T1 over Ethernet. CES8 for Release 5.0* 15 February 2005, Rev. A

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1 Application Note T1 over Ethernet CES8 for Release 5.0* 15 February 2005, Rev. A When migrating from TDM (Time Division Multiplexing) to a converged Ethernet network, it is essential to be able to maintain traditional TDM-based services (like T1) while delivering next-generation packet-based services. This white paper presents a brief primer on running T1 over Ethernet with the Circuit Emulation Service (CES8) service module available in release 5.0 of Allied Telesyn s Multiservice Access Platform (MAP). * The CES8 in R5.0 will also operate in E1 mode. All of the T1 operations described in this paper are applicable in E1 mode. Footnotes will be added where differences between T1 and E1 exist. PAGE 1 of 12

2 T1 over Ethernet CES8 for Release 5.0* 14 January, 2005, Rev. A When migrating from TDM (Time Division Multiplexing) to a converged Ethernet network, it is essential to be able to maintain traditional TDM-based services (like T1) while delivering next-generation packet-based services. This white paper presents a brief primer on running T1 over Ethernet with the Circuit Emulation Service (CES8) service module available in release 5.0 of Allied Telesyn s Multiservice Access Platform (MAP). * The CES8 in R5.0 will also operate in E1 mode. All of the T1 operations described in this paper are applicable in E1 mode. Footnotes will be added where differences between T1 and E1 exist. T1 over Ethernet Introduction For decades, T1 circuits have been used to carry voice and data services, and even today generate tens of billions of dollars in revenue. When planning the evolution of your network, care must be taken to ensure the continuity of this valuable source of revenue. In today s telecommunication networks, the number of T1 digital carrier circuits is second only to the number of POTS circuits. For decades, versatile T1 circuits have been used to carry voice and data services. They are not only used for equipment interconnect inside the Central Office (CO) but because of their reach, are widely used for connecting Outside Plant equipment to the CO; e.g., remote digital loop carriers, channel banks, and wireless base stations. T1s are also used to deliver premium revenue generating voice and data services over PRI and Frame Relay. Although its growth has slowed in recent years, Frame Relay over T1 leased lines still generates tens of billions of dollars of annual revenue in the US alone. Because of a large installed base and valuable source of revenue, T1 s are an important consideration when planning the evolution to packet networks. In a converged network, Ethernet must be transparent to existing T1 services in terms of engineering, provisioning, and maintenance. Equally important is the fact that T1s delivered over Ethernet must not require any change to customer premise equipment or T1 repeater technology. PAGE 2 of 12

3 Allied Telesyn is the only access vendor to offer a solution that incorporates T1with next-generation POTS, FTTH, ADSL2+, G.SHDSL and Ethernet. Services. Allied Telesyn recognizes the importance of this ubiquitous telecom building block and is the only access vendor to offer a solution that seamlessly weaves T1 carriers into an Ethernet fabric while using a platform that natively supports POTS, FTTH, ADSL2+, G.SHDSL and Ethernet services. The 8 port Circuit Emulation Service (CES8) service module is available in release 5.0 of Allied Telesyn s Multiservice Access Platform (MAP). The CES8, shown in Figure 1, gives the MAP the ability to provide transparent transport for DS1 rate circuits over an Ethernet network. It is a single slot service module that can be deployed in either an x400 (3RU high) or x700 (9RU high) chassis and terminates up to 8 T1s on its front panel RJ-21 connector. LEDs provide at-a-glance status of each T1 individual circuit. The 8 port Circuit Emulation Service (CES8) service module is available in release 5.0 of Allied Telesyn s Multiservice Access Platform (MAP). Indicators for T1 circuit status: In Service In Loopback LOS Degraded Service RJ-21 connector provides support for up to 8 T1 circuits Figure 1: 8 port Circuit Emulation Service (CES8) Service Module The single-slot CES8 service module operates in unstructured mode, tunneling the T1 across the Ethernet network over a pseudo span. In Release 5, the CES8 operates in unstructured mode. In this mode, the entire T1 (framing, signaling, and payload) is carried transparently across the Ethernet network. Only the line encoding (AMI or B8ZS) terminates on the CES8. In unstructured mode, the CES8 tunnels the T1 across the network over a pseudo-span. The interworking method used by CES8 is designed for compliance to industry standards for TDM Circuit Emulation Service over Packet (CESoP) being defined by the IETF, Metro Ethernet Forum, MPLS Forum, and ITU-T. A simplified view of this operation is illustrated in Figure 2. Future MAP software releases will allow structured mode transport for manipulation of individual 64kbps channels in a T1. Figure 2: CES8 Clear Channel T1 Operation PAGE 3 of 12

