AlliedTelesyn T1 Over Ethernet a Wp

8/8/2019 AlliedTelesyn T1 Over Ethernet a Wp

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2005 Allied Telesyn, Inc.

------------------------------------------------------------------------------------------------------------------------------------------------T1 over Ethernet , 15 February 2005, Rev. Awww.alliedtelesyn.com PAGE 1 of 12

Application Note

T1 over EthernetCES8 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 (likeT1) while delivering next-generation packet-based services. This white paper

presents a brief primer on running T1 over Ethernet with the Circuit EmulationService (CES8) service module available in release 5.0 of Allied TelesynsMultiservice Access Platform (MAP).* The CES8 in R5.0 will also operate in E1 mode. All of the T1 operations described in this paper areapplicable in E1 mode. Footnotes will be added where differences between T1 and E1 exist.


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T1 over EthernetCES8 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 (likeT1) while delivering next-generation packet-based services. This white paper

presents a brief primer on running T1 over Ethernet with the Circuit EmulationService (CES8) service module available in release 5.0 of Allied TelesynsMultiservice Access Platform (MAP).* The CES8 in R5.0 will also operate in E1 mode. All of the T1 operations described in this paper areapplicable in E1 mode. Footnotes will be added where differences between T1 and E1 exist.

T1 over EthernetIntroduction

For decades, T1circuits have beenused to carry voiceand data services,and even today generate tens of billions of dollars inrevenue.

When planning theevolution of your network, care must betaken to ensure thecontinuity of thisvaluable source of revenue.

In todays 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. Theyare not only used for equipment interconnect inside the Central Office (CO)but because of their reach, are widely used for connecting Outside Plantequipment to the CO; e.g., remote digital loop carriers, channel banks, andwireless base stations. T1s are also used to deliver premium revenuegenerating voice and data services over PRI and Frame Relay. Although itsgrowth has slowed in recent years, Frame Relay over T1 leased lines stillgenerates tens of billions of dollars of annual revenue in the US alone.

Because of a large installed base and valuable source of revenue, T1s arean important consideration when planning the evolution to packet networks.In a converged network, Ethernet must be transparent to existing T1services in terms of engineering, provisioning, and maintenance. Equallyimportant is the fact that T1s delivered over Ethernet must not require anychange to customer premise equipment or T1 repeater technology.


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Allied Telesyn is theonly access vendor tooffer a solution that incorporates T1withnext-generationPOTS, FTTH,

ADSL2+, G.SHDSLand Ethernet.Services.

The 8 port Circuit Emulation Service

(CES8) servicemodule is available inrelease 5.0 of Allied Telesyns Multiservice

Access Platform(MAP).

The single-slot CES8 service moduleoperates inunstructured mode,tunneling the T1across the Ethernet network over apseudo span.

Allied Telesyn recognizes the importance of this ubiquitous telecom buildingblock and is the only access vendor to offer a solution that seamlesslyweaves T1 carriers into an Ethernet fabric while using a platform thatnatively 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 Telesyns Multiservice Access Platform (MAP). TheCES8, shown in Figure 1, gives the MAP the ability to provide transparenttransport for DS1 rate circuits over an Ethernet network. It is a single slotservice 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-21connector. LEDs provide at-a-glance status of each T1 individual circuit.

Figure 1: 8 port Circuit Emulation Service (CES8) Service Module

In Release 5, the CES8 operates in unstructured mode. In this mode, theentire T1 (framing, signaling, and payload) is carried transparently across

the Ethernet network. Only the line encoding (AMI or B8ZS) terminates onthe CES8. In unstructured mode, the CES8 tunnels the T1 across thenetwork over a pseudo-span. The interworking method used by CES8 isdesigned for compliance to industry standards for TDM Circuit EmulationService over Packet (CESoP) being defined by the IETF, Metro EthernetForum, MPLS Forum, and ITU-T. A simplified view of this operation isillustrated 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

RJ-21 connector provides support for

up to 8 T1 circuits

Indicators for T1 circuit status:In Service

In Loopback LOS

Degraded Service


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The CES8 performsTDM to Packet conversion. Changepacketization lengthto affect performanceof the T1 service.

T1s that terminate onthe CES8 are mapped to separate Ethernet

packet streams that can reside on thesame or different MAPs.

T1s can carry serviceto subscribers outsidethe cabinet, or can beused within thecabinet to deriveanalog services from

T1-fed channel banks,or special servicesshelves.

