Asynchronous Transfer Mode
Asynchronous Transfer Mode |
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LAN emulation (LANE) is a group of software components that allows ATM to work with legacy networks and applications. With LAN emulation, you can run your traditional LAN-aware applications and protocols on an ATM network without modification.
LAN emulation makes the ATM protocol layers appear to be an Ethernet or Token Ring LAN to overlying protocols and applications. LAN emulation provides an intermediate step between fully exploiting ATM and not using ATM at all. LANE can increase the speed of data transmission for current applications and protocols when ATM is used over high speed media; unfortunately, LANE does not take advantage of native ATM features such as QoS. However, LANE does allow your current system and software to run on ATM, and it facilitates communication with nodes attached to legacy networks.
LANE consists of two primary components: the LAN emulation client and the LANE services. The LANE client allows LAN protocols and LAN-aware applications to function as if they were communicating with a traditional LAN. It exposes LAN functionality at its top edge (to users) and native ATM functionality at its bottom (to the ATM protocol layers).
The LANE services are a group of native ATM applications that hide the connection-oriented nature of ATM from connectionless legacy protocols. These services maintain the databases necessary to map LAN addresses to ATM addresses, thus allowing the LANE clients to create connections and send data.
The LANE services components can reside anywhere on an ATM network, but most ATM switches are included with LANE services components installed. Therefore, for practical purposes, LANE services reside on an ATM switch or group of switches.
The three primary LANE services are the LAN emulation configuration server (LECS), the LAN emulation server (LES), and the Broadcast and Unknown server (BUS). The LECS distributes configuration information to clients, allowing them to register on the network. The LES manages one or more Emulated LANs (ELANs); it is responsible for adding members to the ELAN, maintaining a list of all the ELAN's members, and handling address resolution requests for the LANE clients. The BUS handles broadcast and multicast services, as shown in Figure 14.11.
Figure 14.11 LANE Client, LECS, LES, and BUS
The following section describes how a LANE client joins and navigates an ATM network running LANE services.
When the LANE client seeks to join the network, the first thing it must do is find the LECS because the LECS gives the client the address of the LES managing the ELAN that it seeks to join. Without the LES address, the client cannot communicate with other members of the ELAN. Unfortunately, at initialization the client has not established a connection to any ATM switch, let alone to the switch or other entity containing the LECS. The client must establish an ATM connection, preferably a connection directly to the configuration server.
If the ATM network only has a single ATM switch, and the switch contains all the LANE services, then finding the LECS is easy. However, if the network has multiple switches, the local switch to which the LANE client has immediate access might not have LANE services running on it. Fortunately, LANE includes several established mechanisms for a LANE client to discovery the LECS.
The LANE client can use any of the following techniques when attempting to connect to the LECS:
Both the well-known ATM address and the well-known VC are standardized. Most switches and clients are preconfigured with this information. In most cases, the LANE client can find the LECS using one of these methods. However, if the well-known values have been changed at the endstation or at the switch, either type of discovery becomes unsuccessful.
If this happens, the LANE client can fall back on ILMI, a protocol standard (similar to Simple Network Management Protocol) designed for ATM administrative and configuration purposes. ILMI provides a query function that the LANE client can use to find the LECS address, and then set up a VC to it.
After the client has discovered the LECS and connected to it, the client asks the LECS to provide configuration information to allow it to connect to a particular ELAN. It does this by sending one or more pieces of information about the desired ELAN, such as the LAN type (Ethernet or Token Ring), the maximum packet size, and the name of the LAN.
The LECS takes the information from the LANE client and looks in its table of ELANs, trying to find a match. When it locates the correct ELAN, it returns that address to the LANE client.
With the information provided by the LECS, the LANE client can now join the ELAN. To do this, it sends an emulated LAN address and its true ATM address to the LES. The LES registers this information. From this point on, the LANE client can send and receive data over the ATM network as if it were using a normal LAN.
