Lane Emulation Over ATM

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PRACTICAL-7

AIM: - Lane Emulation Over ATM Network

LANE was defined by the ATM Forum in 1995 as a way to allow legacy networks such asEthernet, Token Ring, and FDDI to use an ATM network as backbone connections. In July

of1997, the ATM Forum ratified an enhanced LANE specification called LANE 2.0, which

adds support for QoS and other necessary features.

rather than frame-based or that it uses virtual connections rather than a connectionless

scheme. Integrating ATM with legacy LANs is not easy. Keep in mind that ATM is a

connection-oriented technology. It requires that virtual circuits exist between source and

destination before any data can be sent. Data is transmitted in fixed-length cells. Legacy

LANs transmit data in variable-length frames over a shared connectionless network.

What LANE does is automate SVC (switched virtual circuit) setup across ATM networks forLAN clients. Before LANE, administrators had to manually configure PVCs (permanent

virtual circuits) between hosts.

Another thing LANE does is map MAC (Medium Access Control) addresses to ATM

addresses. It also defines a scheme for encapsulating higher-level protocol datagrams intoATM cells and delivering them across the ATM backbone. Since LANE emulates layer 2

protocols (data link layer), it can transport higher-layer protocols such as TCP/IP and

SPX/IPX without modification. This allows existing LAN applications to be used without

change. They don't need to know that the underlying network is cell-based

Since LANE operates in layer 2 (the MAC layer), it is limited to creating bridged networks

(and not routed networks) over the ATM switching fabric. You can create multiple ELANs,

but if you want clients in those ELANs to talk to each other, you'll need to implement

external routers. This external router is often called the "one-arm router."

MPOA (Multiprotocol over ATM) is a related technology that provides inter-ELAN routing

directly on the ATM network so that separate routers are not needed. MPOA adds a cut-

through routing service that allows clients on different VLANs to connect with one another

using the routes learned by the MPOA routing service. MPOA is derived from LANE. The

problem with LANE is the requirement that traffic go through an external router when the

underlying ATM network is fully capable of creating a direct connect between two devicesconnected to different VLANs. MPOA adds this capability.

LANE Configurations

A LANE configuration consists of a number of special servers and processes that help end

systems obtain SVCs across the ATM networks. LANE also gives the ATM network the

ability to simulate the broadcast nature of LANs. Because ATM is connection oriented, it

does not have the ability to broadcast to every end system, so LANE adds this functionality.

The three servers are described below. Note that these "servers" are really processes that can

be located on the same physical piece of equipment:


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LES (LAN emulation server) Manages address resolution, similar to ARP in theIP/Ethernet environment.

BUS (broadcast and unknown server)Manages broadcast and multicast traffic.This process is required since ATM does not have broadcast capabilities. Basically,

the BUS maintains a connection with all the LECs in a particular broadcast group and

sends broadcast messages to them as individual messages. LECS (LAN emulation configuration server)Manages host assignments to LANs.

It maintains a database of information about the LANE network, including the ATM

addresses of the LESs that control each ELAN. Network administrators make entries

into the database about which ELAN a LEC should join. When a LEC first starts, it

retrieves this information first, then joins the ELAN using the procedure described

later.

The final piece in the LANE configuration is the LEC (LAN emulation client). The LEC is

responsible for interfacing between the LAN interface (Ethernet, Token Ring, etc.) and the

ATM network. The LEC process usually runs in a switch, but it may also run in a router or

any other device that has an ATM interface.

The LANE configuration is pictured in Figure L-1. The typical scenario is to connect an

Ethernet LAN to an ATM edge device such as a switch, which has Ethernet ports on one side

and ATM uplinks on the other side. Also note that the LES, LECS, and BUS may be

integrated into a single server or may run in individual servers. Also note that data servers are

connected directly to the ATM backbone.

Figure 1: LANE configuration

As mentioned, an ATM network can support multiple ELANs, but you cannot mix different

LAN types (Ethernet, token ring) in the same LANE network. Each ELAN acts like a

broadcast domain and is managed by a LES and BUS process. The LECS keeps track of each

ELAN and LES/BUS combination in its database. Keep in mind that LANE does not provide


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Figure 2: Virtual LANs in the LANE environment

Each ATM/LAN switch runs a LEC for each ELAN. A separate LES is needed for each ELAN/VLAN.

Servers can run a LEC for each ELAN, allowing users from any ELAN to access thesame server.

Routing between ELANs/VLANs is accomplished in this configuration with externalrouters, or with a software router in the server that implements LANE at both of its

interfaces.

As mentioned, routers are required to move packets from one emulated network to another.

As always, moving packets through a router is inefficient, especially when you consider the

fact that the underlying ATM network can provide adirect virtual circuitbetween two end

nodes, even if they are on different ELANs. The LANE specification does not accommodate

this, but the MPOA (Multiprotocol over ATM) topic does.

LAN Emulation (LANE)

LANE is a standard defined by the ATM Forum. Its 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 bandwidthservers and routers to be connected directly via ATM .


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

Figure 3 : 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 doesnt 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:

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).


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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.

LAN Emulation connectivity requirements


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(ILMI meansnterim Local Management Interface, and thats a set of SNMP-based procedures

used to manage the user-network interface (UNI) between an ATM end system and an ATM

switch).

The LANE Protocol Architecture

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.

LAN Emulation Service's Functions

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


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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 frames cells must be

received by the BUS before the frame can be forwarded to its destination(s). This means that

cells of different frames mustnt 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.

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Figure 4: LAN Emulation Components

Figure 5 : An ELAN consists of clients, servers, and various intermediate nodes


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LAN Emulation Connection Types

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 thatwant 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 theBUS.

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

Figure 6 : LANE data connections use a series of VCLs to link a LAN switch and ATM

hosts.


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LANE Configuration and Control connections

Figure 7 : LANE control connections link the LES, LECS, LAN switch, and ATM

host.

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

The LAN Emulation Operation

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 thats 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.


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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 thats needed

to connect to its target ELAN. That information includes: ATM address of the LES, type ofLAN 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. Thats 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 cant 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

signalling packet.

When the BUS receives the signalling 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 istransferred, sometimes using a flush procedure.


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Address resolution:

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 sendVCC).

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 destinations 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.

Data transfer and flushing:

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.

LANE 2.0

LANE 2.0 adds QoS features and provides support for multicasting. Additional

enhancements include the ability to multiplex multiple emulated LANs over a single ATM

virtual circuit. LANE 1.0 sends traffic using the UBR (unspecified bit rate) service, which

does not guarantee any bandwidth. UBR and other ATM services are discussed under the

ATM heading. What LANE 2.0 adds is support for the other service classes:

CBR (Constant bit rate) Provides a fixed amount of bandwidth that is alwaysavailable for streaming data such as voice and video.

VBR (variable bit rate) This is similar to CBR in that a peak cell rate is specified,but network bandwidth is only used when data is sent.


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ABR (Available Bit rate) Provides a specific bandwidth out of what is availableand allows client to use more bandwidth for bursts if it is available.

Of course, these services must be requested, which means that end systems must be ATM

aware in order to take advantage of them. Switches must also be LANE 2.0 aware.

Documents

Lane Emulation Over ATM