What is LANE ? LAN Emulation	Presented by:	Copyright 2000©

Asynchronous Transfer Mode (ATM) is standard for connection oriented networks which use cell relay communication, meaning that information for multiple service types, such as voice, video, or data, is packed and transmitted in small, fixed-size cells.

Figure 1: Example of a private ATM network and a public ATM network carrying voice, video, and data traffic.

Circuit switching has the benefits of supporting high speed data as well as low and constant transmission delays.

On the other hand, packet switching is efficient for bursty traffic since the bandwidth is used only when there’s something to transmit.

ATM is a technology of cell-switching and multiplexing that has both benefits of circuit switching and packet switching. It also provides range of bandwidth from a few megabits per second (Mbps) to many gigabits per second (Gbps).

ATM Cell Basic Format

The information that transfers through ATM is packed in fixed-size units called cells. Each cell contains 53 bytes. The first 5 bytes contain cell-header information, and the remaining 48 contain the user information.

Voice and video types of traffic can’t stand transmission delays, therefore ATM’s small and fixed-length cells serve well such type of traffic.

Figure 2: The basic format of an ATM cell.

Move the mouse over the picture

An ATM network consists of an ATM switch and ATM end systems.

The ATM switch handles transmission of cells through the ATM network. Its functions are: accepting the incoming cell from an ATM end station or another ATM switch; reading and updating the cell-header information and switching the cell toward its destination.

The ATM end system contains an ATM network interface adapter. Examples of such

end systems are workstations, routers, and LAN switches.

Figure 3: An ATM network.

An ATM network is maid of ATM switches connected by ATM interfaces.

There are two main types of interfaces: UNI and NNI.

The UNI connects ATM end systems to an ATM switch.

The NNI connects two ATM switches.

UNI and NNI are also subdivided into public and private UNIs and NNIs.

A private UNI connects an ATM end system and a private ATM switch.

A public UNI connects an ATM end system or private switch to a public switch.

A private NNI connects two private ATM switches.

A public NNI connects two public ATM switches.

Figure 4: ATM interfaces.

LANE is a standard defined by the ATM Forum. It’s a service that emulates the operation of traditional LANs such as Ethernet or Token-Ring over an ATM network, and this way enables the migration from existing LANs to an ATM environment that provides benefits such as significant bandwidth increases for the backbone and server connections , starting at 155 Mbps or even 622 Mbps today.

LANE provides the backbone infrastructure that allows existing Ethernet and Token Ring- attached stations to keep their current application interfaces while allowing high bandwidth servers and routers to be connected directly via ATM .

The term which describes an emulated LAN over ATM is ELAN.

Figure 5 : A physical LAN Vs. an ELAN.

The LANE protocol defines a service interface for network layer protocols that is identical to the one in the traditional LANs, so that the data sent across the ATM network is packed in the appropriate LAN MAC packet format

LANE does not attempt to emulate the actual MAC protocol of the LAN and it doesn’t require any modifications to higher-layer protocols to enable their operation over an ATM network.

In order to develop an emulated LAN which satisfies the above objectives, it is necessary to settle the differences between the following environments:

1. LANs are connectionless as opposed to the ATM approach which is connection-oriented, meaning it requires the establishment of a connection between end systems before data is transferred.

2. Broadcast and multicast operations are an integral part of a LAN’s function, since every frame sent on the medium is received by every station.
However, in ATM’s connection-oriented approach, broadcasting to all addresses is harder to achieve and a different approach is required.

3. LAN MAC addresses are based on manufacturing serial numbers which are burned into the LAN adapter card at the time of manufacture. That makes the addresses independent of the network geographic location.
The ATM address of an end system is determined by the ATM switch to which the end systems is connected. If an end system moves from one ATM switch to another, then its ATM address will change.
Any LAN emulation system will need to use the real LAN addresses for some functions and therefore, there’s a need for a database that allows mapping from LAN addresses to ATM addresses.

