What is ATM?
Asynchronous Transfer Mode (ATM) is a technology designed for the high-speed transfer of voice, video, and data through public and private networks using cell relay technology. ATM is an International Telecommunication Union Telecommunication Standardization Sector (ITU-T) standard. Ongoing work on ATM standards is being done primarily by the ATM Forum, which was jointly founded by Cisco Systems, NET/ADAPTIVE, Northern Telecom, and Sprint in 1991.
A cell switching and multiplexing technology, ATM combines the benefits of circuit switching (constant transmission delay, guaranteed capacity) with those of packet switching (flexibility, efficiency for intermittent traffic). To achieve these benefits, ATM uses the following features:
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Fixed-size cells, permitting more efficient switching in hardware than is possible with variable-length packets
Fixed-size cells, permitting more efficient switching in hardware than is possible with variable-length packets •Connection-oriented service, permitting routing of cells through the ATM network over virtual connections, sometimes called virtual circuits, using simple connection identifiers
Connection-oriented service, permitting routing of cells through the ATM network over virtual connections, sometimes called virtual circuits, using simple connection identifiers •Asynchronous multiplexing, permitting efficient use of bandwidth and interleaving of data of varying priority and size
Asynchronous multiplexing, permitting efficient use of bandwidth and interleaving of data of varying priority and size The combination of these features allows ATM to provide different categories of service for different data requirements and to establish a service contract at the time a connection is set up. This means that a virtual connection of a given service category can be guaranteed a certain bandwidth, as well as other traffic parameters, for the life of the connection.
ATM Basics
To understand how ATM can be used, it is important to have a knowledge of how ATM packages and transfers information. The following sections provide brief descriptions of the format of ATM information transfer and the mechanisms on which ATM networking is based.
ATM Cell Basic Format
The basic unit of information used by ATM is a fixed-size cell consisting of 53 octets, or bytes. The first 5 bytes contain header information, such as the connection identifier, while the remaining
48 bytes contain the data, or payload (see Figure 1-1). Because the ATM switch does not have to detect the size of a unit of data, switching can be performed efficiently. The small size of the cell also makes it well suited for the transfer of real-time data, such as voice and video. Such traffic is intolerant of delays resulting from having to wait for large data packets to be loaded and forwarded.
48 bytes contain the data, or payload (see Figure 1-1). Because the ATM switch does not have to detect the size of a unit of data, switching can be performed efficiently. The small size of the cell also makes it well suited for the transfer of real-time data, such as voice and video. Such traffic is intolerant of delays resulting from having to wait for large data packets to be loaded and forwarded.
Figure 1-1 ATM Cell Basic Format
ATM Device Types
An ATM network is made up of one or more ATM switches and ATM endpoints. An ATM endpoint (or end system) contains an ATM network interface adapter. Workstations, routers, data service units (DSUs), LAN switches, and video coder-decoders (CODECs) are examples of ATM end systems that can have an ATM interface. Figure 1-2 illustrates several types of ATM end systems—router, LAN switch, workstation, and DSU/CSU, all with ATM network interfaces—connected to an ATM switch through an ATM network to another ATM switch on the other side.
Note In this document the term ATM switch is used to refer generically to the network device that switches ATM cells; the term ATM switch router is used to refer to the Catalyst 8540 MSR, Catalyst 8510 MSR, andLightStream 1010 ATM switch.
In this document the term ATM switch is used to refer generically to the network device that switches ATM cells; the term ATM switch router is used to refer to the Catalyst 8540 MSR, Catalyst 8510 MSR, andLightStream 1010 ATM switch. Figure 1-2 ATM Network Devices
ATM Network Interface Types
There are two types of interfaces that interconnect ATM devices over point-to-point links: the User-Network Interface (UNI) and the Network-Network Interface (NNI), sometimes called Network-Node Interface. A UNI link connects an ATM end-system (the user side) with an ATM switch (the network side). An NNI link connects two ATM switches; in this case, both sides are network.
UNI and NNI are further subdivided into public and private UNIs and NNIs, depending upon the location and ownership of the ATM switch. As shown in Figure 1-3, a private UNI connects an ATM endpoint and private ATM switch; a public UNI connects an ATM endpoint or private switch to a public switch. A private NNI connects two ATM switches within the same private network; a public NNI connects two ATM switches within the same public network. A third type of interface, the Broadband Inter-Carrier Interface (BICI) connects two public switches from different public networks.
Your ATM switch router supports interface types UNI and NNI, including the PNNI routing protocol. For examples of UNI and NNI, see "ATM Network Interfaces."
Figure 1-3 ATM Network Interfaces
Figure 1-3 also illustrates some further examples of ATM end systems that can be connected to ATM switches. A router with an ATM interface processor (AIP) can be connected directly to the ATM switch, while the router without the ATM interface must connect to an ATM data service unit (ADSU) and from there to the ATM switch.
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