T-Carrier Terms
To understand this section, and the T1/T3 section, a few terms must be defined:
IMPORTANT - although there are subtle differences described below - DS1 and T1 are used interchangeably, as are DS3 and T3.
| T-Carrier Systems | North American | Japanese | European (CEPT) and ROW (Rest Of World) |
| 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 (30 user channels) (E1) |
| (Intermediate level, US. hierarchy only) | 3.152 Mbit/s (DS1C) (48 Ch.) | - | - |
| Second level | 6.312 Mbit/s (DS2) (96 Ch.) | 6.312 Mbit/s (96 Ch.), or 7.786 Mb/s (120 Ch.) | 8.448 Mbit/s (120 Ch.) |
| Third level | 44.736 Mbit/s (DS3) (672 Ch.) (T3) | 32.064 Mbit/s (480 Ch.) | 34.368 Mbit/s (480 Ch.) (E3) |
| Fourth level | 274.176 Mbit/s (DS4) (4032 Ch.) | 97.728 Mbit/s (1440 Ch.) | 139.268 Mbit/s (1920 Ch.) |
| Fifth level | 400.352 Mbit/s (5760 Ch.) | 565.148 Mbit/s (7680 Ch.) | 565.148 Mbit/s (7680 Ch.) |
NOTE: in America, only Level's 0, 1, and 3 are used
as access circuits.
Only America and Japan use the T1, while only America uses the T3.
Europe and the rest of the world use E1 and E3.
TDM (Time-division multiplexing) - this is the method in which the T-carrier system can send mutiple data streams across a given T1 or T3 span. It is digital multiplexing in which two or more apparently simultaneous channels are derived from a stream of bits, by interleaving pulses representing bits from different channels. For example, in a T1 frame, if we ignore the framing bits, we have 24 channels, with 192 bits of data per frame. So, the first 8 bits are transmitted and assigned as "channel 1, or time-slot 1. The next 8 bits are channel 2, etc - up to the last 8 bits, which are channel 24.
PDH and SDH systems - PDH (Plesiosychronous Digital Hierarchy) systems is now being replaced by SDH (Synchronous Digital Hierarchy) equipment in most telecommunications networks. The most prevalent SDH by far, is SONET based networks.
Plesiochronous - derived from Greek plesio and chronos, meaning near time. Different parts of the network are almost, but not quite perfectly, synchronised. This has been a long-standing problem with the old systems used with T-carriers, because their clocks never matched exactly.
DS0 (Digital Signal 0) - also called a channel, or time-slot. In T-carrier systems Digital signal 0 (DS0) is the basic unit. It has a digital signaling rate of 64 kb/s, corresponding to the capacity of one voice-frequency-equivalent channel. The DS0 rate may support twenty 2.4-kb/s channels, or ten 4.8-kb/s channels, or five 9.67-kb/s channels, or one 56-kb/s channel, or one 64-kb/s clear channel. However, most DS0's are used for one, 56k or 64k channel.
NOTE: the DS0 is a global unit !! Both the T1 (America) and the E1 (Europe) use the DS0 as their basic unit of data. One T1 is 24 DS0's, and one E1 is
DS-1 (Digital Signal 1) signals, at 1.544 Mbps, are frequently used to connect equipment within a facility. It is a multiplexed (muxed) combination of 24 DS0's.
DS-3 (Digital Signal 3 - see diagram below) signals, at 44.736 Mbps, are frequently used to connect equipment within a facility. It is a multiplexed (muxed) combination of 28 DS1's, or 672 DS0's. DS-3 signals are almost exclusively used within buildings, for interconnections and as an intermediate step before being muxed onto a SONET circuit. This is because a T-3 circuit can only go about 600 feet between repeaters. When a customer orders a DS-3, they usually get a (much faster) SONET circuit run into the building and a multiplexer mounted in a big cabinet. The DS-3 is delivered in its familiar form, two coax cables with BNC connectors on the ends.
DSX (Digital Signal crossconnect) - refers to a digital signal crossconnect, and it's essentially a patch panel allowing easy interconnection.
DSX-1 (Digital Signal crossconnect 1) - remember that DSX is a connection standard, not a signal !! However, when it is used with the "-1" descriptor, it describes a signal. Therefore a DSX-1 is the low-level signal (typically 6 volts peak-to-peak differential) that occurs at the DSX.
T1 (Terrestrial 1) - when a DS-1 leaves the building, it becomes a T-1 and is referred to as a span. The signal is boosted to a higher level and superimposed on a DC voltage, enabling repeaters in the field to be powered from the span itself. Repeaters are placed every few thousand feet, to clean up and strengthen the signal.
T3 (Terrestrial 3) - a T3 is a circuit which uses the DS3 signal standard, at 44.736 Mbps. Since it cannot travel far between repeaters, virtually all T3's are quickly muxed onto higher-speed circuits, and then placed on SONET rings or fiber. The DS3 is derived by concatenating 28 DS-1 signals together. What few people realize, is that this involves two invisible, intermediate steps:
Step 1) 28 DS1's are combined into 7 DS2's
Step 2) 7 DS2's are combined into 1 DS3.
