Transmuxing

Creating an Efficient Bridge Between Legacy DS3 Transport Equipment and Today's SONET Capabilities

Posted: 10/1997

Transmuxing

Creating an Efficient Bridge Between Legacy DS3 Transport Equipment and Today's SONET Capabilities

by Jay Prichard

In an ideal world, carriers would do their network handoffs using SONET (synchronous optical network) signals, either STS-1 or optical OC-n signals. Yet many of them still prefer to hand off an asynchronous DS-3 at network demarcation points, either because of the large embedded base of async equipment or the lower tariffs on DS-3 facilities. There is now a way for them to get the benefits of SONET even as they continue to do async handoffs.

Initially, SONET standards did not address the need to accept DS-1 signals at one multiplexer (mux) in the network and hand off a DS-3 signal at another point. This embedded asynchronous M13 multiplexing function on a SONET mux is known as virtual tributary (VT) transmuxing.

Transmuxing achieves efficient DS-3 handoffs in a SONET ring. A ring with transmuxing collects DS-1s from customer sites, transports them survivably to the central office, and presents a fully groomed DS-3 directly to the interexchange carrier (IXC) without using external M13 multiplexers or digital crossconnects. Transmuxing creates an efficient bridge between legacy DS-3 transport equipment and today's SONET capabilities. It also allows integrated network operations support, and in multiplexers such as Fujitsu Network Communications' FLM 150 ADM, is easily added by installation of a low-speed card.

The local exchange carrier (LEC) practice of collecting DS-1 traffic from customers, but multiplexing it into DS-3 signals for the interexchange carrier (IXC), originated prior to the development of SONET. At that time, manufacturers developed fiber optic transmission systems which were used to collect several DS-3 signals and remultiplex them to the vendor's proprietary fiber optic transmission code as a method of bulk transport.

Because of the popularity of these transmission systems, there was a heavy investment in DS-3 hardware among carriers. That is why, despite the advantages ushered in by SONET, the IXCs still often require DS-3 handoffs today.

LECs use SONET rings to transport DS-1 services from customer locations and digital loop carrier equipment to the IXC or other carrier point of presence (POP). However, if the other carrier requires a DS-3 handoff, the LEC must multiplex the DS-1 traffic gathered from customer sites into a DS-3 signal before handing it off to the IXC. This can be done (Figure 1) using traditional M13 multiplexers or digital crossconnect systems that are external to the SONET network.

Although it provides the necessary DS-3 interface, external M13 equipment presents a significant operations challenge and means additional boxes in the network. Requiring an additional M13 mux for every DS-3 at the IXC POP is an unwelcome consumer of valuable space.

As an alternative, a LEC can locate an M13 within the central office before transmission to the IXC. But inserting an M13 mux between the customer SONET access ring and the LEC's SONET interoffice facility (IOF) ring breaks the SONET continuum. The LEC no longer can deliver end-to-end SONET service from the customer to the IXC, losing SONET advantages such as survivability.

Rather than use an M13 multiplexer, LECs sometimes achieve a DS-3 handoff with digital cross-connect systems (DCCS). The cost of DCSs makes it impractical to deploy them at each site that requires a DS-3 handoff. The LEC often transports DS-3s through the SONET ring and backhauls them to a central DCS.

The DS-3s enter the DCS from the ring, are groomed at the DS-1 level, and re-enter the ring as DS-3s. Although not as expensive as the first DCS option, this method wastes considerable ring bandwidth backhauling the DS-3s to the centrally located DCS.

A SONET ring with integrated transmuxing avoids these problems. The ring collects DS-1s from customer sites, transports them survivably to the central office and presents a fully groomed DS-3 directly to the IXC or other carrier. All this is accomplished without external M13 muxes or digital crossconnects. Integrating transmux into a SONET ring also streamlines network operations and maintenance and offers a rich interface for managing network demarcation.

DS-1s are transported within the VT1.5s from customer sites or from next generation digital loop carrier equipment (Figure 2). The VT paths enjoy SONET survivability to the handoff node and are groomed at the handoff node using VT time slot assignment.

