LAN Emulation over ATM (LANE)
Asynchronous Transfer Mode (ATM) as a new important networking technology offers many benefits (higher capacity, bandwidth, scalability etc.). Given the vast installed base of Local Area Networks (LAN), the success of ATM lies largely in its ability to allow interoperability between LAN technologies and ATM. LANE is a standard developed by the ATM Forum, that allows legacy networks, legacy protocols and applications to operate unchanged over ATM. This paper shortly describes LANE basic principle and functions.
2.0 LANE architecture
The main function of the LANE protocols is to emulate a LAN on the top of the ATM network. Specifically, mechanisms have been defined for emulating of either an IEEE 802.3 Ethernet or an 802.5 Token Ring. LANE protocol does not define mechanism for emulation of FDDI networks. LANE protocol defines a service interface for higher layers (network layer)protocols. The packets of these networks protocols are sent across the ATM network to be encapsulated in the appropriate LAN MAC packet format. The LANE protocols make an ATM network look and behave to network protocols like an Ethernet or token ring LAN, but operating at higher speed than a real network. The LANE services present the same interface of existing MAC protocols to network layer drivers, hence no changes are required in those drivers. In this way the IP protocol (and others) that is running previously on Ethernet and Token Ring, requires no modifications to operate over the ATM network. This helps to accelerate deployment of the ATM.
The basic function of the LANE protocol is to map MAC addresses into ATM addresses. The goal of LANE is to perform such address mappings so that LANE end systems on LANs can connect to other end systems in LANs, as well as to ATM-attached servers, routers, and switches.
3.0 LAN Emulation Components
The LANE protocol defines operation of a single Emulated LAN (ELAN) only. There are two types of Emulated LAN: Ethernet/IEEE 802.3 or IEEE 802.5 (Token Ring). Each Emulated LAN is composed of a set of LAN Emulation Clients (LECs) and a single LAN Emulation Service (LE Service). This LE Service consists of one or more LE Configuration Servers (LECS), one or more LE Servers (LES), and one or more Broadcast and Multicast Servers (BUS) [LANE2]. Multiple ELANs may coexist simultaneously in a single ATM network. However, each of the ELANs is logically independent from the others and for communication between them, some type of interconnection device (router, bridge) is required.
LAN Emulation Client (LEC):
The LEC is the entity of an end system that performs data forwarding and address resolution, and other control functions for a single end system within a single ELAN. Each of the LECs has a unique ATM address, and is associated with one or more MAC addresses, which are reachable through this ATM address.
LAN Emulation Server (LES):
The LES implement control co-ordination function for the Emulated LAN. The LES provides facility for registering and resolving of unicast and multicast MAC addresses and/or to route descriptors to ATM addresses.
Broadcast and Unknown Server (BUS):
The Broadcast and Unknown Servers handle data sent by clients to the broadcast MAC address, multicast data and initial unicast data which are sent by a LEC before the data direct connection is built between two LECs.
LAN Emulation Configuration Server (LECS):
The LECS is an entity that assign individual LANE clients to particular ELANs by giving them the appropriate LES ATM address, that corresponds to the ELAN.
4.0 LAN Emulation Connections
The LANE entities communicate with each other using series of ATM connections. LECs maintain separate connections for data transmission and for control traffic. LANE v2 ads support for LLC-multiplexed Virtual Circuit Connections (VCCs).
The control connections are:
1. Configurations Direct VCC:
a bi-directional VCC set up by the LEC (or other entity) is used to obtain configuration information from a LECS.
2.Control Direct VCC:
a bi-directional, point-to-point VCC, set up by the LEC to the LES for sending control traffic.
3. Control Distribute VCC:
unidirectional point-to-multipoint VCC that may be optionally established by the LES to distribute control traffic to the LEC.
The data connections are:
1. Data-Direct VCC:
a bi-directional point-to-point VCC, which is set up between LECs.
2.Multicast Send VCCs:
A LEC sets up one or more bi-directional point-to-point VCCs to the BUS, they are used for all unknown, broadcast and multicast data.
3. Multicast Forward VCCs:
unidirectional point-to-point VCCs, which the BUS may additionally set up to the LECs.
5.0 LAN Emulation Operation
The LANE system and components operation may be described through the following stages:
5.1 Initialization and Configuration
Upon initialization (such power up) a LEC has to contacts the LECS. The mechanisms used to find a LECS are the following: 1) to configure the ATM address of the LECS into the LEC. 2) to get it through Interim Local Management Protocol (ILMI). 3) to use well-known address [LANE2] to open a configuration VCC. After finding the location of the LECS, the LEC will establish the Configuration Direct VCC to the LECS. Through configuration protocols it informs the LECS about its ATM addresses, type of the LAN, name of the ELAN and the maximum frame size. The LECS provides to the LEC the information that is required for connection into its target ELAN. This includes the ATM address of the LES, the type of ELAN, the maximum frame size and the ELAN name.
5.2 Joining and Registration
After the LEC obtains the LES ATM address, it sets up the control direct VCC to the LES. The LEC registers its ATM address, the MAC address, the type of LAN, the maximum data frame size, the ELAN name and whether it works as proxy. The LES sets up the control distribute VCC back to the LEC and assigns its own a unique LEC ID (LE Client identifier) to the LECS, the ELAN name, the maximum frame data size, and the LAN type. After this join phase LEC may attempt to register any number unicast and multicast MAC addresses (if the LEC is a proxy) for which it is responsible.
