Actually, DS1 and T1 aren’t exactly the same thing, although most of the time the terms are used interchangeably. They are defined in the same family of technical specifications developed by Bell Labs. This is the T-carrier digital line service introduced as a replacement for analog carrier telephony. While designed for the telephone industry, DS1 and T1 have since greatly expanded their application into data connections for point to point private lines and dedicated Internet access.
Just what does DS1 mean? The term DS1 or DS-1 stands for Digital Signal 1. It is a time division multiplexed set of channels that are also referred to as timeslots. Each channel is 8 contiguous bits. The collection of 24 channels lined up end to end amounts to 192 bits. These are send out as a serial bitstream from transmitter to receiver. How do you tell the channels apart? One additional bit is added to the front of the pack to act as a synchronizing or framing bit. The sync bit says “start a new frame here.” The first 8 bits are channel 1, the second 8 are channel 2, and so on.
The DS1 frames repeat 8,000 times per second. This frequency isn’t arbitrary. It was selected to be just fast enough to adequately sample an audio signal that ranges from 300 to 3,300 Hz. You’ll recognize that as the voice range allocated to telephone calls. To avoid distortion, you need to sample at least twice the highest frequency with a little extra margin for filtering. The combination of 8 bits or 256 different levels times an 8 KHz sampling rate gives each channel a bandwidth of 64 Kbps. The string of 24 channels equals 1.536 Mbps. Add the framing bit (1 bit times 8 KHz) for another 8 Kbps and the grand total is 1.544 Mbps. There is where the T1 bandwidth of 1.5 Mbps (rounded) comes from.
This is what is going on inside a DS1. Each of those 24 channels is called a DS0. You can use them separately to carry 24 separate telephone conversations or combine their capacity to carry 1.536 Mbps of data payload. There is a special service called Integrated T1 that uses some of the channels for voice and some for data. This gives you both telephone and broadband Internet service on the same T1 line.
DS1 is the digital signal that rides on a T1 line. The line is a physical transmission system. It was originally specified as two twisted pair copper phone lines, one for transmit and one for receive. The signal on the line consists of alternating positive and negative pulses that represent the logical “1” level and zero voltage representing a logical “0. Nowadays, other modulation schemes like HDSL and HDSL2 may be used to increase transmission distance and reduce the number of pairs needed from two to one. Regardless, these are all called T1 lines.
There is now a competing line service called Ethernet over Copper (EoC). It uses the same twisted pair copper wiring as T1 lines, although the termination equipment on each end is different. Both T1 and EoC can combine multiple copper pairs to increase bandwidth. Ethernet, itself, is not a channelized protocol. Instead, it uses packets to transmit voice, data or video.
T1 is highly reliable, reasonably priced and almost universally available due to its deployment on the telephone infrastructure over half a century. Ethernet over Copper is also highly reliable and less expensive than T1, where available. You can typically get 2 to 3 Mbps EoC for the same price as 1.5 Mbps T1. Ethernet bandwidths can easily go up to 20 or 30 Mbps, while bonded T1 lines are limited to around 12 Mbps. The limitation of Ethernet is that the signal fades rapidly with distance. You need to be fairly close to the telco central office that terminates your copper pair to get the higher bandwidths. In some areas, Ethernet services are still not readily available.
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