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During the 1980s, the telephone network infrastructure began to take on significant amounts of nonvoice traffic from corporate WANs and upstart Internet backbones To help ensure service levels for everybody, the ATM protocol was introduced in the early 1990s Since then, ATM links (with speeds up to 622 Mbps) have become the staple for high-speed WAN links, especially for ISP backbones ATM transport is still relatively unknown to end users, remaining in the background because it rarely extends all the way to the desktop Suffice it to say, though, that when you connect to a Web page or an FTP site, your download almost certainly passes through at least one ATM network along the way ATM has many QoS-enabling features built in, most notably the fixed-sized 53-byte cell it uses to transport information This gives the hardware in ATM switches a substantial speed advantage over IP devices, which must process variable-sized packets The variable-size packets force IP routers and switches to spend extra CPU cycles figuring out exactly where fields begin and end The fixed cell size is what allowed ATM hardware to operate at speeds of 622 Mbps when Ethernet was still at 100 Mbps Just as importantly, the fixed cell's inherent predictability lets ATM network managers meter out services with much greater precision than is possible with IP links Like Frame Relay, ATM is a switched-network technology that deploys UNI signaling to manage connections end to end within ATM networks In contrast to Frame Relay, however, most ATM connections travel over switched virtual circuits (SVCs), on which connections are set up and torn down in response to UNI signaling That is, an ATM SVC spans the entire network without the need to wire end-nodes into the network cloud via permanently connected physical phone loops One of the principal attractions of ATM is its ability to deliver telephone network like certainty without the need for PVCs, thereby offering both high performance and flexibility The ATM protocol spans layers 2 and 3 of the OSI model, so it's incompatible with IP at the native level In other words, IP cannot operate over ATM as transparently as it does over Ethernet, Token Ring, and FDDI IP can, however, interoperate with ATM, but only through emulation Even so, the use of ATM for Internet backbone links is growing because of its inherently strong service quality ATM is technically feasible for Internet backbone service because IP-to-ATM adapter technology is field proven, economically viable, and very fast
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Because of its speed and manageability, ATM is considered by ISPs to be a desirable option despite its additional complexity and expense Specifically, ATM switches have the capability to operate at these committed service levels: Constant bit rate (CBR) Service at a fixed data-transfer rate comparable to leased-line transfer, such as T1 or T3 (DS3) CBR is used for applications that require precise clocking to guarantee undistorted delivery Variable bit rate (VBR) Service at a variable bit rate to deliver a specified throughput capacity, but without sending the data evenly as in CBR VBR is a popular choice for VoIP and videoconferencing applications Available bit rate (ABR) Service that guarantees a minimum capacity, but allows traffic to burst to higher levels when the bandwidth is available ABR operation is similar to Frame Relay Unspecified bit rate (UBR) Used to carry traffic that does not rely on time synchronization UBR has no mechanism to dynamically adjust the amount of bandwidth available to a connection
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The important point here is that IP cannot specify any bit rate at all (at this juncture, at least) Because of its inherent QoS, ATM is especially capable for multimedia traffic, which must have CBR or real-time VBR to operate effectively But ATM's QoS is also important for regular traffic, because it enables operators to tightly manage bandwidth in real time Most Tier 1 ISPs use ATM to operate their Internet backbone trunks, because Tier 1's can cut a contract with a Tier 2 ISP or a big enterprise and use ATM QoS to deliver on their commitments, and also have detailed records documenting services rendered ATM switching works by combining multiple virtual circuits (VCs) into a virtual path (VP) Multiple VPs are in turn combined within a physical circuit, usually a fiber-optic cable operated by the Synchronous Optical Network (SONET) protocol at the physical level, as depicted in Figure 15-1
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Figure 15-1: ATM multiplexing allows multiple discrete traffic paths over a single circuit Each ATM cell identifies itself with a VCI and a VPI (VC and VP identifiers) to tell ATM switches how the cell should be handled As a cell hops between switches, its VCI and VPI values are rewritten to direct the cell to its next hop, thereby combining path integrity and adaptability A big part of ATM's ability is a second level of signaling through the Private Network-to-Network Interface (PNNI) protocol, which distributes topology information among switches in an ATM network This is how ATM networks are able to calculate optimal end-to-end paths through ATM networks PNNI maintains topology attribute information on individual ATM links to compute a cumulative score that ranks end-to-end ATM paths PNNI is roughly similar to a link-state routing protocol in IP network, such as the Open Shortest Path First (OSPF) protocol What makes ATM so economically compelling is its proven ability to operate deterministic discrete paths (using SVCs) over a single cable ATM's value is emphasized by the fact that fiber-optic bandwidth is more expensive over WAN links than on local networks
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