Source: Communications Network Test and Measurement Handbook in Software

Creating Code 128C in Software Source: Communications Network Test and Measurement Handbook

Introduction Asynchronous Transfer Mode (ATM) is a network technology designed to meet the needs of the world s communications services, both today and into the future It has been designed to allow integrating a wide range of services having diverse traffic characteristics and service quality requirements into a single network infrastructure It is a highly scalable technology and can be used over physical interfaces with rates from as low as 15 Mbps to 24 Gbps and beyond ATM can be deployed in both local area network (LAN) and wide area network (WAN) environments, and therefore is suitable for both private enterprise and public carrier networks With standards defining the internetworking of ATM with other technologies, such as Ethernet and frame relay, ATM can be deployed as part of a gradual migration to high-bandwidth integrated networks

ATM is a connection-oriented technology using fixed-length packets It effectively sits at layer 2 of the OSI protocol stack as a Data Link technology, although it also can be used to carry other layer 2 technologies (such as LANs), and additionally could be used as a layer 3 network technology (Figure 111) In its most popular current use, ATM is playing a significant part in the infrastructure growth of the Internet, where it is firmly placed as a layer 2 technology for IP Each ATM packet, or cell, is 53 bytes long, with a 5-byte header and 48-byte trailer In an ATM network, cells are used to carry service data on a virtual connection (VC) over a predefined route through a network of switches Cells from multiple services are multiplexed together with management and empty cells into a contiguous cell stream, ie, the first bit of one cell follows immediately after the last bit of the

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ATM Testing 238 Wide Area Networks LAN Emulation Layer 7 Application Layers 5 and 6 Session and Presentation Layer 4 Transport Layer 3 Network Layer 2 Data Link Application API (Application Programming Interface) TCP, UDP etc IP, IPX etc Ethernet or token ring MAC layer LAN Emulation Classical IP Classical IP over ATM Application API ATM-aware API TCP, UDP etc IP Native ATM applications Application

preceding cell Service data is adapted, using the ATM Adaptation Layer (AAL) to fit into as many cells as required The short, fixed-length cell structure allows fast switching in hardware, a key performance benefit over the software processing of router-based networks The cell also is short enough to allow delay control on real-time services (such as voice), while at the same time long enough to transfer data services efficiently (such as long, variable-length packets from a LAN) An ATM service uses cells only when it has data to send, a process referred to as bandwidth on demand This is the asynchronous part of ATM and contrasts with synchronous Time Division Multiplexing (TDM) networks, where a fixed bandwidth is reserved for the duration of a connection Bandwidth on demand results in variable bit rate (VBR) traffic profiles in addition to services (particularly many real-time services) that remain constant bit rate (CBR) VBR profiles allow a resource allocation effect known as statistical multiplexing Statistical multiplexing works by assuming that, over time, the high-bandwidth periods on some services will correspond with the low-bandwidth periods of other services Rather than reserving bandwidth at the maximum rate for each service, the network operator can reserve a lower average rate, allowing more services to be accommodated This results in the sum of the maximum bandwidths of all services on a link actually being greater than the bandwidth of the link itself This process has to be managed carefully if congestion is to be avoided

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