DS3 Circuits and T3 Lines Defined

DS3 Circuits and T3 Lines Defined How DS3 and T3 work and the new competition from copper and fiber Carrier Ethernet.

Companies that have outgrown their T1 lines often upgrade to DS3 bandwidth. Let’s take a look at what DS3 has to offer, how it relates to T3 lines, and recent competition from Ethernet over Copper services.

What is DS3?

DS3 stands for Digital Signal level 3. The specification for it is found in the T-carrier standards developed by Bell Labs for the telephone industry. The way it works is that DS0 is the smallest unit with a bandwidth of 64 Kbps. That’s just the right size to transport one digitized telephone call using PCM (Pulse Code Modulation), the original digital phone standard. You combine 24 DS0s to create a DS1 that runs on a T1 line. DS3 is the equivalent of 28 DS1s or 672 DS0s.

Are DS3 and T3 The Same?

You’ll often hear DS3 referred to as T3, like DS1 is called T1. The fine line of difference between DS3 and T3 is that DS3 refers to the actual structure of the data stream and T3 refers to the physical transmission layer. Most of the time, they are treated as equivalent. The reason that the digital signal has its own definition is that you can multiplex DS3 as well as DS1 to make much larger bandwidth services. At the far end, you can then demultiplex the signals to recover the original DS3 or DS1.

Types of DS3 Circuits

Combine the 672 voice channels plus the bits needed for synchronization and line maintenance and you have a bandwidth of 44.736 Mbps. That’s commonly referred to as 45 Mbps. You can order DS3 as a telephone trunk line if you have a high capacity phone systems like a PBX serving a major corporation. You can also order DS3 as a data line with one big chunk of bandwidth, namely 45 Mbps.

The Coax Connection

If you order a T3 line or DS3 service, it will be delivered to you on a pair of 75 ohm coaxial cables using BNC connectors. Look at a DS3 router card and you’ll see the connectors, one for receive and one for transmit. Some cards support more than one DS3 connection so you’ll see multiple sets of connectors.

Distance Limitations

The coaxial cable used to connect your DS3 router can be no more than 450 feet in length (only 225 feet if using small diameter coax). That’s fine for connecting to the telco demarc in your building, but how does the DS3 get to the central office?

DS3 over SONET

The actual provisioning of DS3 services can be via fixed wireless transmission or SONET fiber optic service. DS3 can be easily encapsulated into a SONET STS-1, as they are both telco standards designed to be compatible. An OC-3 service can carry 3 STS-1s for 3 DS3s.

Getting Higher Bandwidth Using T1 Lines

What if you are not in range for fixed wireless and there’s no fiber in the area? Are you stuck with the 1.5 Mbps T1 bandwidth? No, not really. T1 lines can be bonded to create larger bandwidths. This works well up to 10 or 12 Mbps. Bridging the gap between bonded T1 and DS3 is Ethernet over Copper. This technology also uses multiple copper pair instead of fiber strands, but can deliver much higher bandwidth than T1 technology.

How About Ethernet?

Ethernet over Copper (EoC) and Ethernet over Fiber bandwidth starts at typically 2 Mbps and goes up to 10 Gbps or even 100 Gbpsin some areas. This gives you a way to grow your bandwidth incrementally beyond T1and avoid the 45 Mbps ceiling on DS3 bandwidth. You should know that Ethernet over Copper is distance limited, so getting 45 or 50 Mbps using copper pair is limited to locations nearly a telco office equiped with EoC terminal equipment. Ethernet over Fiber is not available in all locations yet, but if your building one nearby is lit for fiber optic service, you may have an extensive range of bandwidth options available.