Information Technology Reference
In-Depth Information
Foundation Topics
Physical Layer Access
This section reviews definitions of physical technologies and the speeds available for each. You
need to understand the basic characteristics of synchronization, T1/E1 encoding and line speeds,
SONET/SDH, and DTP/SRP.
Synchronous Lines
Most WAN lines run over physical time-division multiplexed (TDM) networks. These circuits
are synchronous, meaning that they are dependent on having the same clock so that the
receiving side knows exactly when each frame bit is received.
Many synchronous line speeds are available, which are based on the basic digital signal level 0
(DS-0) rate of 64 kbps. In North America, the most common is the T1 carrier. It has a capacity
of 1.544 Mbps and can carry 24 DS-0s at 24
×
64 kbps = 1.536 Mbps of bandwidth. Each DS-0
carries data traffic.
In Europe and other countries, the E1 is most common. It has a capacity of 2.048 Mbps and can
carry 30 DS-0s at 30
×
64 kbps = 1.920 Mbps.
For both T1s and E1s, less than a full range of individual channels might be used. Fractional T1
or E1 service uses one or more 64 kbps and the DS-0 channel to provide the desired bandwidth.
A T3 is a dedicated phone connection that supports data rates of approximately 45 Mbps. A T3
line actually consists of 672 individual DS-0 channels. The T3 is also commonly called DS-3
and carries 28 T1 lines.
Encoding Schemes of T1s and E1s
Bipolar 8-zero substitution (B8ZS) is a T1 carrier line code in which bipolar violations are
deliberately inserted if the data contains a string of 8 or more consecutive 0s. B8ZS ensures a
sufficient number of transitions to maintain system synchronization when the user data stream
contains an insufficient number of 1s. The European hierarchy uses B8ZS at the T1 rate.
Alternate mark inversion (AMI) is a T1 carrier line code in which 1s are represented by positive
or negative voltage, alternatively. A 0 is represented by 0 volts.
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