The CES8 performs TDM to packet conversion by taking slices of the TDMdigital stream and encapsulating each slice with packet overhead. Thenumber of TDM bytes used to create an Ethernet packet is user configurableand can be set from as low as 16 bytes to as large as 1023 bytes (62 T1frames). This packetization length is one of several parameters that affectthe 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 Ethernetpacket streams (pseudo-spans) allowing individual T1s to be switched todifferent destination CES8 cards. The destination CES8 cards can be either in the same MAP or in different MAPs. Pseudo-span connectivity isillustrated in Figure 3. When the pseudo-span is created, its destination isset as a part of the provisioning process by entering a unique address thatidentifies endpoint CES8 and the corresponding T1 port. There are severalformats for setting pseudo-span endpoint addresses. For details, pleaserefer to the MAP User Guide for R5.0.

Figure 3: CES8 Point to Point Switching

As mentioned earlier, future releases of Allied Telesyn MAP software willadd the capability for the CES8 to individually switch DS0s. This structured(or channelized) T1 capability is a software-enabled feature and will notrequire new CES8 hardware

A common application for the CES8 will be to transport T1s to remotecabinets. Figure 4 shows a traditional configuration for T1 services deployed

from remote cabinets. Business voice via PBX, data, and leased lines aremultiplexed together in the CO onto a SONET transport system that feeds aremote Digital Loop Carrier (DLC) cabinet. At the cabinet, the services arede-multiplexed from the SONET fiber and delivered to subscribers. Inaddition to the T1s that carry service to subscribers outside the cabinet,other T1s are used inside the cabinet to derive special analog servicesfrom T1 fed channel banks, or special services shelves. Typical services thatare delivered in this manner are legacy analog interfaces such as coinphone, analog 4-wire, and analog PBX. The CES8 enables the replacement


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of expensive SONET transport with a cost-effective Ethernet access platformthat 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 T1services equivalent to those shown in Figure 4. This is a simplified picturefocused only on T1 services and ignores other services such as video andinternet data. T1s in the CO are terminated directly on the CES8 whichconverts the TDM traffic into packet traffic for the Ethernet network. Thepseudo-spans are switched through the network to destination MAPs, wherethe CES8 regenerates the original T1 from the packet flow. The T1s can beused directly from the CES8 for delivery inside or outside the cabinet.

Figure 5: Cabinet Resident T1 Service Delivery with CES8


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Note: In Figure 5, T1 spans have a maximum reach of 6000ft. When thedistance from the remote cabinet to the subscriber exceeds this distance, aT1 repeater must be used. Repeaters are often line powered directly fromthe T1 equipment, however the CES8 does not provided line power. For thisreason, if a repeatered T1 service is required, a separate Span TerminationShelf 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 emulationcapability is shown in Figure 6. In this example, the three MAP nodes areconnected in a protected Ethernet Protection Switched Ring (EPSR)configuration. MAP nodes 1 and 3 are in the CO and have Gigabit Ethernetlinks into the voice, T1, data, and video service networks. MAP node 2 islocated in a remote outdoor cabinet. In this example, only Nodes 1 and 2terminate T1s with CES8.

Figure 6: Full Services Network


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TDM to EthernetPacket Network Considerations

Designed for simplicity, theinterworking technology used for TDM conversion isrobust and designed to compensate for error conditions and challenges posed by T1 delivery.

Allied Telesyn

recommends that applications set the

packetization size to193 bytes to avoid wasting packet bandwidth or adding delay in the TDM circuit.

The T1 circuit emulation service provided by the CES8 is designed to besimple to engineer, configure, deploy, and maintain. The interworkingtechnology used for the conversion between TDM and packet is extremelyrobust and overcomes the challenges of delivering T1 services over a packetnetwork. Transport robustness is important since many characteristics of packet networks are not conducive to constant bit rate services. Packetnetwork congestion, blocking, QoS prioritization, multiple paths with varyinglatency, all can lead to jitter, lost packets, duplicate packets, and out of sequence packet delivery. All of these error conditions can be compensatedby the CES8, some automatically and some through user adjustableconfiguration parameters.

The CES8 automatically corrects sequence errors in received packetstream. The CES8 can detect and correct the following packet sequenceerrors:

1. Out of order packet sequence2. Late arrival of a packet. This is effectively the same as a packet

arriving out of sequence. The maximum variation in transit delay isdetermined by the depth of the setting of the jitter buffer.

3. Duplicate packets

All of these errors are corrected without requiring any special configuration.

