This introductory text provides practicing engineers, managers, and students with a useful guide to the latest developments and future trends of three major technologies: SONET, SDH, and ATM, and a brief introduction to legacy TDM communications systems.
This concise book features easy-to-follow illustrations, review questions, worked examples, and valuable references. An accompanying CD-ROM provides the key figures in full color, suitable for easy cut-and-paste presentations.
Since the turn of the twentieth century, telecommunications has shifted from traditional voice transport to data transport, although digitized voice is still a large contributor. Instead of an evolution of existing transport standards, a revolution was necessary in order to enable additional data-related transport.
Equally, generic framing procedure GFP , the methodology that efficiently transports asynchronous, or variable bit-rate data signals over a synchronous or constant bit-rate, is explored in detail. It will also be an invaluable resource for postgraduate students on network communications courses. It clearly explains the use of models in the ITU-T and ETSI standards, the transport networks and the transport equipment in the definition, implementation and deployment phase.
For multiple sites, each router would need a single access link to the ATM network, with a VC between sites as needed. The other big difference is that ATM does not forward frames—it forwards cells.
Just like packets and frames refer to a string of bits that are sent over some network, cells are a string of bits sent over a network.
Packets and frames can vary in size, but ATM cells are always a fixed bytes in length. ATM cells contain 48 bytes of payload and a 5-byte header. The users of a network typically connect using Ethernet, and Ethernet devices do not create cells. So, how do you get traffic off an Ethernet onto an ATM network? When a router receives a packet and decides to forward the packet over the ATM network, the router creates the cells. The creation process involves breaking up a data link layer frame into byte-long segments.
Each segment is placed in a cell along with the 5-byte header. Figure shows the general idea, as performed on R2. As you will read more about in Chapter 5, "Fundamentals of IP," routers forward IP packets, but they must add a data-link header and trailer to the packet before sending it. R2 takes the packet, adds a data-link header appropriate for ATM, and then also segments the frame into cells before sending any data.
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