When the LANE client receives a request from a protocol (such as TCP/IP, IPX, or NetBEUI) to send information to another point in the ELAN, it sends the destination LAN address to the LES. The LES looks for a match in its database, and then returns the true ATM address to the LANE client. The client then sets up a normal VC between itself and the destination, and subsequent data traffic is sent directly on this VC without any further intervention by the LES or the other LANE services. While this address resolution request is being processed, interim traffic is sent to the BUS and copied from there to all stations in the ELAN.
If the LES does not find a match for the destination address, the data is sent to the Broadcast and Unknown server (BUS). The BUS attempts to deliver the data to the unknown client, as described in the following section.
The BUS does two different things: it handles distribution of data to unknown clients and it emulates LAN broadcast services. If the LES cannot find a particular ELAN client, the data is sent to the BUS for distribution, and the BUS forwards it to all the clients of the ELAN.
The BUS also handles broadcasts; it registers its address with the LES identical to any other client. It registers under the address of F (x16), which is the normal LAN address for a broadcast message. When a LANE client protocol wants to broadcast a message to the entire LAN, it addresses the message to F (x16) and passes it on. The LEC sends this address to the LES for resolution, and the LES returns the ATM address of the BUS. The LEC can then send the message to the BUS. The BUS maintains a list of all clients on the ATM network and sends the message to all clients. The BUS service is usually co-located (in the same piece of equipment) with the LES.
The Integrated Local Management Interface (ILMI) resides on an ATM switch and provides diagnostic, monitoring, and configuration services to the User-Network Interface. The ILMI is defined by the ATM Forum, and uses the Simple Network Management Protocol (SNMP) and a management information base (MIB). It runs over AAL3/4 or AAL 5 with a default VPI/VCI of 0/16.
The ILMI MIB contains data describing the physical layer, the local VPCs and VCCs, network prefixes, administrative and configuration addresses, ATM layer statistics, and the ATM layer itself. The most common client-oriented function of the ILMI is to assist a client during LECS discovery. For more detailed information about the specifics of the ILMI MIB, see RFC 1695.
ATM often serves as an efficient, high-speed backbone for an emulated LAN. Here are a few tips to set up an ELAN's clients and keep them operating.
Complete information about configuring an ATM adapter card and LANE client can be found in Windows 2000 Server Help. The information provided there describes the step-by-step process required to add a new client to an existing ELAN.
Before upgrading from Microsoft® Windows NT® version 4.0 to Windows 2000, it is important to note the following configuration information for each of the LAN emulation clients you plan to upgrade:
In Control Panel, double-click Network. Next, in the Network and Dial-Up Network Connections dialog box, click the Identification tab. Record the ELAN name from the Domain Name field.
To find these addresses, open Control Panel and double-click Network. Next, in the Network and Dial-Up Network Connections dialog box, click the Adapters tab. Open the Hardware Properties dialog box, the same place that lists the Ethernet network adapter.
To view this listed information, in Control Panel, double-click Network, and then click the Adapters tab. Open the Windows NT 4.0 Properties dialog box.
After you note these configuration parameters, use the LECS interface on your ATM switch to configure ELANs and their associated parameters. This includes the ELAN name, media type, LES and BUS addresses, and maximum packet size.
Next, install Windows 2000 and configure the ELAN name for each LEC. For information about configuring the ELAN name, see "Configure a LAN Emulation Client" in Windows 2000 Server Help. Windows 2000 LANE clients are preconfigured with the "default" or "unspecified" ELAN, and this (ELAN) is usually enabled and configured in ATM switches sold today. This eases the configuration for small networks.
If the LECS or LES fails, the Windows LANE client completely restarts its initialization at the point of LECS discovery. Therefore, if the LANE servers fails and then restarts, the LANE client automatically reregisters itself properly without any interaction from users. Ultimately the fault tolerance responsibility lies primarily with the LECS and LES. The LANE client only detects a fault and restarts.
Some switches allow a backup LECS or LES to be ready and waiting to come online if the current server goes down. If this happens, the backup LECS can register at the same well-known address as the failed LECS, and all clients can find it.
