The DS1 DS3 Relationship

If you are involved with business telecommunications networking or need to be, you have probably heard the terms DS1 and DS3 bandied about. But what do these terms really mean and where did they come from?

The DS in DS1 and DS3 stands for Digital Signal level. It is a designation that is used to specify the capacity of a digital line in the T-carrier system. You know the most popular T-carrier service as T1. Another popular service is T3. In simple terms, DS1 goes along with T1 and DS3 goes along with T3.

Now, let’s see what’s behind these DS levels and T-carriers. The history of this system goes back to the 1950’s, when Bell Labs recognized the advantage of changing from an all-analog telephone system to digital connections between telephone company offices. Digital transmission carries many conversations on few wires and eliminates the noise and cross-talk that were commonly heard on long distance calls at that time.

The most basic service is one two-way telephone conversation. That was made the most basic element of the T-carrier system and given the designation DS0 or Digital Signal level zero. A DS0 signal is the digital representation of a single voice call. It has a bandwidth of 64 Kbps formed by sampling the audio conversation at a rate of 8 Kbps with 8 bits of data. Those numbers are critical, because they represent the minimum bandwidth needed to give the same audio quality as an equivalent analog phone line.

If a DS0 is one phone call and consumes 64 Kbps of bandwidth, then what is a DS1? DS1 is a collection of 24 DS0 “channels” packed together in a bundle. That’s 24 separate telephone conversations. The bandwidth for this bundle is 1.536 Mbps. When you add 8 Kbps for synchronization, the total bandwidth is 1.544 Mbps. It’s commonly referred to as 1.5 Mbps. A T1 line is a DS1 digital signal package carried on a very specific physical network consisting of 2 pair of twisted pair telco wires with a defined voltage and waveform characteristic.

That’s what DS1 means. Now, how about DS3? If you bundle 28 DS1 signals, the equivalent of 672 DS0 voice channels, you get a DS3. The bandwidth of this signal is 44.736 Mbps, often referred to as 45 Mbps. A T3 line carries a DS3 signal on coaxial cable, although T3 service may also be sent by microwave.

Now, here’s where things get interesting. You can generate the DS digital signal protocol but not necessarily use a T1 line or T3 line for transport. In a process called multiplexing, multiple DS1 and DS3 signals can be combined and transported over fiber optic cable. The SONET fiber optic services were designed to directly accommodate this. The lowest bandwidth SONET service generally available is OC3, which carries 3 multiplexed DS3 signals with a total bandwidth of 155.52 Mbps. When the signals get to where they are needed, they are separated or demultiplexed to get back the 3 DS3 signals, which might be constructed of 28 DS1 signals each.

When used to transport data packets instead of telephone calls, the individual channels are generally invisible to the end user, although they exist within the T-carrier and SONET systems. DS1 and T1 can be said to have a bandwidth 1.5 Mbps. DS3 and T3 can be said to have a bandwidth of 45 Mbps. It is these numbers that you can use to compare pricing with competing telecom services , such as Metro Ethernet.