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Understanding DS3 and T3, a Tutorial
DS3 T3 Tutorial offers the technical background and applications for DS3 over T3 lines.

By: John Shepler

If you need a lot more bandwidth than the 1.5 Mbps delivered by a T1 line, it might be time to step up to DS3 service delivered on a T3 line. How much more bandwidth can you get? How about as much as 28 T1 lines or 45 Mbps? That's enough throughput for broadcast quality video transport, high volume call centers or Internet Service Provider backbones. If you have a really large organization, perhaps you want that 45 Mbps as YOUR Internet service connection.

The Relationship of DS3 to T3
So what is DS3 and how does it relate to T3? As you might guess, T3 and T1 lines are related. They were designed as part of family of digital network services for the telephone companies called the T-Carrier system. This isn't exactly new technology. The first T1 lines went into service in the late 1950s. What is new is the availability and reasonable pricing of T3 and DS3 services to non-telecom carrier businesses.

DS3 stands for Digital Signal level 3. That's the definition of the protocol that runs on the physical T3 line. DS3 is a TDM or Time Division Multiplexed synchronous format. It runs at 44.736 Mbps, which we usually call 45 Mbps for short. The transmission rate of 44.736 Mbps isn't some arbitrary number. It has to be that speed so that DS1 signals that are carried on T1 lines can also be carried on T3 lines.

There are actually two types of T3 circuits. The oldest is channelized T3 which is used to transport digitized telephone calls. Each channel is 64 Kbps wide and carries one phone call. A T1 line can carry 24 of these DS0 channels. A T3 line carries 672 of them or 28 DS1s. A device called an M13 Multiplexer combines the 28 DS1s and gets them aligned in the proper time slots to make a DS3, which can then be carried on a T3 line.

The other type of T3 format is called unchannelized T3. If you don't need all those little channels to support phone calls or modem connections, unchannelized T3 gives you one big data pipe with a payload rate of 44.210 Mbps. This is similar to an unchannelized T1 line but with a much higher bandwidth.

The Physical Layer
The physical connection from your equipment to the phone line Network Interface Unit consists of two 75 ohm coaxial cables with BNC connectors. That's different from a T1 line which gives you an RJ-48 jack at the NIU for use with unshielded twisted pair cable. One of the T3 coaxial cables is transmit, the other is receive. T3 is a full duplex circuit, meaning that transmit and receive are independent and both run at the 45 Mbps rate.

Outside your building the T3 line might stay as coaxial cable, but it is more likely to be carried on a fiber optic cable between your location and the carrier's office. It will likely be multiplexed onto a larger fiber optic carrier, even an OC48 or OC192, for long haul transmission.

But what if a T3 line has way more bandwidth than you need and a T1 line isn't nearly enough? In between, there are a number of options. If you need 3 to 9 Mbps your lowest cost option may well be to bond several T1 lines together to get the equivalent of a single larger circuit. Above that, fractional DS3 may be the better option until you need a full T3 or multiple T3 lines. Once you get to needing a 100 Mbps connection to carry your network traffic, OC3 or 100 Mbps Ethernet service on fiber optic cable makes sense.

Competition from Carrier Ethernet
In recent years, a newer technology called Carrier Ethernet has started replacing both T1 lines, DS3 and SONET OCx fiber. Ethernet over Copper (EoC) uses the same twisted pair wiring at T1 and is available from 2 to 45 Mbps. Ethernet over Fiber (EoF) is highly scalable in the range from 10 Mbps to 10 Gbps, with 100 Mbps and Gigabit Ethernet being very popular service levels. Ethernet is far more scalable than earlier technologies and enjoys a cost advantage as well.

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