As the VTs are dropped at the handoff node, their DS-1 payloads are extracted and then multiplexed onto a DS-3 tributary interface. The DS-3 can be handed over directly to the other carrier or transported over an EC-1 SONET interface.

As the VT1.5s reach the handoff node (Figure 3), those containing the DS-1s which are destined for the carrier are dropped from the ring. The user gathers one to 28 of the VT1.5s on the ring and drops two copies of each VT1.5 to a path selector. Each path selector chooses the best VT1.5 based on VT path level parameters. This guarantees the survivability of all the DS-1s that make up the handed-off DS-3.

The selected VT1.5s are then terminated and their component DS-1s are extracted. The DS-1s that are demapped from the VT1.5 are immediately multiplexed into a DS-3 by the M13 function before being dropped out of the handoff node.

The user, in effect, grooms the DS-3 by grooming the VT1.5s either at the handoff node or at the remote nodes, or both. The user simply drops the VT1.5s to adjacent drop ports (for example, drop ports one through 28 of a low-speed group) and the M13 function organizes the DS-1s into a DS-3, based on the port assignment.

The DS-3 can then be dropped directly from the ring. Optionally, the DS-3 can be mapped into an STS-1 so that the drop interface from the ring is an EC-1. A section data communications channel could be supported on this EC-1 interface.

LECs have begun offering customers premier service, in which DS-1s are put on a SONET ring from start to handoff. Customers get a higher quality, more reliable signal from their site to the local central office to the IXC, and some are willing to pay a premium for this quality service.

Transmuxing can be taken one step further, if desired, for transmission of the DS-3 within a DS-3-mapped EC-1.

Combining transmux with dual interconnection provides SONET from start to handoff and provides 100 percent survivability (Figure 4). The network has no single point of failure. Each step of this network can survive at least one fiber or node failure and, in some cases, more than one. Using an M13 multiplexer or DCS within the central offices to create the DS-3 signal would invalidate the end-to-end survivability, because an M13 or DCS does not support SONET automatic protection mechanisms.

Suppose DS-1 traffic needs to be transported between two carriers, each requiring a DS-3 handoff. DS-3s can be transported intact through the ring, but each consumes an entire STS-1 of ring bandwidth, even if the DS-3 is not completely filled with 28 DS-1s. As an alternative, the DS-3 signals can be demultiplexed by the VT transmux function into DS-1 signals at each handoff site. If fewer than 28 DS-1 signals are carried by the DS-3s, the DS-1 signals can be groomed into a single STS-1. This approach conserves ring bandwidth.

By integrating the M13 multiplexer's functionality, transmux has several important operations benefits. It offers key advantages in monitoring, provisioning and survivability and results in a greatly reduced operations network (Figures 5 and 6).

Clearly, a SONET ring with integrated transmuxing has important advantages. By integrating the M13 function on the multiplexer's drop side, the transmux-equipped SONET ring provides efficient DS-3 handoffs without external equipment. In addition, transmuxing conserves ring bandwidth when transporting DS-1 traffic between two carriers that each require DS-3 handoffs. Bandwidth is conserved because transmuxing allows the DS-1s between carriers to be individually transported and groomed within the ring, rather than as an entire DS-3.

In addition to bandwidth savings, transmuxing in a SONET ring streamlines operations and maintenance by eliminating external M13 multiplexers and their associated management interfaces. All DS-3 interface monitoring is done as a part of the SONET ADM, and no alarm correlation is needed between the SONET ring and the M13. Provisioning of the DS-3 interface can be handled remotely from the SONET network operations center.

Transmuxing also provides a rich interface for managing network demarcation by providing integrated DS-3 and DS-1 performance monitoring and C-bit parity formatted DS-3s. Finally, the DS-3 drop can be formatted into an STS-1 for an EC-1 handoff to the interoffice facility ring. SONET performance monitoring is then available on the handoff EC-1 interface.

With a SONET ring using integrated transmuxing, it is no longer a headache for a LEC to make an efficient DS-3 handoff to the interexchange carrier. The task becomes another highly streamlined network operation.

Jay Prichard is marketing manager for SONET high-bit-rate products, Fujitsu Network Communications, Inc., Richardson, Texas. For more information, call (972) 690-6000.

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