ATM Protocols
The protocols used in ATM have been specifically designed to support high−speed networks at speeds
ranging up to gigabits per second (Gbps). Other physical LAN topologies, such as Gigabit Ethernet, provide
high−speed networking and work very well in LANs. ATM, on the other hand, can handle network Gbps
traffic in both LAN and WAN environments and could care less about the type of physical media being used.
ATM works on the theory that it is possible to expect upper−layer protocols to use a connectionless service to
communicate with the lower layers. LANE is used to allow an upper−layer protocol to make connections to
lower−layer ATM connection−oriented services. Thus, LANE provides a switching service that is transparent
to the 802.x networks.
Traditional methods of transporting data use one of two ways to send data: character−based or frame−based.
ATM is a cell−based switching technology that uses both circuit switching and frame switching to move
packets through the network. Let’s take a closer look at ATM’s method of cell−based circuit switching.
ATM Circuit Switching
ATM is an efficient, high−bandwidth switching and multiplexing technology that also utilizes the benefits of
circuit switching. Circuit switching is the process of using straight−through circuits between two points to
ensure minimal transmission latency and guarantee equal bandwidth availability. Let’s take a look at the ATM
technology components used in ATM circuit switching services:
Circuit emulation (CE)—A connection−oriented, constant bit rate ATM transport service. This
service handles the heavy−duty, end−to−end timing requirements for the user’s chosen bandwidth and
QoS requirements for establishing a connection. This is typically a dedicated line for applications
such as video conferencing and multimedia.
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Frame Relay—A widely used industry standard for WAN traffic that works by switching Data Link
layer data. It uses multiple virtual circuits by implementing High−Level Data Link Control (HDLC)
encapsulation between connected devices.
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Switched Multimegabit Data Services (SMDS)—A high−speed, packet−switched, datagram−based
WAN technology typically offered by telephone companies.
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Cell relay services (CRS)—The basis for networking protocols, including ATM, SMDS, and IEEE
802.6. This networking technology uses small, fixed−length cells that can be switched in hardware at
very high speeds.
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Frame Relay, SMDS, and CRS are fastpacket transmission technologies used in today’s network. Most
standard ATM platforms can support all three of these fastpacket technologies. Typically, these transmission
technologies support two types of network connections:
Permanent virtual circuit (PVC)—A logical physical connection between two communicating ATM
peers. This type of connection remains active (static) between two endpoints regardless of whether
data is being transmitted over the connection. PVCs are typically used for interconnectivity between
two fixed locations, such as a data center or company locations. This type of connection allows the
network bandwidth to be predictable and constant.
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Switched virtual circuit (SVC)—A switched connection that is established by means of a defined and
standardized ATM signaling protocol. Such connections are set up dynamically and are activated only
when data must be sent to the other end of the logical link. The connection is made on demand and is
then terminated.
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ATM Cells
ATM transports network data in fixed−sized units commonly called cells. Each cell is 53 bytes in length and
is divided into a 5−byte header and 48 bytes of data. The 53−byte size of the cell, illustrated in Figure 8.1, is a
compromise between the voice, data, and video advocates—one side wanted small cells (32 bytes) and
another wanted larger packets (64 bytes). The final decision was to add the defaults (32 + 64 = 96) and divide
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