Huawei Technologies

The TDM overThe TDM overThe TDM overThe TDM overThe TDM overPacket networkPacket networkPacket networkPacket networkPacket network

58NOV 2005 ˙ ISSUE 20

By Zhang Shifa

The TDM overThe TDM overThe TDM overThe TDM overThe TDM overPacket networkPacket networkPacket networkPacket networkPacket network

Here we discuss the importance, technical architecture, timing, and QoSissues of the TDM over Packet network. The conclusion shows that TDMover Packet network is a specialist solution supported by Huawei that providesthe key to building a unified All-IP network while saving in the CAPEXand OPEX areas.

Perspectives of the next generation network

ixed infrastructure providers are facing tremendous pressure toreduce their cost base. Voice services are being substitutedby voice over IP and mobile. Even now, broadband IPservices are growing fast, but such competition means amuch lower profit margin.

The legacy network infrastructure is stovepipes style forvarious services, e.g. PSTN, lease line, broadband Internet,ATM, FR, X.25, etc. Labor cost to maintain the parallelnetworks is high. The next negation network is based on aunified pure IP platform which can significantly reduceOPEX. This pure IP platform is perfect for next generationtelecom service, e.g. IP multimedia communication, IPTVand broadband Internet etc. Unfortunately, it is impossiblefor all telecom subscribers to move from legacy service toIP-based services overnight. The present telecom servicesare likely to last up to ten years, even after the core networkis migrated to IP. So, it is mandatory for a packet network tosupport the legacy services, including TDM services.

The advanced IP network can transport priority traffic acrossthe network with low packet loss rate, low latency, and smalljitter. The priority traffic can be used to carry the TDM byte stream.This technology is known as TDM over Packet (TDMoP), or TDMover IP (TDMoIP), or Circuit Emulation Service over Packet SwitchNetwork (CESoPSN). This paper introduces the TDMoP solutionsupported by Huawei.

F

Leading EdgeThe TDM over packet network

57 DEC 2005 ˙ ISSUE 20

The challenges facingTDM over Packet

The packet networks have, by nature,packet loss, reordering, transportation latencyand a packet delay variant. These characteristicsof the packet network can affect the TDMservice carried over the network.

Another challenge of TDM over packetis the synchronization. The TDM servicerequires both ends of the TDM connectionto be synchronized. If they are not, bufferoverflow or underflow would occur, leadingto frame dropped or inserted, and the qualitydecreases. Synchronization is necessarybecause the packet network can use the flowcontrol to adapt the bit rate of sender andreceiver. There is no global synchronizationon current IP/Ethernet networks. Thismakes it difficult to synchronize the TDMequipments over the packet network.

Huawei has developed a TDM overPacket solution with leading partners to solveall these issues.

The architecture of aTDM over packet network

As shown in Figure 1, the solutionprovided by Huawei solves packet loss, miss-ordering and packet jitter issues of the mostcarrier-class IP networks. It also supportshigh accuracy and stability clock recoveryfor either network reference clock or serviceclock.

In this solution, there are several com-ponents including CLE, access node, gate-way and management system.

CLE is customer located equipmentwhich provides various user interfaces (e.g.X.21, V.35, V.36), G.703/704, and xDSLor fiber uplink interfaces. The CLE performsthe TDM over IP function complying withthe standard draft from IETF.

The access node is DSLAM or MultiService Access Node (MSAN) equipment whichprovides xDSL and fiber access to CLE. Theaccess node supports the network referenceclock recovering from IP network. The net-

work reference clock can be transported tothe CLE over NTR protocol of G.SHDSL.

The gateway has a similar TDM over IPfunctionality of CLE. It interoperates withthe legacy TDM network to provide theTDM service across IP and TDM network.

The management system supports end-to-end service configuration for the TDMover Packet service.

Synchronization solution

In PSTN/SDH networks, there is onenode called the clock master, which providesa time reference to the other, called the slave.Somewhere in the network there is at leastone extremely accurate primary referenceclock, with long term accuracy of one partin 1011. This node, the accuracy of which iscalled stratum 1, provides the reference clockto secondary nodes with stratum 2 accuracy,and these in turn provide a time referenceto stratum 3 nodes.

