Serve your network a six-pack of Jolt Cola
In the old days, networks ran little more than a word processor, database and spreadsheet programs, and email. Files were small, and standard Ethernet was more than adequate. Today, end users have a dizzying array of bandwidth-hungry applications that shoot far more data over the network and send network managers scurrying to find ways to speed up the throughput.
Asynchronous transfer mode (ATM) is one solution for increasing your network bandwidth. ATM is a high-speed WAN technology for connecting LANs, even different kinds of LANs. ATM offers transmission speeds of up to 155Mbits per second (Mbps), compared with standard Ethernet's 10Mbps maximum. Adding ATM technology to your network is like giving it a six-pack of Jolt Cola.
Besides speeding communication, ATM optimizes performance by handling different types of network traffic in different ways. ATM applies specific parameters for cell loss ratio, cell transfer delay, and cell delay variation to four service classes. You can choose the appropriate class of service for your data from the following:
*Class A: Constant bit rate (CBR). For applications that require a small amount of bandwidth at all times and are sensitive to delay and cell loss. Example: voice traffic connected to a PBX.
*Class B: Variable bit rate--Realtime (VBR-RT). For applications that require high bandwidth and are sensitive to delay and cell loss. Example: compressed voice and video applications,
such as videoconferencing or imaging.
*Class C: Variable bit rate--Non-realtime (VBR-NRT). For applications with a greater tolerance for delay. Example: video playback and video mail.
*Class D: Unspecified bit rate (UBR) and available bit rate (ABR). For services that do not have specific traffic parameters. Example: email and file transfer.
ATM is a versatile solution. With ATM you can send a wide variety of traffic. You can also minimize traffic congestion through ATM's monitoring and control techniques. So if you're in the market for a high-speed network, take a look at how ATM works and how ATM LAN emulation (LANE) software lets ATM and non-ATM networks communicate.
ATM Basics
ATM is available now, and you can deploy it over a site's existing Category 5 unshielded twisted pair cabling. ATM is a connection-oriented technology: It requires a specific path to be established between two network endpoints before you can move data between them. You establish these connections through dynamic ATM routing protocols, such as a private network-to-network interface. PNNI disseminates topology information to each switch on the network; each switch then calculates the best path between endpoints. PNNI handles link failures by defining alternative routes.
In a connectionless LAN such as Ethernet and Token Ring, the amount of bandwidth available to each client decreases as you add nodes to the network. On an ATM network, however, you can dedicate a specific amount of bandwidth to each device on the network. For example, you can allocate 25.6Mbps to a desktop application and the full 155Mbps to a corporate server. Allocating bandwidth to specific circuits is a function of the service class and corresponding Quality of Service (QoS) setting.
ATM is well suited to connecting LANs over a wide area. Traditional Ethernet does not scale well geographically; standard Ethernet doesn't work beyond a cable length of 1640 ft per segment. Even Fast Ethernet (100Mbps) is limited to about 5 miles. You cannot expand Ethernet indefinitely with multiple repeaters; the Carrier Sensing Multiple Access with Collision (CSMA/CD) protocol that prevents collisions breaks down in large networks. The result is significant performance degradation.
ATM and LANE
ATM's LANE feature makes it possible to send data from an ATM LAN to a non-ATM (Ethernet or Token-Ring) LAN. LANE lets a destination PC without an ATM, network interface card (NIC) see an originating system's ATM NIC as an Ethernet or Token-Ring adapter card. Thus, users on the two LANs can communicate seamlessly.
Data travels over an ATM network in cells--individual fixed-length units of 53 bytes (5 units for the header and 48 for data). In contrast, data on Ethernet networks moves in packets--larger, variable-length units. LANE converts ATM cells destined for an Ethernet network to packets and converts Ethernet packets headed for the ATM network to cells.
The ATM Forum, the international standards body that oversees development of ATM technology, established the LANE specification. LANE consists of two parts: LANE services and LANE clients. LANE services can reside on one server or can be distributed among multiple servers. Some vendors (CrossComm, FORE Systems) implement LANE services from within the ATM switch. LANE services map an ATM endpoint's address to the non-ATM endpoint's native network address. A LANE client can be a workstation, server, bridge, or router. LANE clients reside within each end station on the emulated LAN.
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