Metropolitan Area Networks
MAN APPLICATION,
METROPOLITAN AREA ETHERNET,
The telecommunications industry categorizes networks as local, metropolitan,and wide area or global. The distinction between WAN and MAN is vague because many of the same protocols and services are used without regard to the span ofthe network.
A MAN is a public network that bridges LAN and WAN, typically covering a span of 5 to 50 km. The area is roughly the size of a city, and in larger metropolitan areas multiple providers offer MAN services.
A MAN has many of the same characteristics as a WAN, and shares some of the same aplication. It could be considered simply as a scaled-down version of the WAN, but many of its requirements are distinctly different.
MAN technologies are borrowed from both the LAN and WAN and work primarily at the datalink layer. MAN services are delivered over fiber optics if possible, but copper and wireless alternatives also exist.
Legacy technologies used for MANs include frame relay, ATM, Fiber Distributed Data Interface (FDDI), Fibre Channel, Distributed Queue Dual Bus (DQDB), and Switched Multimegabit Data Service (SMDS). Newer networks work over gigabit and 10G Ethernet applied directly to fiber and some of the wireless technologies we discussed in Chapter 21. Besides point-to-point circuits riding on various levels of TDM, The ILECs offer frame relay with bandwidths of up to DS-3. The ILECs’ frame relay networks generally span a LATA which, depending on the territory,
may encompass a metropolitan area or an entire state. The following are typical applications for a MAN:
- LAN-to-LAN connectivity—sometimes called transparent LAN service
(TLS)
- Storage area networks (SANs)
- Telemedicine
- Delay-sensitive data such as VoIP and video conferencing
- Low bit-rate synchronous and asynchronous data such as telemetry
and alarm services
- WAN access services
- High bit-rate data such as video-on-demand
- Traditional data and voice leased-line services
None of the legacy protocols is a good match for all of these applications,several of which are bursty. Conventional networks, usually provisioned over SONET/SDH are composed of fixed-bandwidth circuits that are optimized for voice, but do not scale well for data. Fixed-bandwidth circuits are acceptable for many applications, but emerging services have a high ratio of peak-to-average data flow
.
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