In the case of the packet network, there

Figure1 TDM over IP end-to-end solution by Huawei

Legacy SDH

E1/E3/STM-1 E1/STM-1

Gateway to PSTN/LL/SDH

xDSL/Fiber xDSL/Fiber

CLE CLE CLE

E1/nx64K E1/nx64KE1/ISDN PRA

PBX

Phone Fax/Modem

CLE to CLE Pseudo Wire

CLE

to

CLE

Pse

udo

Wire

E1/nx64K

DTEDTE

BSS/OSS

TL1/Coba

iManager

Pseudo Wire Solution

Legacy Digital Leased Line Legacy PSTN

IP Network

MA/UA SeriesMA/UA Series

Huawei Technologies

DEC 2005 ˙ ISSUE 20 58

is no network reference available for TDMemulated service. To provide synchroni-zation among TDM equipments, sometiming schemes of GPS, such as the out-band clock network, or in-band clockrecovery must be implicated. The timingschemes are shown in Figure2.

The GPS scheme provides an accurateand stable clock independently to the packetnetwork. The out-band timing schemeconstructs a parallel synchronizationnetwork to provide clock to TDM equip-ment. In in-band P2P adaptive timingscheme, clock is carried by the TDM traffic,and is recovered by adaptive mode by thereceiver. In in-band common referencetiming scheme, a common reference clockis used. The reference clock is sent over packetnetwork by TDMoP traffic to the TDMoPterminals or the access equipment. The GPSand out-band timing scheme can satisfy thesynchronization requirement. However,sometimes the two schemes are notacceptable because of high cost. Theperformance of adaptive timing scheme ishighly dependant on the performanceof the packet network. In the case of a com-mon reference clock, some advanced tech-nologies can be implemented to improve theclock performance in a cost effective way.

Test results showed the advance timingscheme over packet satisfies the synchro-nization requirement of ISDN signaling, fax,modem, GSM and 3G BTS.

QoSThe packet loss, reordering and jitter of

the packet network decreases the quality ofTDM service. The TDM over Packetsolution must solve this problem.

The packet loss leads to an interruptionin TDM traffic. The TDMoIP system hasthe capability to recognize the packet lossand correct the error by inserting stuffingpacket. Otherwise, the end to end latencychanges. If extremely high quality is re-quired, two copies of each TDMoP packet aresent at the same time. If an incidental packetis lost, the receiver can use the second copyof the packet to retrieve the TDM stream.

The packet reordering can be detected byexamining the sequence number. Thereceiver has the ability to sort the packet ina correct sequence.

In-band P2P Adaptive Timing Scheme

GPS Timing Scheme

Out-band Timing Scheme

Even if the packet arrives through the samepath, the arrival time may be different, due topacket delay variant or packet jitter. The re-ceiver equipment has a jitter buffer to smooththe arrival time difference of various packets.

Latency of TDM service is critical becauselong latency may cause echo and difficultyin conversation. The latency of TDM overPacket service is composed of packing timeand transportation delay. It is impossible forthe TDM over Packet terminal to shorten thetransportation delay. So, to lower the latencyover packet network, the same priority sche-me must be implemented in the packet network.

AdvantagesThe TDM over Packet solution can be

used for PBX/E1/ISDN access to PSTN viabroadband access network. This would help

CLEC cut down the CAPEX to constructTDM network for those services.

The TDM over Packet solution can alsohelp ILEC migrate to NGN smoothly. TheILEC can provide TDM service over all-IPnetworks, decoupling the migration of thenetwork and service migration to IP serviceof subscriber. The unified all-IP based packetnetwork will help ILEC significantly cutdown OPEX.

The solution can provide an accurate clockfor most TDM based services, e.g. ISDNBRI/PRI, fax or modem over ISDN, GSMBTS and 3G BTS for voice access. Thesolution can guarantee the quality of TDMservice and fully comply with internationalquality requirements of TDM service de-fined in the ITU-T series standards. In a word,TDM over Packet technology is now matureenough for operator implementation.

In-band Common Reference Timing Scheme

Figure 2 The timing scheme for TDM over Packet

GPSGPS

CLKCLK

NTR

CLE TDMoP

CLE TDMoP

PBX / Router Access Node

IP Network

Access Node PBX / Router

NTR

CLE TDMoP

CLE TDMoP

PBX / Router Access Node

IP Network

Access Node PBX / Router

External Clock Distribution

Network

GW TDMoP

CLE TDMoP

PSTN / SDH

CLKAdaptive

Clock Mode

Access Node

IP NetworkCLK

Access Node PBX / Router

CLE TDMoP

CLE TDMoP

PBX / Router

NTR

Access Node

TDMoP

Stratum 1 master reference clock

IP Network

Access Node PBX / Router