4 The CES8 performs TDM to Packet conversion. Change packetization length to affect performance of the T1 service. T1s that terminate on the CES8 are mapped to separate Ethernet packet streams that can reside on the same or different MAPs. The CES8 performs TDM to packet conversion by taking slices of the TDM digital stream and encapsulating each slice with packet overhead. The number of TDM bytes used to create an Ethernet packet is user configurable and can be set from as low as 16 bytes to as large as 1023 bytes (62 T1 frames). This packetization length is one of several parameters that affect the performance of the T1 service. These settings are discussed later in this paper. The 8 T1s that terminate on the CES8 are mapped to separate Ethernet packet streams (pseudo-spans) allowing individual T1s to be switched to different destination CES8 cards. The destination CES8 cards can be either in the same MAP or in different MAPs. Pseudo-span connectivity is illustrated in Figure 3. When the pseudo-span is created, its destination is set as a part of the provisioning process by entering a unique address that identifies endpoint CES8 and the corresponding T1 port. There are several formats for setting pseudo-span endpoint addresses. For details, please refer to the MAP User Guide for R5.0. Figure 3: CES8 Point to Point Switching T1s can carry service to subscribers outside the cabinet, or can be used within the cabinet to derive analog services from T1-fed channel banks, or special services shelves. As mentioned earlier, future releases of Allied Telesyn MAP software will add the capability for the CES8 to individually switch DS0s. This structured (or channelized) T1 capability is a software-enabled feature and will not require new CES8 hardware A common application for the CES8 will be to transport T1s to remote cabinets. Figure 4 shows a traditional configuration for T1 services deployed from remote cabinets. Business voice via PBX, data, and leased lines are multiplexed together in the CO onto a SONET transport system that feeds a remote Digital Loop Carrier (DLC) cabinet. At the cabinet, the services are de-multiplexed from the SONET fiber and delivered to subscribers. In addition to the T1s that carry service to subscribers outside the cabinet, other T1s are used inside the cabinet to derive special analog services from T1 fed channel banks, or special services shelves. Typical services that are delivered in this manner are legacy analog interfaces such as coin phone, analog 4-wire, and analog PBX. The CES8 enables the replacement PAGE 4 of 12

5 of expensive SONET transport with a cost-effective Ethernet access platform that enables advanced broadband services such as IP Video. Figure 4: T1s in Remote Cabinets Figure 5 shows how the MAP with the CES8 can be used to deliver T1 services equivalent to those shown in Figure 4. This is a simplified picture focused only on T1 services and ignores other services such as video and internet data. T1s in the CO are terminated directly on the CES8 which converts the TDM traffic into packet traffic for the Ethernet network. The pseudo-spans are switched through the network to destination MAPs, where the CES8 regenerates the original T1 from the packet flow. The T1s can be used directly from the CES8 for delivery inside or outside the cabinet. Figure 5: Cabinet Resident T1 Service Delivery with CES8 PAGE 5 of 12

6 Note: In Figure 5, T1 spans have a maximum reach of 6000ft. When the distance from the remote cabinet to the subscriber exceeds this distance, a T1 repeater must be used. Repeaters are often line powered directly from the T1 equipment, however the CES8 does not provided line power. For this reason, if a repeatered T1 service is required, a separate Span Termination Shelf is required to convert the T1 signal from the CES8 to the appropriate power levels for line power remote T1 repeaters. A typical configuration for a full services network with circuit emulation capability is shown in Figure 6. In this example, the three MAP nodes are connected in a protected Ethernet Protection Switched Ring (EPSR) configuration. MAP nodes 1 and 3 are in the CO and have Gigabit Ethernet links into the voice, T1, data, and video service networks. MAP node 2 is located in a remote outdoor cabinet. In this example, only Nodes 1 and 2 terminate T1s with CES8. Figure 6: Full Services Network PAGE 6 of 12

