Digital signal X is a term for the series of standard digital transmission rates or levels based on DS0, a transmission rate of 64 kilobits per second (Kbps), the bandwidth normally used for one telephone voice channel. Both the North American T-carrier system and the European E-carrier system of transmission operate using the DS series as a base multiple. The digital signal is what is carried inside the carrier system.
DS0 is the base for the digital signal X series. DS1, used as the signal in the T1 carrier, is 24 DS0 (64 Kbps) signals transmitted using pulse-code modulation (PCM) and time-division multiplexing (TDM). DS2 is four DS1 signals multiplexed together to produce a rate of 6.312 Mbps. DS3, the signal in the T3 carrier, carries a multiple of 28 DS1 signals or 672 DS0s or 44.736 Mbps.
Digital signal X is based on the ANSI T1.107 guidelines. The ITU-TS guidelines differ somewhat. The following table summarizes the set of signals and relates them to the T-carrier and E-carrier systems.
| Digital Signal Designator | Data Rate | DS0 Multiple | T-Carrier | E-Carrier |
|---|---|---|---|---|
| DS0 | 64 Kbps | 1 | - | - |
| DS1 | 1.544 Mbps | 24 | T1 | - |
| - | 2.048 Mbps | 32 | - | E1 |
| DS1C | 3.152 Mbps | 48 | - | - |
| DS2 | 6.312 Mbps | 96 | T2 | - |
| - | 8.448 Mbps | 128 | - | E2 |
| - | 34.368 Mbps | 512 | - | E3 |
| DS3 | 44.736 Mbps | 672 | T3 | - |
| - | 139.264 Mbps | 2048 | - | E4 |
| DS4/NA | 139.264 Mbps | 2176 | - | - |
| DS4 | 274.176 Mbps | 4032 | - | - |
| - | 565.148 Mbps | 4 E4 channels | - | E5 |