Monday, February 11, 2008

Virtual Concatenation for Ethernet Transport

Carrier Ethernet services for wide area and metropolitan area networks are becoming the transport technology of choice for many companies. Ethernet offers the advantages of direct compatibility with local area networks, offering higher bandwidths than traditional T1 (1.5 Mbps) and DS3 (45 Mbps) telecom services. Traffic levels are increasing as more content becomes video and high resolution images in addition to email and static Web pages. Ethernet (10 Mbps), Fast Ethernet (100 Mbps) and Gigabit Ethernet (1,000 Mbps) are often the best-price solutions to bandwidth expansion.

That's all well and good if you are a carrier that has deployed long haul IP networks. But how about incumbents with legacy investments in TDM networks? When requests for Ethernet were few and far between, the solution was simply to load the Ethernet packets into existing signals with enough bandwidth to accommodate the desired rate. Conversions to and from Ethernet are done are the network edges. It works, but a lot of bandwidth is lost to inefficiency. Digital networks designed to efficiently carry telephone conversations don't directly support Ethernet data streams.

Here's why. Digital telephony is all based on individual telephone conversations that consume 64 Kbps each. One phone call goes into one channel in a circuit switched network. Channels can be concatenated or bundled to create digital trunk lines, such as a T1 line with 24 channels. TDM or Time Division Multiplexing is the technology that keeps the channels separate and synchronized end to end.

You can also build higher speed services by bundling 28 T1 lines into a T3 carrier. That gives you 672 telephone channels. When transporting data, the entire line capacity is used as packet bandwidth less a bit of overhead needed to keep everything running smoothly. A T1 line offers about 1.5 Mbps and a T3 has 45 Mbps of payload. Both of these are copper-based trunk lines. Higher speeds require fiber optic carriers.

Fiber has its own digital signal hierarchy, but it is still based on channels and a TDM technology called SONET for Synchronous Optical NETwork. The smallest service is equivalent to the T3 payload of 45 Mbps. This is called STS-1. This was done deliberately to multiplex T3 signals, also known as DS3, onto fiber optic carriers.

In practice, the lowest bandwidth SONET carrier is the OC3 which consists of 3 STS-1's for a total of 155 Mbps bandwidth. Higher capacity carriers that are typically available are OC12 at 622 Mbps, OC48 at 2.5 Gbps and OC-192 at 10 Gbps. These higher bandwidths are achieved by stringing together or contiguously concatenating the necessary number of STS-1 signals.

Note that Fast Ethernet at 100 Mbps fits into an OC3 carrier at 155 Mbps, while a Gigabit Ethernet service is too big for an OC12 and wastes more than half of an OC48. Contiguous concatenation is like a freight train of a fixed size. If many of the cars are empty, they still have to come along for the ride.

Another process that is just starting to be deployed by major carriers, including AT&T, is called VCAT or Virtual Concatenation. VCAT allows mixing and matching of available STS-1's to create just the right size bandwidth for the customer's payload. Using the Synchronous Transport Signal as the basic building block rather than choosing only the standard OCx carrier levels makes for a more efficient mapping from Ethernet to SONET and back again. It also opens the possibility of easily adding or subtracting STS-1's to increase or decrease bandwidth as desired.

Virtual concatenation is among the upgrades that legacy carriers can implement to make their already deployed and highly reliable TDM networks more effective with the IP-based traffic generated by today's enterprise customers. Your company can benefit from these technology changes and the recent proliferation of competitive carriers offering voice and data services from T1 lines through Gigabit Ethernet.

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