Packet SizePacket 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 16to 1023 bytes. Increasing the number of TDM bytes per packet increasesend to end latency, and thus adds delay in the TDM circuit. Excessivelatency can degrade both voice and data services carried over the packetnetwork between CES8 line cards. In voice applications, sufficient amountsof delay can result in the need for echo cancellation. At the other extreme,lower numbers of TDM bytes per packet decrease transport efficiency byincreasing the percentage of overhead bytes associated with the packet.This results in higher bandwidth consumption in the packet network. Whenthere is not enough TDM data to fill a minimum Ethernet frame of 64 bytes,the packet is filled with dummy information, wasting packet bandwidth. AlliedTelesyn recommends that most applications set the packetization size to193 bytes (about 1ms) 1.

Packet Delay VariationTDM operates at a constant bit rate. When converting from packet to T1, theCES8 must have at least one queued packet to play out to the TDM T1. Itcannot 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).


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The CES8 providesbuffers that absorbsup to 60ms of PDV,and reducing jitter and improving latency.

For delay-sensitiveapplications, such asvoice, it is important toconsider the transport

system from end toend.

Such considerationshould include:interworking delay,

packet size, switchtransit delay and PDV buffering.

Unfortunately, in a packet network transit time varies from packet to packet.These variations, or jitter in the arrival time of the incoming packets, arereferred to as Packet Delay Variation (PDV). PDV is a parameter that ismeasured in milliseconds. PDV has many origins: multiple routes withdifferent 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 toensure that another packet has arrived before the current packet has beenplayed out. The CES8 provides buffers that can be configured to absorbvalues of PDV that are dependent on the configuraed payload size.Increasing the depth of these buffers adds latency to the T1. As mentionedin the discussion on packet size, excessive latency can degrade both voiceand data services.

End-to-End LatencyMany services carried over T1 are delay sensitive. For example, in voiceapplications, end to end transport latency greater than 25ms generallyrequires the use of echo cancellers to remove audible reflections from far

end 4 to 2 wire hybrid circuits used in telephones. Thus, in someapplications, it is important to understand the total end-to-end transportdelay when engineering a T1 over Ethernet circuit.

Calculations of end-to-end latency must include the following:

1. The intrinsic delay associated with the interworking function. Thedelay is not symmetric; converting TDM to packet has less delaythan converting packet to TDM.

2. Packet size. The number of bytes from the T1 stream that are usedto 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 tothe end to end delay.

The total latency is determined by adding the contribution of each the factorsabove. For example, consider the network shown in Figure 5. This networkis an EPSR configuration and therefore the end to end delay variesdepending on which direction traffic is circulating on the ring. The total end toend calculation is based on the model shown in Figure 6.

Figure 7: End-to-End Delay Model


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Packet overhead is asmall percentage of overall bandwidth,but should beconsidered. A fully-

loaded CES8 linecard will require * * 2.264Mbps or 18.112Mbps of packet bandwidth..

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 be6.42ms. Note that the MAP Central Fabric Controllers (CFCs) add negligibledelay to the path.

Ethernet Bandwidth Requirements

Packetization of the TDM streams adds protocol overhead to the 1.544Mbpsbandwidth of the T1. The structure of a pseudo-span packet is shown in

Figure 7.

Figure 8: Pseudo-span Packet Format

While the overhead is a small percentage of the overall bandwidth, theremay be instances, especially in congested networks, where the total packetbandwidth must be considered. From the figure above, protocol overhead(Ethernet + IP + RTP) is 90 bytes. If the packet size is set for 193 bytes (asrecommended by Allied Telesyn) then each pseudo-span packet will be 283bytes. In this configuration, overhead is approximately 32% of the packet

bandwidth. Therefore, at a packet size of 193 bytes, a single T1 will require2.264Mbps (32% more than 1.544Mbps) of packet bandwidth. A fully loadedCES8 linecard will require 8 * 2.264Mbps or 18.112Mbps of packetbandwidth.


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T1 over EthernetTiming & Synchronization

Both ends of the link must operate at thesame clock rate to

prevent underflow or overflow. The CES8 coordinates clock rates with a built-intiming mechanism.

As with all T1 applications, both ends of the link must operate at the sameclock rate. Otherwise the receiver will either overflow or underflowdepending on the relative timing between the transmitter and receiver.

Figure 9 shows two pieces of standard T1 equipment connected over anEthernet network using the CES8. Bits are clocked out of T1 Equipment A ata rate set by the clock f service . The CES8 collects the T1 frames andencapsulates 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 thefar-end CES8s queue will overflow or underflow depending on the relativetiming between the two clocks.

Figure 9: T1 over Ethernet Timing

The CES8 has a built-in timing mechanism that allows the far-end CES8 torecover a clock based on time stamps carried in the packet stream. Therecovered 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 therequirements of ANSI T1.403.