7 TDM to Ethernet Packet Network Considerations Designed for simplicity, the interworking technology used for TDM conversion is robust and designed to compensate for error conditions and challenges posed by T1 delivery. The T1 circuit emulation service provided by the CES8 is designed to be simple to engineer, configure, deploy, and maintain. The interworking technology used for the conversion between TDM and packet is extremely robust and overcomes the challenges of delivering T1 services over a packet network. Transport robustness is important since many characteristics of packet networks are not conducive to constant bit rate services. Packet network congestion, blocking, QoS prioritization, multiple paths with varying latency, all can lead to jitter, lost packets, duplicate packets, and out of sequence packet delivery. All of these error conditions can be compensated by the CES8, some automatically and some through user adjustable configuration parameters. The CES8 automatically corrects sequence errors in received packet stream. The CES8 can detect and correct the following packet sequence errors: 1. Out of order packet sequence 2. Late arrival of a packet. This is effectively the same as a packet arriving out of sequence. The maximum variation in transit delay is determined by the depth of the setting of the jitter buffer. 3. Duplicate packets All of these errors are corrected without requiring any special configuration. Allied Telesyn recommends that applications set the packetization size to 193 bytes to avoid wasting packet bandwidth or adding delay in the TDM circuit. Packet Size Packet size is directly related to the number of T1 bytes used to create a packet for the pseudo span. This is user-configurable and can range from 16 to 1023 bytes. Increasing the number of TDM bytes per packet increases end to end latency, and thus adds delay in the TDM circuit. Excessive latency can degrade both voice and data services carried over the packet network between CES8 line cards. In voice applications, sufficient amounts of delay can result in the need for echo cancellation. At the other extreme, lower numbers of TDM bytes per packet decrease transport efficiency by increasing the percentage of overhead bytes associated with the packet. This results in higher bandwidth consumption in the packet network. When there is not enough TDM data to fill a minimum Ethernet frame of 64 bytes, the packet is filled with dummy information, wasting packet bandwidth. Allied Telesyn recommends that most applications set the packetization size to 193 bytes (about 1ms) 1. Packet Delay Variation TDM operates at a constant bit rate. When converting from packet to T1, the CES8 must have at least one queued packet to play out to the TDM T1. It cannot wait for a packet to arrive without causing errors in the T1. 1 For E1 applications, packet size should be set for 256 bytes (about 1ms). PAGE 7 of 12

8 The CES8 provides buffers that absorbs up to 60ms of PDV, and reducing jitter and improving latency. For delay-sensitive applications, such as voice, it is important to consider the transport system from end to end. Unfortunately, in a packet network transit time varies from packet to packet. These variations, or jitter in the arrival time of the incoming packets, are referred to as Packet Delay Variation (PDV). PDV is a parameter that is measured in milliseconds. PDV has many origins: multiple routes with different transit times, switch/node congestion, mixed packet sizes, contention with traffic marked for higher QoS, and router loading. Generally, more switching or routing nodes in a pseudo span path, mean greater PDV. In order to compensate for PDV, the CES8 must buffer enough packets to ensure that another packet has arrived before the current packet has been played out. The CES8 provides buffers that can be configured to absorb values of PDV that are dependent on the configuraed payload size. Increasing the depth of these buffers adds latency to the T1. As mentioned in the discussion on packet size, excessive latency can degrade both voice and data services. End-to-End Latency Many services carried over T1 are delay sensitive. For example, in voice applications, end to end transport latency greater than 25ms generally requires the use of echo cancellers to remove audible reflections from far end 4 to 2 wire hybrid circuits used in telephones. Thus, in some applications, it is important to understand the total end-to-end transport delay when engineering a T1 over Ethernet circuit. Calculations of end-to-end latency must include the following: Such consideration should include: interworking delay, packet size, switch transit delay and PDV buffering. 1. The intrinsic delay associated with the interworking function. The delay is not symmetric; converting TDM to packet has less delay than converting packet to TDM. 2. Packet size. The number of bytes from the T1 stream that are used to form the Ethernet packet. 3. Switch transit delay. The delay incurred as a packet is switched or routed through nodes in the Ethernet network. 4. PDV Buffering. The depth of the jitter buffer on the CES8 adds to the end to end delay. The total latency is determined by adding the contribution of each the factors above. For example, consider the network shown in Figure 5. This network is an EPSR configuration and therefore the end to end delay varies depending on which direction traffic is circulating on the ring. The total end to end calculation is based on the model shown in Figure 6. Figure 7: End-to-End Delay Model PAGE 8 of 12