5.3 Address Resolution
Address resolution is a procedure by the means of which the LEC associates LAN MAC destination address with the particular ATM address of another LEC or BUS. For this purpose (already established) Control Direct VCC and Control Distribute VCC between the LEC and the LES are used. If the LEC needs to know the ATM address of the destinations, the LEC formulates LE_ARP request (LAN Emulation Address Resolution Protocol) and sends it to the LES. If the LES recognises this, it reply to the relevant LEC. If the LES does not know mapping, the LE_ARP is distributed to all LECs by the means the Control Distribute VCC.
The LES might not know mapping – it is because of the address of a node is behind the LEC, that is a type of a bridge – in a side of a network, which the bridge connect to the ATM network. The LEC may not have registered all these addresses, for which it works as proxy. The LECs working as proxy nodes have been registered to the LES in joining phase as a proxy, hence redirection LE_ARP requests have to sent only these LECs.
When the LES obtains from the LEC this ATM address, which is proxying for that MAC address, the LES will forward the response to the requesting LEC, or optionally to all LECs, that can learn and cache the particular address mapping.
5.4 Connecting to the BUS
The LE_ARP mechanism explained above is used by the LEC to determine the ATM address of the BUS. It does it by sending a LE_ARP request for the « all ones » MAC broadcast address to the LES, which respond with the BUS’s ATM address. The LEC then sets up the Multicast Send VCC to the BUS and the BUS set up back to the LEC the Multicast Forward VCC.
5.5 Data Transfer
The LEC may receive either network layer packets to transmit from a higher layer protocol (in the case of NIC), or it receives a MAC packet to forward across a LAN port (in the case a LAN switch). In the first case, the LEC search in its internal cache table (if any exist) for MAC address to ATM address mapping, that it learns through a LE_ARP.
In the case, when the LEC has no ATM address of a destination, it formulates a LE_ARP request and sends it to the LES. Waiting for a response, the LEC forwards also the frame to the BUS that will forward the frame to at least all proxy clients. This must be done, because in an Emulated LAN the case can arise, when the LEC does not know where the MAC address is located (when the destinations of frame are reached through a passive device). Similarly, as in a case of a learning bridge, a LEC will learn the location of the device if and only if it responds to the flooded frame. Additionally, resolving a LE_ARP or establishing a Data Direct VCC may take some time and many network protocols are intolerant of either loss (if the LEC choose to discard the frame while waiting to the LE_ARP response), or latency (if the frame is buffered).
If the LEC already knows the ATM address (an LE_ARP respond is received), the LEC then sets up a Data Direct VCC to the destination node. Before it, the LEC must use the LANE
to ensure that all frame previously sent to the BUS were delivered to the destination prior to using of the data direct VCC. For this purpose, a control cell is sent down the first transmission path, following the past packet. The Flush procedure allows the sender to avoid the possibility delivering frames out of order that can cause multiple paths. Since Token Ring and Ethernet LANs do not permit delivery of frames out of order, applications designed to run over LANs do not always have robust mechanisms for dealing with out of order frames.
The BUS is also used for broadcast and multicast frames, which are forwarding to all LECs. It can cause that the source LEC may receive a copy of its own frame. Since some protocols cannot tolerate it, the LANE packets use a prefix with the LEC ID. The LECs then filter this field for all frames received from the BUS to ensure that it never receives its own frames.
A LEC locally caches any address mappings that it learns through the LE_ARP. When the LEC receives packets and has mappings for a particular MAC address, it uses them rather then sending of another LE_ARP. Such entries will be cleared aged out over configurable time period.
LANE is a possible way how to run existing network applications across the ATM network, and how to interconnect the installed base of the Ethernet and Token Ring LANs with the ATM network. But it has also some disadvantage, such as preventing of the user from taking advantage of some of ATM’s feature (for example limited ability to specify Quality of Service). LANE is the first step in transition of networks to ATM connectivity.
[LANE] The ATM Forum – "LAN Emulation Over ATM Specification – Version 1", ATM Forum Specification, January 1995
[LANE2] The ATM Forum -"LAN Emulation Over ATM Version 2 – LUNI Specification", ATM Forum Specification, July 1997
C.Rigault, P.Martins, T.Kováčiková: Signaling Requirements and Network Information Models for Multimedia Session and Call Control”, ICT’98 – International Conference on Telecommunications, Porto Carras, Greece, 22.-25.June 1997
T. Kováčiková – COST257: “Impacts of New Services on the Architecture and Performance of Broadband Services”, ročná hodnotiaca správa medzinárodného projektu za rok 1997, Žilina 9.1.1997
LANE je jednou z možností ako zabezpečiť beh sieťových aplikácii nad ATM sieťou a ako prepojiť už inštalované LAN siete typu Ethernet a Token Ring so sieťou ATM. Ale takisto toto riešenie má určité nevýhody, ktoré bránia užívateľovy využiť niektoré z vlastnosti ATM (napríklad LANE poskytuje užívateľovy limitované možnosti špecifikovať QoS). LANE je prvý krok pri prechode sietí smerom k ATM konektivite.
LANE ist eine aus den Möglichkeiten, wie kann man der Betrieb den Netzaplikationen über ATM Netz zu sichern und wie kann man schon instalierte LAN Netze von den Typen Ethernet und Token Ring zu verbinden. Diese Lösung hat bestimmte Urteile, die dem Benutzer nicht ermöglicht einige von den ATM Eingenschaften zu benutzen (Zum Beispiel LANE bietet dem Benutzer die limitierenden Möglichkeiten für die QoS Spezifikation. LANE ist der erste Schritt bei dem Übergang der Netze in der Richtung zur ATM Konektivität.