Virtual concatenation packs SDH to boost data transport

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Virtual concatenation

The financial necessity to use incumbent voice-based SONET/SDH networks for data transport is leading to the use of virtual concatenation.

By Mark Telford

Demand for data traffic is still growing and the dominant data protocol in local-area networks is Ethernet, which is in increasing demand from businesses for metro- and wide-area network access. However, cash-strapped carriers need to use existing infrastructure, which is mainly SONET/SDH-based for TDM-based voice traffic.

But a Gigabit Ethernet data stream is too big for a SONET OC-12 (622Mbit/s) line and must be carried on an OC-48 (2.5Gbit/s) line, leaving 1.5Gbit/s unused. So, to offer data services while keeping investment low, there is a need to adapt SONET/SDH infrastructure to deliver packet-based data services without wasting bandwidth.

Virtual concatenation standards
A recent standard aiming to reduce bandwidth waste is virtual concatenation (VCat; ITU G.707 2000), which maps the payloads into SONET/SDH.

Rather than using contiguously concatenated payloads, VCat breaks down data packets by mapping them into the base units of TDM frames: STS1 (51Mbit/s) for SONET; AU4 for SDH (155Mbit/s). It then groups them in multiple data flows of varying size to create a larger "right-sized" aggregate payload which will better fill the SONET/SDH pipe capacity.

Methods of encapsulating packet-based data in a SONET/SDH frame include the Generic Framing Protocol (GFP; ITU G.7041), used mainly for mapping Ethernet onto SONET in North America, and Link Access Procedure for SDH (LAPS, ITU-T X.86) for Ethernet-over-SDH in Asia (particularly China, where LAPS is a requirement) and, increasingly, in Europe.

Use in conjunction with the ITU G.7042 Link Capacity Adjustment Scheme (LCAS) boosts efficiency by allowing dynamic reconfiguration of the VCat groups to allocate bandwidth according to user need. Until recently, this took several days. Reducing provisioning delay allows dial-up of "bandwidth on demand". Different bandwidths can be can assigned to the same user at different times (e.g. for night-time back-up of storage area networks).

VCat framer-mapper ICs
Networks evolution should therefore lead to strong demand for framer-mapper ICs with VCat.

Most suppliers target existing OC-48, OC-12 and OC-3 networks, with Agere Systems, Agilent Technologies, Cypress Semiconductor and PMC-Sierra all offering OC48 (2.5Gbit/s) ICs, such as for Gigabit Ethernet over SONET.

Agere has been shipping VCat ICs the longest. Its Detroit chip set of 1998 implemented early SDH technology and used both VCat and GFP. The more recent TADM042G5 and the latest TADMVC2G5 "system-on-chip add-drop multiplexers" integrate a cross-connect to break traffic into STS-1 packets.

PMC-Sierra's PM5397 Arrow-2xGE is a two-port framer-mapper that uses VCat to aggregate two GbE feeds onto a 2.5Gbit/s OC-48 line by treating each as an STS-21 (1.09Gbit/s) worth of data, allow the remainder to be filled with voice links or smaller data connections. Meanwhile, Cypress' POSIC2GVC uses VCat to allocate up to 16 channels of voice and data.

However, while many suppliers have been producing ICs for next-generation SONET, support for SDH has lagged. With three times the granularity, it is less efficient on utilisation. At SuperComm 2002, several suppliers un-veiled framer/mapper ICs that support LAPS.

Agilent's HDMP-3002 framer/mapper for OC-48 (2.5Gbit/s), set for production by end-2002, is the second member of its MPIC multi-protocol IC family following January's launch of HDMP-3001 for OC-3 (155Mbit/s). Agilent claims it is also the first EoS mapper to integrate SerDes, clock data recovery and OC-3 to OC-48 framers. By providing full-duplex mapping of Fast Ethernet and Gigabit Ethernet frames encapsulated using GFP or LAPS, it can connect up to four GbE feeds into one STS-48/STM-16 (2.5Gbit/s), four STS-12/STM-4 (622Mbit/s), or four STS-3/STM-1 (155Mbit/s) channels.

TranSwitch's EtherMap-48 framer/mapper IC for OC-48, due for customer trials in Q1/2003, supports up to 24 Fast Ethernet and four GbE ports. As well as supporting LCAS for SDH, on-chip Gigabit Ethernet MACs equipped with flow control allows support of oversubscription of Ethernet streams.

Multilink Technology Corp's VCat-10 MTC6210, sampling in Q4/2002, is its first product from its Intelligent Edge product family and its first mapper-framer IC, offering both GFP and LAPS. Multilink also claims it is the first 10Gbit/s SONET/SDH device to offer VCat.

While some suppliers say that 10Gbit/s VCat chips are not needed, Multilink says it is targeting use closer to the metro core than the customer. Agere also plans to extend VC and GFP to 10Gbit/s. The MTC6210 is also one of the first chips to support LCAS. The only other announced, from TranSwitch, runs at 155Mbit/s, Multilink says.

West Bay Semiconductor claims that its WB4500 is the only chip that brings the full benefit of VCat to SDH. It overcomes the granularity limitation of SDH by fabricating a chip with TUG-3 (Tributary Unit Group-3) mapping functionality that enables SDH VCat at the VC-3-Xv (51Mbit/s) level, which is the same size as an STS1. Simultaneous processing and mapping for TDM and data transmission protocols including GFP and LAPS allows equipment makers to develop single-box multi-application next-gen SONET/SDH systems for the metro area network as well as at the edge of the wide area network, it says.

In July, Groupe Sagem SA, the second largest telecoms equipment supplier in France, said it will be deploying West Bay's chip in metro core and access equipment in a five-year, EUR10m deal, showing that virtual concatenation for SDH is poised to take off.

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