Understanding the network.pdf - Back to Home

DTE status indicators. Most integrated DSU/CSU provide a serial configuration and
testing interface and support SNMP management.
High-Speed Digital Transport: T-Carrier (DS-3/DS-4)
and SONET/SDH (OC1/OC3)
To meet the growing demand for voice and data communications services,
telephone carriers require interoffice trunk lines that can provide transport rates
and DS0 destinations beyond that of the primary rate (DS1) trunk carrier circuit. To
provide these transport services, one of two digital transport solutions is employed:
T-carrier or SONET.
DS3 Service over T3
To provide DS3 service over T-carrier, an asynchronous T3 transport circuit is used.
It is asynchronous transport because timing is derived from the two output circuits
independently from one another. T3 transmission circuits operate over microwave
radio, multimode fiber, satellite, and coaxial cable (for short distances only). The
formation of a T3 utilizes 28 T1 circuits, multiplexed together using a two-step
multiplexing process called M13 multiplexing.
The first step multiplexes the 28 T1s into seven T2 circuits. Each T2 channel is
constructed out of four T1s and consists of the 6.176Mbps from the four T1s, plus
136KBs of framing and stuff bits to accommodate the timing differences between
the four T1s. This is called plesiochronous multiplexing, where two or more
bitstreams operating at the same bit rate are controlled by different timing sources.
The actual T2 frame is formed by interleaving the T1 bits together. It consists of four
subframes, each consisting of six blocks of 49 bits each. Figure 5.5 illustrates this
process.
Figure 5.5. T1-to-T2 multiplexing.
The second step takes the seven T2 circuits and multiplexes them together to form
the T3 circuit. The combination of the seven T2 circuits is also a plesiochronous