A typical CES8 configuration is shown in Figure 9. In this configuration, theCO CES8 is configured to use the incoming T1 CO1 as the timing master. Inthis mode, packets entering PSEUDO SPAN 1 will carry timing informationfrom T1 CO1 . At the remote CES8, the port serving T1 REM1 is configured toextract timing from PSEUDO SPAN 1, making T1 REM1 a timing slave toT1 CO1 . Note that each of the 8 ports on the CES8 can be independentlyconfigured to derive timing from either the T1 or the pseudo span.

The accuracy of the recovered clock is dependent on the behavior of thepacket network. The amount of packet loss and packet delay variation can

significantly influence time interval error. However, the clock recoverymechanism used by the CES8 is very robust and can compensate for a widerange of network behaviors. It has been shown to meet synchronizationperformance requirements even in poorly engineered packet networkswhere PDV can be as high as 60ms and packet loss can be as much as0.5%.


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Figure 10: Typical CES8 Application with Timing

CES8 T1 SpecificationsPhysical

Number of Ports 8Connector RJ-21Interface Voltage 2.4VInterface Impedance 100ohmsReach (Line Build Out) Short Haul (feet) 0-133, 133-266, 266-399, 399-533, 533-655.

Long Haul (dB) 0, -7.5, -15.0, -22.5.Line Rate 1.544MbpsLine Code AMI, B8ZSFraming In unstructured mode, all framing types are supported in that they are

transparently passed through the network.

Physical Layer Alarms LOSIn unstructured mode, all framing types are supported in that they aretransparently passed through the network.

Power 8WOperating Temperature -40C to 65CStorage Temperature -40C to 75CHumidity 5% to 90%Safety UL60950Emissions FCC Part 15 Class A

InterworkingStandards Compliance IETF SAToP (Structure Agnostic TDM over Packet)

Packet Size 1 to 1488 bytesPDV Buffer Max 60ms

Timing and SynchronizationTiming source Loop timed or Pseudo-span timed

Jitter ANSI T1.102, T1.403, GR-499-CORE

Wander T1.403

Holdover Accuracy Stratum 4 local oscillator.


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Company OverviewAllied Telesyn

A global company with nearly two

decades of continuous

profitability.

Allied Telesyn focusesentirely on end-to-end, purpose-built Ethernet applications.

A world-classengineering and support organizationspanning fivecontinents and morethan 30 countries.

The ideal choice for cost-consciousService Provider

professionals who arelooking for high-

quality, feature-richnetwork solutions.

Founded in 1987 with the goal of producing feature-rich, reliable, standards-based networking products, Allied Telesyn has a proven track record inbridging 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 globalmanufacturing competencies, weve evolved a market-focused approach tosystem development that is geared entirely to applications, rather thanindividual components. And by concentrating on battle-tested, end-to-endsolutions for vertical market applications we avoid the scattershot, company-focused approach common in the industry. Our tagline: Its our Network,too is a testament to our high-level of accountability and to our investmentin our customers bottom line success.

Allied Telesyn focuses entirely on end-to-end, purpose-built Ethernet and IPapplications; with a complete line of networking products that includes Layer

2 switches, Layer 3 switches, carrier class fiber/copper Multiservice AccessPlatforms, wireless access points, wireless adapter cards, and residentialgateways. No other networking vendor can match Allied Telesyns breadthand depth of Ethernet productswe are the leading manufacturer of mediaconverters, unmanaged Fast Ethernet switches and hubs, fiber optic networkadapters and other feature-rich interconnectivity products, worldwide.Additionally, Allied Telesyn has developed a world class systemsengineering and support organization that ensures networks are designedand implemented to handle the stress of providing voice, video and dataservices.

With engineering, manufacturing, sales, and distribution divisionsstrategically located throughout the Americas, Europe, Asia and Japan,Allied Telesyn is able to deploy solutions anywhere in the world, quickly andefficiently. And by rigorously testing products in design and support centersand leveraging our design and manufacturing competencies, Allied Telesynis able to offer solutions for the access edge that are both customized andplug-and-play. This ideal combination helps our customers keep costs low,speed network deployment and maximize network uptime.

Our customer-driven approachcombined with a pragmatic, value-basedpricing scheme and a superlative service organizationhas made AlliedTelesyn a global networking leader, with more than 17 years of continuousprofitability and products deployed in more than 50,000 companies in 30countries and five continents. Allied Telesyn: the ideal choice for cost-

conscious Service Provider professionals who are looking for high-quality,feature-rich network solutions at a lower price.

www.alliedtelesyn.com

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