ATM WANs

ATM WANs are made up of a set of ATM switches under control by the same network administration. Figure 9-1 illustrates an ATM WAN supporting direct access by routers, PBXs (with Voice-over-ATM [VoATM] modules), file servers, and a video server.

Figure 9-1. ATM WAN

ATM WANs are a connection-oriented service, meaning that a communications path must be provisioned between endpoints prior to the exchange of traffic across the circuit. ATM WANs also can be set up as a connectionless service with the use of Switched Virtual Circuits (SVCs), using X.121 or E.164 addressing for device addressing. ATM SVC service operates in a similar fashion to SMDS (Switched Multimegabit Data Service), a similar dial-on-demand broadband access technology.

ATM works by transmitting all traffic as fixed-length, 53-byte cells, shown in Figure 9-2. These fixed-length cells enable very fast switches to be built, because it is much faster to process a known datagram (packet or cell) size than to figure out the start and end of variable-length datagrams. These small ATM cells ensure that voice and video can be inserted into the stream often enough for real-time transmission.

Figure 9-2. ATM Cell

ATM networks are made up of switches, interfaces, and endpoints (often customer sites). ATM interfaces are the connection points between two ATM switches. These interswitch connections are called Network-to-Network interfaces (NNI). Connections between an ATM switch and an endpoint (customer site) are called User Network Interfaces (UNI). UNIs and NNIs can be privately owned by customers or publicly owned by NSPs. These private and public connections are known as private or public UNIs and NNIs. When two network service providers interconnect ATM switches, a broadband intercarrier interface (B-ICI) is used.