9 From the figure the delay is calculated from the following parameters: 1. TDM to packet conversion delay (t tdm2pkt ) 2. Packet size set on the CES8 in node 1 (t PacketSize ) 3. Switch transit delay of the CFC in node 1 (t CFC ) 4. Switch transit delay of the CFC in node 3 (t CFC) 5. Switch transit delay of the CFC in node 2 (t CFC ) 6. PDV buffer in the CES8 in node 2 (t PDV ) 7. Packet to TDM conversion delay (t pkt2tdm ) In this example, if we assume that t tdm2pkt = 125us, t PacketSize = 1ms, t CFC = 15us, t PDV = 5ms, and t pkt2tdm = 250us, the end to end delay would be 6.42ms. Note that the MAP Central Fabric Controllers (CFCs) add negligible delay to the path. Ethernet Bandwidth Requirements Packetization of the TDM streams adds protocol overhead to the 1.544Mbps bandwidth of the T1. The structure of a pseudo-span packet is shown in Figure 7. Figure 8: Pseudo-span Packet Format Packet overhead is a small percentage of overall bandwidth, but should be considered. A fullyloaded CES8 linecard will require * * 2.264Mbps or Mbps of packet bandwidth.. While the overhead is a small percentage of the overall bandwidth, there may be instances, especially in congested networks, where the total packet bandwidth must be considered. From the figure above, protocol overhead (Ethernet + IP + RTP) is 90 bytes. If the packet size is set for 193 bytes (as recommended by Allied Telesyn) then each pseudo-span packet will be 283 bytes. In this configuration, overhead is approximately 32% of the packet bandwidth. Therefore, at a packet size of 193 bytes, a single T1 will require 2.264Mbps (32% more than 1.544Mbps) of packet bandwidth. A fully loaded CES8 linecard will require 8 * 2.264Mbps or Mbps of packet bandwidth. PAGE 9 of 12

10 T1 over Ethernet Timing & Synchronization Both ends of the link must operate at the same clock rate to prevent underflow or overflow. The CES8 coordinates clock rates with a built-in timing mechanism. As with all T1 applications, both ends of the link must operate at the same clock rate. Otherwise the receiver will either overflow or underflow depending on the relative timing between the transmitter and receiver. Figure 9 shows two pieces of standard T1 equipment connected over an Ethernet network using the CES8. Bits are clocked out of T1 Equipment A at a rate set by the clock f service. The CES8 collects the T1 frames and encapsulates them into packets. At the other end of the Ethernet network, another CES8 queues up the incoming packets to be played out as T1 data. The clock used by the far-end CES8 must be equal to f service otherwise the far-end CES8 s queue will overflow or underflow depending on the relative timing between the two clocks. Figure 9: T1 over Ethernet Timing The CES8 has a built-in timing mechanism that allows the far-end CES8 to recover a clock based on time stamps carried in the packet stream. The recovered clock in the far-end is a close approximation to f service. The quality of the recovered clock-- in terms of jitter and wander-- meets the requirements of ANSI T A typical CES8 configuration is shown in Figure 9. In this configuration, the CO CES8 is configured to use the incoming T1 CO1 as the timing master. In this mode, packets entering PSEUDO SPAN 1 will carry timing information from T1 CO1. At the remote CES8, the port serving T1 REM1 is configured to extract timing from PSEUDO SPAN 1, making T1 REM1 a timing slave to T1 CO1. Note that each of the 8 ports on the CES8 can be independently configured to derive timing from either the T1 or the pseudo span. The accuracy of the recovered clock is dependent on the behavior of the packet network. The amount of packet loss and packet delay variation can significantly influence time interval error. However, the clock recovery mechanism used by the CES8 is very robust and can compensate for a wide range of network behaviors. It has been shown to meet synchronization performance requirements even in poorly engineered packet networks where PDV can be as high as 60ms and packet loss can be as much as 0.5%. PAGE 10 of 12

