Page 19 - From GMS to LTE

Global System for Mobile Communications (GSM) 5

1.3 Transmission Speeds

The smallest transmission speed unit in a classic circuit‐switched telecommunication
network is the digital signal level 0 (DS0) channel. It has a fixed transmission speed of
64 kbit/s. Such a channel can be used to transfer voice or data, and thus it is usually not
called a speech channel but simply referred to as a user data channel.
The reference unit of a telecommunication network is an E‐1 connection in Europe
and a T‐1 connection in the United States, which use either a twisted pair or coaxial
copper cable. The gross datarate is 2.048 Mbit/s for an E‐1 connection and 1.544 Mbit/s
for a T‐1. An E‐1 is divided into 32 timeslots of 64 kbit/s each, as shown in Figure 1.3
while a T‐1 is divided into 24 timeslots of 64 kbit/s each. One of the timeslots is used for
synchronization, which means that 31 timeslots for an E‐1 or 23 timeslots for a T‐1,
respectively, can be used to transfer data. In practice, only 29 or 30 timeslots are used
for user data transmission while the rest (usually one or two) are used for SS‐7 signaling
data (see Figure 1.3). More about SS‐7 can be found in Section 1.4.
A single E‐1 connection with 31 DS0s is not enough to connect two switching centers
with each other. An alternative is an E‐3 connection over twisted pair or coaxial cables.
An E‐3 connection is defined at a speed of 34.368 Mbit/s, which corresponds to
512 DS0s.
For higher transmission speeds and for long distances, optical systems that use the
synchronous transfer mode (STM) standard are used. Table 1.1 shows some datarates
and the number of 64 kbit/s DS0 channels that are transmitted per pair of fibers.

31 timeslots with 8 bits (1B) each for user data or
SS-7 signaling

Synchronization

Repetition interval: 8000 Hz
Speed: 32 timeslots × 8 Bit × 8000 1/s=2.048 Mbit/s

Figure 1.3 Timeslot architecture of an E‐1 connection.

Table 1.1 STM transmission speeds and number of DS0s.

Approximate number
STM level Speed (Mbit/s) of DS0 connections
STM‐1 155.52 2300
STM‐4 622.08 9500
STM‐16 2488.32 37,000
STM‐64 9953.28 148,279