Technical Requirements

The technical requirements needed from the LAN emulation service are as follows:

Connectionless Services

As mentioned before, LAN stations can send data without previously establishing connections, therefore LANE has to provide the appearance of such a connectionless service to the participating end systems.

Multicast Services

In LANs, end systems share the same media which makes broadcasting an integral part of the service, therefore, the LANE service must support the use of multicast MAC addresses (meaning broadcast, group, or functional MAC addresses).

MAC Device Driver Interface

The main goal of the LANE service is to enable existing applications to access an ATM network through higher layer protocols (such as TCP/IP, NetBIOS, etc.) as if they were running over traditional LANs. Since these protocol stacks are configured to communicate with a MAC driver, the LANE service has to support the same MAC driver service.

There are several standard interfaces between MAC device drivers and higher layer protocol stacks, such as NDIS (Network Driver Interface Specification), and ODI (Open Data-Link Interface). LAN emulation must provide these interfaces and services to the upper layers.

Connectivity

The LAN emulation service has to enable connectivity between ATM-attached stations and LAN-attached stations. This includes connectivity both from ATM stations to LAN stations as well as LAN stations to LAN stations across ATM, as pictured in figure 6.

Figure 6: LAN Emulation connectivity requirements

The LANE service uses some of the ATM network services such as the following:

ATM Adaptation Layer

Since the LANE service is a frame-oriented data transfer service, it uses the services of the ATM adaptation layer (AAL) to fragment and reassemble its protocol data units.

The preferred type of AAL is AAL Type 5.

Point-To-Point Virtual Channel Connections

The ATM network must provide point-to-point virtual channel connections (VCCs) as permanent virtual connections (PVCs) or as switched virtual connections (SVCs). For SVCs the ATM network must provide the means for establishing, maintaining and releasing the Point-to-Point VCCs.

Multicast Virtual Channel Connections

As mentioned before, the LANE service must support broadcast/multicast traffic.

If the ATM network provides multicast services (for example, point-to-multipoint VCCs) they will be used by LANE for multicasting.

If no such service is provided by the network, the LANE service would provide an equivalent service, for example by sending the same message to all members of the LAN, one by one.

ATM Registration Procedure

In order to use Direct VCCs to connect end system in the LANE service, ATM-attached end systems must know their ATM address. Therefore, the ATM network must provide a registration procedure in which an end system can discover its own ATM address. An example of such a registration procedure using ILMI is defined by the ATM Forum.

(ILMI means “Interim Local Management Interface”, and that’s a set of SNMP-based procedures used to manage the user-network interface (UNI) between an ATM end system and an ATM switch).

The main function of the LANE protocol is to translate MAC addresses into ATM addresses so that LANE end systems can set up direct connections with each other and then transmit data.

There are two types of ATM-attached equipment that implement the LANE protocol: ATM network interface cards (NICs) and LAN switching equipment.

1. ATM NICs implement the LANE protocol and interface to the ATM network but present the current LAN service interface to the higher-level protocols in the attached end system. The network-layer protocols on the end system keep working as if they were on a LAN by using known procedures, but now they can use the bigger bandwidth of ATM networks.

2. ATM-attached LAN switches and routers, create an ELAN service. The ports on the LAN switches are assigned to particular ELANs independently of physical location.

The LANE service consists of the following five functions:

Initialization

In the initialization function, the end system obtains access to the Default VCC, connecting the LANE layer in an end station or in an ATM-LAN bridge - to the LANE server, in order to exchange control and user information.

Address Registration

The address registration function provides the MAC addresses to the LANE layer in ATM end systems for local filtering of incoming LAN frames.

Address Management and Resolution

The address management and resolution function provides a method which allows the ATM end system to learn a destination ATM address in order to establish a Direct VCC for the exchange of LAN frames. This method includes: learning the ATM address of a target station, mapping the MAC address to an ATM address, storing the mapping in a table which for the end system is known as Destination Address Association Table or DAAT, and managing that table.