11 Figure 10: Typical CES8 Application with Timing CES8 T1 Specifications Physical Number of Ports 8 Connector RJ-21 Interface Voltage 2.4V Interface Impedance 100ohms Reach (Line Build Out) Short Haul (feet) 0-133, , , , Long Haul (db) 0, -7.5, -15.0, Line Rate 1.544Mbps Line Code AMI, B8ZS Framing Physical Layer Alarms Power Operating Temperature Storage Temperature In unstructured mode, all framing types are supported in that they are transparently passed through the network. LOS In unstructured mode, all framing types are supported in that they are transparently passed through the network. 8W -40C to 65C -40C to 75C Humidity 5% to 90% Safety UL60950 Emissions FCC Part 15 Class A Interworking Standards Compliance IETF SAToP (Structure Agnostic TDM over Packet) Packet Size PDV Buffer Timing and Synchronization Timing source Jitter 1 to 1488 bytes Max 60ms Wander T1.403 Holdover Accuracy Loop timed or Pseudo-span timed ANSI T1.102, T1.403, GR-499-CORE Stratum 4 local oscillator. PAGE 11 of 12

12 Company Overview Allied Telesyn A global company with nearly two decades of continuous profitability. Allied Telesyn focuses entirely on end-toend, purpose-built Ethernet applications. A world-class engineering and support organization spanning five continents and more than 30 countries. The ideal choice for cost-conscious Service Provider professionals who are looking for highquality, feature-rich network solutions. Founded in 1987 with the goal of producing feature-rich, reliable, standardsbased networking products, Allied Telesyn has a proven track record in bridging the gap left by other Ethernet networking manufacturers, whose solutions are often limited in scope or cost-prohibitive. By taking cues directly from our customers and leveraging our global manufacturing competencies, we ve evolved a market-focused approach to system development that is geared entirely to applications, rather than individual components. And by concentrating on battle-tested, end-to-end solutions for vertical market applications we avoid the scattershot, companyfocused approach common in the industry. Our tagline: It s our Network, too is a testament to our high-level of accountability and to our investment in our customers bottom line success. Allied Telesyn focuses entirely on end-to-end, purpose-built Ethernet and IP applications; with a complete line of networking products that includes Layer 2 switches, Layer 3 switches, carrier class fiber/copper Multiservice Access Platforms, wireless access points, wireless adapter cards, and residential gateways. No other networking vendor can match Allied Telesyn s breadth and depth of Ethernet products we are the leading manufacturer of media converters, unmanaged Fast Ethernet switches and hubs, fiber optic network adapters and other feature-rich interconnectivity products, worldwide. Additionally, Allied Telesyn has developed a world class systems engineering and support organization that ensures networks are designed and implemented to handle the stress of providing voice, video and data services. With engineering, manufacturing, sales, and distribution divisions strategically located throughout the Americas, Europe, Asia and Japan, Allied Telesyn is able to deploy solutions anywhere in the world, quickly and efficiently. And by rigorously testing products in design and support centers and leveraging our design and manufacturing competencies, Allied Telesyn is able to offer solutions for the access edge that are both customized and plug-and-play. This ideal combination helps our customers keep costs low, speed network deployment and maximize network uptime. Our customer-driven approach combined with a pragmatic, value-based pricing scheme and a superlative service organization has made Allied Telesyn a global networking leader, with more than 17 years of continuous profitability and products deployed in more than 50,000 companies in 30 countries and five continents. Allied Telesyn: the ideal choice for costconscious Service Provider professionals who are looking for high-quality, feature-rich network solutions at a lower price. PAGE 12 of 12