clock synchronization between the two endpoints • Maintain time slot ordering and preserve DS0 alignment
The AXN Pseudo-Wire Gateway
Axerra Networks’ family of AXN Pseudo-Wire Gateways and Access Devices™ enables service providers to offer legacy services (including leased-line services using CE-PW, as well as Frame Relay, ATM, and full-featured voice services) alongside emerging services over packet-based infrastructures. The AXN product family offers the industry’s most wide-ranging set of multiservice Pseudo-Wire services. Using standards-compliant service
adaptation technology, the AXN can adapt any voice or data service for packet transport and deliver consolidated traffic flows to the service provider’s IP/MPLS, Carrier Ethernet, or HFC network. As shown in Figure 1, Axerra Networks’ AXN Pseudo-Wire Gateway is deployed at the edge of the next-generation network, adapting both legacy and emerging services for transport across any kind of gigabit packet-based networks being installed today. The AXN accepts enterprise subscriber traffic over T1/E1 or T3/E3 lines from the TDM-based equipment already installed at the customer premise. The AXN utilizes standards-based service adaptation technology to implement the CES to IP function. It then delivers the traffic to the network via several optional high-speed network interface types, including Fast Ethernet, Gigabit Ethernet, and STM-1/4 Packet over SONET (POS). A wide range of AXN platforms are available that address applications from small, low-cost CPE
installations, to MTU/CLE sites, and even high-density, fault-tolerant CO/PoP applications.
Figure 1. CE-PW functional blocks
CE-PW Solution Description
Figure 2 illustrates from a protocol perspective the CE-PW solution, showing the protocol stacks for each of the network elements outlined in Figure 1. Traffic flows as a constant bit-rate (CBR) stream from the CPE and enters into the CES-to-Packet function of the AXN. In order to remain compatible with all applications and protocols (both voice and data), the AXN’s CES-to-Packet function does not depend on protocol patterns or formats within the bit stream of the CBR traffic. Instead, after receiving the requisite number of bits, the CES-to-Packet function chops the bit stream into a fixed-length payload, encapsulates the payload using the appropriate header with the destination address and information concerning synchronization and multiplexing, then adds the appropriate layer-2 header (for transport over Gigabit Ethernet or SDH, for example), and finally transfers the packet over the uplink to the packet-based network.
At the receiving side the CES-to-Packet interworking function (IWF) receives the packet from layer-2 (i.e., after the layer-2 header is removed), unpacks the payload by removing the address and synchronization information, inserts the packet payload to a jitter buffer, adjusts the receiver clock, and generates a CBR bit stream.
Figure 2. CE-PW Protocol Stack
Transparency for Seamless Interoperability
By not depending on protocol patterns or formats within the CBR bit stream, the CE-PW service maintains seamless transparency, allowing applications and protocols already in use to continue to function. In fact, because CE-PW is completely agnostic to the protocol right down to the bit level, even applications using proprietary protocols remain completely functional. This transparency is a key benefit because it facilitates interoperability with all types of CPE, including PABXs, FRADs, CSU/DSUs, integrated access devices (IADs), and routers. The existing applications running over such CPE have become critical to the very operation of many businesses, and most enterprises cannot afford the cost and risk of an immediate replacement to these applications.
However, transparently carrying the bit streams from end-to-end is not by itself sufficient to ensure that the applications and CPE will continue to operate properly. Applications
developed to run over leased lines typically require the strict performance criteria of a circuit-switched network in order to operate satisfactorily. These performance criteria are more restrictive than those developed for applications that are more tolerant of packet-network behavior.
In particular, applications and CPE developed to operate in conjunction with leased lines are often very sensitive to timing issues, requiring the smooth delivery of data with consistent and predictable delays. In addition, voice traffic, which is often carried over leased lines in tie-line applications, can suffer from echo when subjected to excessive delay. To maintain proper timing performance, Quality of Service (QoS) parameters must be met on an end-to-end basis and variations in packet arrival time must be smoothed out. Simultaneously, the service provider should engineer the service within end-to-end delay constraints, while at the same time efficiently utilizing the bandwidth available on network links. Each of these topics is examined more carefully below.
Ensuring QoS
In packet networks, delay occurs when multiple packets contend for use of network links and other resources. Packets carrying CE-PW traffic need to be given higher priority than packets of most other types of traffic in order to minimize these delays, both within the AXN Pseudo-Wire Gateway and in the packet network as a whole. Proper handling of priorities also guards against packet loss for CE-PW traffic should network congestion occur. Within the AXN, packets arriving from a CBR application are classified as having a high-priority Class of Service (CoS). At the uplink to the backbone network, the AXN employs multiple prioritized queues. Each queue is policed and shaped individually,
according to a pre-configured scheduling/queuing mechanism – such as strict priority, WRR (Weighted Round Robin), or a combination of both.
