The Synchronous Optical Network (SONET) is a subset
The Synchronous Optical Network (SONET) is a subset of the Synchronous Digital Hierarchy (SDH), an ITU-T standard. The SDH standard can accommodate both ITU and ANSI PDH signals. Frame relay So far I have been describing voice circuits. When a voice circuit is set up you reserve a bandwidth slot for the duration of the call. If none is available you get a busy tone. The requirements for data are different. The reserved bandwidth is not as important as ensuring the delivery. Data can use the spare capacity as voice traffic changes up and down; data packets can be dispatched as capacity is available. Frame relay is a standard for packet-switched networks that operate at layer 2 the data link layer of the OSI model. A bidirectional virtual circuit is set up over the network between the two communicating devices. Variable-length data packets are then routed over this virtual circuit. A number of virtual circuits can IP networks and telecommunications 27 Table 2.5 Plesiochronous Digital Hierarchies ITU-T standard ANSI standard Signal Data rate Channels Signal Data rate Channels DS0 64 kbit/s E1 2.048 Mbit/s DS1 1.544 Mbit/s 24 DS0 E2 8.45 Mbit/s 4 E1 DS2 6.3 Mbit/s 96 DS0 E3 34 Mbit/s 16 E1 DS3 45 Mbit/s 28 DS1 E4 144 Mbit/s 64 E1 1 24 28 2 24 x DS0 (64 kb/s) 27 x DS1 1 x DS3 (45 Mb/s) DS1 (1.5 Mb/s) MUX MUX Figure 2.5 Voice circuit multiplexing.
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