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One timeslot (TS0) is reserved for framing purposes, and alternately transmits a fixed pattern. This allows the receiver to lock onto the start of each frame and match up each channel in turn. The standards allow for a full Cyclic Redundancy Check to be performed across all bits transmitted in each frame, to detect if the circuit is losing bits (information), but this is not always used. An alarm signal may also be transmitted using timeslot TS0. Finally, some bits are reserved for national use.[3]
| bit number | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
|---|---|---|---|---|---|---|---|---|
| one frame every two frames |
checksum (0) or international usage |
0 | 0 | 1 | 1 | 0 | 1 | 1 |
| the other frame | checksum (CRC4) or international usage |
1 | alarm | national usage |
One timeslot (TS16) is often reserved for signalling purposes, to control call setup and teardown according to one of several standard telecommunications protocols. This includes channel-associated signaling (CAS) where a set of bits is used to replicate opening and closing the circuit (as if picking up the telephone receiver and pulsing digits on a rotary phone), or using tone signalling which is passed through on the voice circuits themselves. More recent systems use common-channel signaling (CCS) such Signalling System 7 (SS7) where no timeslot is reserved for signalling purposes, the signalling protocol being transmitted on a different physical channel.[4]
Link An unidirectional channel residing in one timeslot of a E1 or T1 Line, carrying 64 kbit/s (64'000 bit/s) raw digital data.
Line An unidirectional E1 or T1 physical connection.
Trunk A bidirectional E1 or T1 physical connection.
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E3 lines have a transmission speed of 34.368 Mbit/s. They can be seen as 4 E2 lines, which are themselves made of 4 E1 lines.
The PDH based on the E0 signal rate is designed so that each higher level can multiplex a set of lower level signals. Framed E1 is designed to carry 30 or 31 E0 data channels plus 1 or 2 special channels, all other levels are designed to carry 4 signals from the level below. Because of the necessity for overhead bits, and justification bits to account for rate differences between sections of the network, each subsequent level has a capacity greater than would be expected from simply multiplying the lower level signal rate (so for example E2 is 8.448 Mbit/s and not 8.192 Mbit/s as one might expect when multiplying the E1 rate by 4).
Note, because bit interleaving is used, it is very difficult to demultiplex low level tributaries directly, requiring equipment to individually demultiplex every single level down to the one that is required.
| T-carrier and E-carrier systems | North American | Japanese | European (CEPT) |
|---|---|---|---|
| Level zero (channel data rate) | 64 kbit/s (DS0) | 64 kbit/s | 64 kbit/s |
| First level | 1.544 Mbit/s (DS1) (24 user channels) (T1) | 1.544 Mbit/s (24 user channels) | 2.048 Mbit/s (32 user channels) (E1) |
| (Intermediate level, T-carrier hierarchy only) | 3.152 Mbit/s (DS1C) (48 Ch.) | – | – |
| Second level | 6.312 Mbit/s (DS2) (96 Ch.) (T2) | 6.312 Mbit/s (96 Ch.), or 7.786 Mbit/s (120 Ch.) | 8.448 Mbit/s (128 Ch.) (E2) |
| Third level | 44.736 Mbit/s (DS3) (672 Ch.) (T3) | 32.064 Mbit/s (480 Ch.) | 34.368 Mbit/s (512 Ch.) (E3) |
| Fourth level | 274.176 Mbit/s (DS4) (4032 Ch.) | 97.728 Mbit/s (1440 Ch.) | 139.264 Mbit/s (2048 Ch.) (E4) |
| Fifth level | 400.352 Mbit/s (DS5) (5760 Ch.) | 565.148 Mbit/s (8192 Ch.) | 565.148 Mbit/s (8192 Ch.) (E5) |
Note 1: The DS designations are used in connection with the North American hierarchy only. Strictly speaking, a DS1 is the data carried on a T1 circuit, and likewise for a DS3 and a T3, but in practice the terms are used interchangeably.
Note 2: There are other data rates in use, e.g., military systems that operate at six and eight times the DS1 rate. At least one manufacturer has a commercial system[citation needed] that operates at 90 Mbit/s, twice the DS3 rate. New systems, which take advantage of the high data rates offered by optical communications links, are also deployed or are under development. Higher data rates are now often achieved by using synchronous optical networking (SONET) or synchronous digital hierarchy (SDH).
Note 3: A DS3 is delivered native on a copper trunk. DS3 may be converted to an optical fiber run when needing longer distances between termination points. When a DS3 is delivered over fiber it is still an analog type trunk connection at the termination points. When delivering data over an OC3 or greater SONET is used. A DS3 transported over SONET is encapsulated in a STS-1 SONET channel. An OC-3 SONET link contains three STS-1s, and therefore may carry three DS3s. Likewise, OC-12, OC-48, and OC-192 may carry 12, 48, and 192 DS3s respectively.[citation needed]