The network is made aware of the CE-PW traffic’s high-priority status through QoS markings. The QoS mechanisms used by the AXN in carrier networks today include DiffServ (DS) and 802.1q Ethernet priority tagging. The AXN also supports MPLS to ensure QoS in next-generation networks. In a packet network that uses DiffServ marking, for example, the AXN marks the DS byte in the IP header as the adaptation process generates each packet, and according to its defined CoS, directs the packet to its corresponding queue. In the case of CE-PW, the DS byte is marked as Expedited
Forwarding (EF), which is the highest PHB (Per Hop Behavior) available in DiffServ. Along the packet’s path, any switch/router which is DiffServ-enabled will handle this packet with higher priority than other packets. Thus, the combination of packet classification, DiffServ marking, multiple prioritized queues, and intelligent scheduling result in minimized delay variations and overall end-to-end delay.
Handling Packet Delay Variation
Although overall delay can be minimized with proper management of QoS, delay variations can still remain, inducing jitter at the far end. The AXN handles these packet delay variations by the implementation of a jitter buffer to remove these variations and deliver a CBR bit stream.
The size of the jitter buffer can significantly impact the end-to-end delay and successful operation of the CE-PW service. For example, a large buffer size will increase the overall delay but will also be able to handle large amounts of jitter. On the other hand, a small buffer size will minimize the overall delay but will limit the jitter that can be accommodated.
Thus, setting the optimal size of the jitter buffer for each connection is critical to achieving minimum delay with maximum performance.
The AXN gives the network manager the flexibility to set the ideal jitter buffer size for each CES connection. The jitter buffer size is set according to the characteristics of the specific network, allowing the AXN to provide successful circuit emulation across a range of different operating environments.
In addition, the AXN CES-to-Packet function supports packet reordering. In packet-based networks, even MPLS networks, packets can sometimes arrive out of sequence. The AXN CES-to-Packet function enables “late” packets to jump into their relative location in the queue at the receiving end if they arrive out of order. This packet reordering is crucial to maintaining bit-stream integrity for circuit emulation services.
Bandwidth Utilization
The amount of protocol overhead required to operate over packet networks can have a dramatic impact on overall network bandwidth utilization. Small payload sizes increase the proportion of the packet devoted to overhead and can dramatically decrease the percentage of network bandwidth utilized to carry the payload. However, larger payload sizes require that a larger number of bits arrive at the CES-to-Packet IWF before a packet is formed, resulting in increased network latency. In fact, the size of the packet significantly affects the end-to-end delay and the successful operation of the CE-PW service. Since the packet overhead (as measured in bytes for the header) is constant, the only parameter left to adjust in order to optimize the service is the size of the payload.
The AXN allows the network manager to set the ideal packet size for each circuit connection. The packet size is set according to the specific requirements of the service, giving the network manger the flexibility to optimize the end-to-end performance by giving priority to low delay or to bandwidth efficiency. In addition, the AXN can ensure that packets are not segmented within the packet network by limiting the payload size to a maximum of 1280 bytes.
Maintaining Clock Synchronization
Emulating an end-to-end CBR connection over a packet network requires that the data and the clock between the endpoints A and B (as shown in Figure 3) be synchronized. Any difference between the clocks at the two endpoints will result in overflow or underflow of the receiver buffer, thus disrupting the flow of the traffic.
Clock synchronization can be accomplished in the AXN by one of the following: • Station Clocking – Clock is derived from the carrier’s external station clock
distribution system.
• Adaptive Clock Recovery – The AXN supports reconstruction of the clock from the received packet stream that carries the CES traffic based on Axerra Networks’ CE-PW implementation. With Adaptive Clock Recovery (ACR), the AXN continuously
monitors and dynamically adjusts the receiver clock, thus eliminating the requirement for an external clock source.
Figure 3. Clock recovery in CE-PW
Preserving DS0 Alignment
In order to support voice applications and integrated voice/data applications that utilize the DS0 structure, the CE-PW service preserves time slot alignment and ordering. Traffic to be transmitted over the packet network using CE-PW can be treated using either of the following modes:
• Structured
The structured mode is used to transfer a number of time slots from an E1/T1 or a fractional E1/T1 link. In this mode, the CES-to-Packet function utilizes the DS0 format based on the E3/E1 or T3/T1 framing structure and performs packetization of the incoming CBR stream with attention to the start and the end of each time slot. This allows the receiver to maintain synchronization and reconstruct the original order of the time slot format and even to cross-connect between time slots.