Similarly, this function gives the LANE server a way to support the use of a Direct VCC by an ATM end system. This includes: mapping the MAC address to an ATM address, storing the mapping in a table which for the server is known as Server Address Association Table or SAAT, managing the table, and providing the mapping to ATM end systems.

LAN Frame Transmit

The LAN frame transmit function in ATM end systems involves LAN frame forwarding and VCC control.
LAN frame forwarding includes identification of the VCC type (Default or Direct) through which a LAN frame will be sent using the address management table and encapsulation of the frame.
VCC control includes the establishment, release, and maintenance of Direct VCCs.

Similarly, the LAN frame transmit function of the LANE server determines the outgoing VCC over which a frame is to be forwarded by querying the address management table.

LAN Frame Receive

For a specific ATM station, the LAN frame receive function decides if an incoming data frame should be received by this end station, according to the LAN frame filter controls specified by the higher layers. If the data frame is indeed received, this function performs the de-capsulation of the frame before delivering it to the user.

For the LANE server, the LAN frame receive function delivers every frame it receives to the LAN frame transmit function. The LANE server does not change the contents of received LAN frames.

Although multiple ELANs can simultaneously exist on a single ATM network, the LANE protocol defines the operation of a single ELAN which contains the following components:

The LEC implements the LANE layer in an end system and performs: data forwarding, address resolution, establishment of the various VCCs, registration of MAC addresses with the LAN emulation server (LES) and provision of a standard LAN interface to higher-level protocols on LANs.

An ATM end system that connects to multiple ELANs has one LEC per ELAN.

The above LANE layer functions can be implemented completely in software, in hardware on a specialized LANE ATM adapter, or in a combination of both.

The LES is a central control point to which LECs forward registration and control information.

Its main control function is resolving destination MAC addresses into ATM addresses.

An emulated token-ring LAN cannot have members that are emulating an Ethernet LAN (and vice versa). Thus, there must be an instance of a LES for every type of LAN emulation.

The LES may be physically internal to the ATM network or may be provided as an external device, but logically it is always an external function which simply uses the services provided by ATM to do its job.

Each ELAN must have a LES and only one LES exists per ELAN.

Broadcast and Unknown Server - BUS

The BUS is a multicast server , its main function is to flood traffic with unknown destination address and to forward multicast and broadcast traffic to clients of a specific ELAN.

The BUS works in a store-and-forward mode, which means that all the frame’s cells must be received by the BUS before the frame can be forwarded to its destination(s). This means that cells of different frames mustn’t be intermixed.

The BUS is actually the component which simulates the shared-media LAN over an ATM network.

Each LEC is associated with only one BUS per ELAN.

LAN Emulation Configuration Server - LECS

The LECS provides configuration information to clients.

It manages a database of LECs and their corresponding ELANs. The LECS gets requests from LECs and responds with the correspondent ELAN identifier, meaning, the ATM address of the LES that serves that ELAN.

One LECS per administrative domain is enough to serve all the ELANs within that domain.

The LANE entities communicate with each other by using several ATM VCCs.

LECs have separate connections for data transmission and control traffic.

LANE data connections

The LANE data connections kinds are:

 

1. Data-direct VCC is a bi-directional point-to-point VCC set up between two LECs that want to exchange data. Two LECs usually use the same data-direct VCC to carry all packets between them, this way connection resources are conserved and the time dedicated to connection setup is saved.

    

2. Multicast send VCC is a bi-directional point-to-point VCC set up by the LEC to the BUS.

    

3. Multicast forward VCC is a uni-directional VCC set up by the BUS to the LEC. It’s usually a point-to-multi-point connection, with each LEC as a leaf.

    

Figure 11 : LANE data connections use a series of VCLs to link a LAN switch and ATM hosts.

The LANE control connections are:

1. Configuration-direct VCC is a bi-directional point-to-point VCC set up from the LEC to the LECS.

    

    

2. Control-direct VCC is a bi-directional VCC set up from the LEC to the LES.

    

    

3. Control-distribute VCC is a uni-directional VCC set up from the LES back to the LEC (usually a point-to-multipoint connection).

