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.
You give each client an ATM address and a media access control (MAC) address. An ATM address differs from the native NIC addresses in Ethernet and Token-Ring networks. ATM uses a 20-byte Open Systems Interconnect (OSI) Network Service Access Point (NSAP) address, whereas traditional LANs use the 48-bit MAC address associated with each NIC.
LANE services lets a user on an ATM-enabled network transparently connect to a user on a non-ATM Ethernet or Token-Ring network. You simply map an ATM NSAP address into a MAC address (and vice versa). The two networks are connected with a bridge or switching device. Note that the connection is transparent only to end users; the administrator must configure the connections through the LANE software.
Figure 1 illustrates how LANE's address mapping works. Suppose User A, who is on the ATM network, wants to send a message to User B, who is on an Ethernet network, to find out who won the World Series.
User A's system sends out an address resolution message (ARM) to the system's LANE server. The LANE server sends the message to a bridge or router on the destination LAN (in this example, an Ethernet LAN). This router works like a proxy LANE client, storing the addresses of all Ethernet stations on the destination LAN. The router's action depends on whether it has the MAC address of User B's workstation in its database. If the router has User B's MAC address, the router sends the message directly to User B. User B responds with the good news that the New York Yankees won. The message travels back to the bridge, which uses its own ATM address to send the response back to User A.
If, however, the bridge does not know the MAC address of User B's workstation, the bridge relays the message to a LANE service known as Broadcast/Unknown Server (BUS). The BUS broadcasts the message to all Ethernet stations on the destination LAN, where User B eventually receives the message. User B's response is carried back to the router, which stores the MAC address in its database for future use. Again, the router uses its ATM address to send the response back to User A.
LANE services consists of three parts: the LANE Server, the LANE Configuration Server, and the BUS. The LANE Server resolves the MAC addresses of each LANE client on a non-ATM destination LAN. This address resolution lets LANE clients communicate with the originating LAN via ATM. The LANE Configuration Server provides data about virtual LANs (VLANs) on the ATM network (for an introduction to VLANs, see "Virtual LANs," page 132). Finally, if the LAN bridge doesn't have the destination address, the BUS forwards multicast messages that originate from the ATM LAN to all systems on the emulated LAN. This process is essential for resolving the differences between connection-oriented ATM and connectionless LANs. Because an ATM network can't accommodate multicast traffic, the BUS converts a point-to-point ATM address request and broadcasts it to all systems on the non-ATM destination network.
ForeThought's LANE Software
Most ATM management software is UNIX-based. Some vendors (Net2Net, FORE Systems, and Olicom), however, are releasing versions of their ATM software for Windows platforms.
Establishing an ATM network will soon become easier for Windows NT-based networks. In October, Microsoft and networking vendor FORE Systems announced a joint venture to establish an easy-to-use, interoperable ATM solution for NT and other Microsoft operating systems. As part of this venture, Microsoft has licensed FORE's ForeThought ATM LANE client software and will include ForeThought in the next major release of NT. (Microsoft also has incorporated the ATM Forum's LANE specification in its Windows compatible logo program for testing third-party products designed for NT Server 4.0.) At present, LANE client software implementations are third-party software products that aren't integrated with Windows (ATM adapter card vendors provide the Windows integration). Thus, integrating FORE's LANE client into NT should simplify adding ATM to an NT-based environment and will probably help ATM vendors offer more affordable products and promote interoperability.
Virtual LANs
A virtual LAN (VLAN) is a set of logical network domains that ATM LAN emulation (LANE) software creates. You can use the software application to change these domains as needed.
Any member of the enterprise can become a member of the VLAN, regardless of whether that member's station is on the ATM source network or the non-ATM destination network. Furthermore, one workstation can belong to multiple VLANs. In contrast to physically connected network subgroups, VLANs are subgroups defined by logical relationships. Logical relationships simplify forming or dissolving LAN workgroups on the fly, regardless of their physical location. And you can dynamically configure a LAN (i.e., physically change it, such as by relocating a workstation) without changing VLAN configurations.
An ATM network can support multiple VLANs, and broadcast traffic on a VLAN is isolated from traffic on other VLANs in the same network. VLANs offer a security benefit, too, because an administrator can control access to each VLAN from a central location.