• Unstructured
The unstructured mode is used to transfer the entire contents of a T1/E1, and can even include the framing bit or byte. In this mode, the CES-to-Packet function transfers the CBR stream transparently and does not implement any special function to maintain the format and alignment of the stream over the packet network.
Security
Security and the protection of an organization’s intellectual assets has become an ever-increasing concern in today’s networking environment. The transparent nature of the CE-PW service means that the security measures already in place, that protect the enterprise’s applications, will continue to operate and be effective. In addition, the provider manages the CE-PW service on an end-to-end basis, with access available only at the two customer premise endpoints. As the CBR traffic is transported over the carrier’s packet network, the CE-PW implementation preserves leased lines’ point-to-point, connection-oriented nature. Although packets and addresses may be used internally in the network, this point-to-point nature keeps it secure from unauthorized users. Finally, the AXN validates the format and integrity of the CE-PW headers, effectively preventing someone from hacking through the service adaptation process should they gain access to the network’s IP addresses.
Reliability
End users today depend upon the reliability of leased-line services for the day-to-day operation of their businesses, having built mission-critical applications that run over their leased-line corporate networks. In order to maintain today’s high service-level requirements for CE-PW services, the AXN provides high availability and redundancy, as well as powerful management tools that furnish visibility into the performance of the network.
Designed specifically to meet the exacting demands of service providers and engineered to the highest industry standards, the AXN is a fault-tolerant, carrier-class platform. The AXN architecture protects against failures with a complete set of redundancy features that include redundant network interface cards, common processing cards, and power supply units. This set of redundancy capabilities protects against points of failure, increasing system reliability and minimizing downtime.
Powerful management tools that furnish visibility into alarms and performance monitoring statistics are critical to assuring the highest end-to-end service availability while controlling operating costs. Axerra Networks’ AXNVision™ is a complete, easy-to-use SNMP-based Element Management System (EMS). AXNVision provides the network manager with a complete set of management tools for AXN configuration, management, control, and service provisioning with a feature-rich, Web-based Graphical User Interface (GUI). The AXNVision application is platform-independent and is easily integrated into service providers’ operations systems with centralized management platforms, such as HP OpenView.
A Platform for Emerging Services
While an immediate change in applications and communications services may incur unacceptable risk and disruption, today’s sophisticated business customer is certainly
monitoring the evolution of IP Virtual Private Networks (VPNs), Voice over IP (VoIP), and other emerging services. Through deployments of the AXN Pseudo-Wire Gateway,
forward-looking carriers can build a network platform for both today’s services and the value-added services of tomorrow. The AXN furnishes both the performance and flexibility to enable a smooth transition to new service offerings.
The AXN architecture integrates powerful, dedicated hardware that performs packet forwarding, routing decisions, packet filtering, and classification, thus enabling service adaptation performance at “wire-speed” even in the most demanding traffic situations. The power of the architecture anticipates evolving standards for Pseudo-Wires and multiservice over packet, enabling carriers to modify service offerings to meet the changing requirements of end users.
In addition, the AXN architecture is a highly flexible solution enabling any service on any port and any time slot. For example, the T1/E1 interfaces on each IOM can be easily configured to support any service on any port with DS0 resolution. This allows carriers to adapt to evolving service requirements, while simultaneously simplifying service
provisioning, rationalizing equipment deployment, and reducing maintenance inventories. The AXN’s open hardware and software architecture also ensures easy integration and adaptation of new technologies (including third-party solutions), further enhancing the flexibility of the solution.
Finally, rapid and simple configurations for new services or modification of parameters for an existing service are possible through the intelligent software design of the AXNVision management tools. Adopters of CE-PW can take advantage of the service’s transparency and compatibility with today’s applications and CPE with the assurance that their service
provider is developing a network that will allow it to adapt to users’ changing needs.
Conclusion
Today’s service providers are deploying next-generation networks connecting cities throughout the world using thousands of kilometers of fiber optics. Axerra Networks is partnered with these providers to unlock the power of their packet networks by enabling a full portfolio of services.
Today’s business users have come to rely on a range of legacy communications services, the most widely used being leased-line or private-line services. Using next-generation IP/MPLS, Carrier Ethernet, and HFC networks, forward-looking carriers are deploying Circuit
Emulation Pseudo-Wires, offering customers all the benefits of leased-line services
combined with the advantages of IP/MPLS, HFC, and Ethernet infrastructures. Customers can continue to leverage the transparency, seamless interoperability, security, and reliability of their current leased-line services, while simultaneously taking advantage of IP/MPLS, HFC, and Ethernet’s lower costs on networks that are positioned for new and emerging services.