    

Figure 12 : LANE control connections link the LES, LECS, LAN switch, and ATM host.

In order to explain and understand the operation of a LANE system and components , lets examine the following stages of the LEC operation:

Initialization and Configuration

The initialization process begins when the LEC obtains its own ATM address, which usually happens during the address registration.

Then, the LEC determines the location of the LECS and that’s accomplished by one of the following methods: using a defined ILMI procedure to determine the LECS address; using a well-known LECS address; or using a well-known permanent connection to the LECS.

As soon as the LECS is located, the LEC sets up a configuration-direct VCC to the LECS and sends a

LE_CONFIGURE_REQUEST. If the LECS find a matching entry - it returns a LE_CONFIGURE_RESPONSE to the LEC with the configuration information that’s needed to connect to its target ELAN. That information includes: ATM address of the LES, type of LAN being emulated, maximum packet size on the ELAN, and ELAN name.

Joining and Registering with the LES

A LEC joins the LES and registers its own ATM and MAC addresses by the following three steps:.

1. Once the LEC got the LES’ address, it sets up the control-direct VCC to the LES, and sends through that VCC an LE_JOIN_REQUEST. That’s the way the LEC registers its own MAC and ATM addresses with the LES. This information is managed so that two (or more) different LECs can’t register the same MAC or ATM address.

2. When the LES receives the LE_JOIN_REQUEST, it checks with the LECS through its open connection, verifies the request, and confirms the client's membership.

3. If the verification is successful, the LES adds the LEC to its point-to-multipoint control-distribute VCC, as a leaf, and sends the LEC a successful LE_JOIN_RESPONSE containing a unique LAN Emulation Client ID (LECID) which is used by the LEC to filter its own broadcasts from the BUS.

Finding and Joining the BUS

Once the LEC has successfully joined the LECS, it must find the BUS’ ATM address in order to join the broadcast group and become a member of the emulated LAN.

First, the LEC sends an LE_ARP_REQUEST packet with the MAC address 0xFFFFFFFF on the control-direct VCC to the LES.

The LES responds with the BUS' ATM address on the control- distribute VCC.

When the LEC obtains the BUS' ATM address, it joins the BUS by creating a special signaling packet.

When the BUS receives the signaling request, it adds the LEC to its point-to-multipoint multicast forward VCC, as a leaf.

Now the LEC is a member of the ELAN and it can start transmitting data.

Data Transfer

This final stage, the ATM address of the destination LEC is resolved, and the data is transferred, sometimes using a flush procedure.

When a LEC has to send a data packet to an unknown-destination MAC address, it has to find out the ATM address of the destination LEC that can lead it to that specific address.

In order to do so, the LEC first sends the data frame to the BUS (using the multicast send VCC).

The BUS then forwards it to all LECs on the ELAN (using the multicast forward VCC).

The LEC then sends a LE_ARP_REQUEST control frame to the LES (using a control-direct VCC).

If the LES knows the answer, it translates the destination’s MAC address to its corresponding ATM address, and sends back the response.

If the LES does not know the answer, it floods the LE_ARP_REQUEST to some or all LECs, and if there are switching devices with LEC software in the ELAN - they translate and forward the ARP on their LAN interfaces.

After an LE_ARP is received, the LEC sets up a data-direct VCC to the destination node.

Now, the sending client and the receiving client have two paths between them for unicast frames: one via the BUS and one via the data-direct VCC between them.

A client is supposed to use only one path at a time for a specific LAN destination, although the choice of paths may change over time. Switching between the two paths can cause frames to be delivered out of order to the receiving client.

The Flush protocol is therefore provided to make sure that data frames are delivered in the right order. In the flush procedure, a control cell is sent down the first transmission path (BUS) following the last packet. The LEC waits until the destination acknowledges the receipt of the flush packet before using the second path (VCC) to send packets.

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