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FDDI

Because of its speed, FDDI is used extensively as the backbone to interconnect LANs. FDDI uses a token-passing method to provide 100M-bps of bandwidth. It can also be spread over larger distances (i.e., 200K meters) than bus-type networks and supports up to 500 stations on a ring.

FDDI uses two counter rotating rings for data transfer. The use of two rings ensures against failure of the ring when a single node fails. FDDI has extensive network management support, and its cost has fallen to $1,500 to $3,000 per hub.

FDDI II

FDDI II provides a circuit-switched service while maintaining the token-controlled packet-switched service of the original FDDI. This is done by imposing a fixed-frame structure on the original FDDI and regularly repeating time slots in the frame (i.e., asynchronous transmission). FDDI II provides the continuous sustained data rate needed for multimedia traffic, but total bandwidth still remains at 100M bps.

Suitability for Multimedia Traffic. FDDI II adds constant bit rate traffic to FDDI to support multimedia. Its bandwidth is sufficient for LANs and supports multicasting. To bring FDDI II to the desktop is a costly proposition, however. Like FDDI, FDDI II is used mostly at the backbone to interconnect LAN systems. It is also incompatible with FDDI in the sense that a station with FDDI cannot read a FDDI II frame; the reverse, however, is possible.

ASYNCHRONOUS TRANSFER MODE

ATM is a technology that serves equally well at the LAN and WAN levels. The following sections provide a general description of this still-emerging technology and specifics on its LAN emulation technology.

ATM uses fixed-size 53-byte cells (i.e., a 48-byte information field and a 5-byte header field) and breaks all traffic into these cells to ensure quicker switching and multiplexing. It provides point-to-point and point-to-multipoint connections through virtual circuits. ATM currently supports speeds as high as 155M bps and 622M bps and will reach 10G bps in the future. It operates as a DS3 or T3 line (i.e., 45M bps) or possibly a DS1 line (i.e., 1.544 Mbps).


Exhibit 1.  ATM Layer Architecture.

ATM Architecture

ATM functionality corresponds to the physical layer and data link layer of the OSI model; the architecture is depicted in Exhibit 1. In the physical layer, information is transferred from one user to another by cell-based asynchronous transmission or, more frequently, by an externally framed synchronous transmission structure, typically a SONET structure. Thus in the latter option, the ATM cells are carried inside a SONET structure (i.e., 90 columns and 9 rows of 8-bit bytes) with a bit rate of 51.84M bps (i.e., one frame is transmitted every 125 microseconds — the sampling interval used in digitizing voice for telephone systems).

The ATM layer multiplexes cells over the physical link. The major function of this layer is to complete the ATM cell structure and set up the cell streams for transmissions of outgoing process, receive the incoming cells, and send them to the corresponding stream. The cells are distinguished by the VCI and VPI in the cell header. A table in the switch helps the ATM layer place the cell in the appropriate output link. A generic flow control is used for media access by the user network interface to control the amount of traffic entering the network.

The ATM adaptation layer has the job of adapting higher-level data into the format needed by the ATM layer. When the necessary adaptation is completed, many different kinds of traffic can be carried over the same system, enabling ATM networks to aggregate network traffic to cut costs while simultaneously providing flexible service provisioning.5 AAL identifies four service classes based on the following three parameters:

Exhibit 2. Quality and Application Classes in ATM
Name Priority Negotiation Advantage Applications

Constant Bit Rate High priority “Contract basis” for maximum data rate No cell loss Real-time audio and video
Variable Bit Rate Medium to high Peak cell rate and sustained cell rate QOS guaranteed for cell loss and bandwidth availability Not for LAN traffic but for most others
Available Bit Rate Medium Negotiates bandwidth availability and cell loss Reliable delivery of bursty data LAN traffic
Unspecified Bit Rate Low to medium No QOS Efficient use of bandwidth Noncritical applications

  The timing relation between source and destination (i.e., yes/no).
  Constant, variable, or available (C, V, A) bit rate.
  Connection mode (connection-oriented or connectionless).

Suitability for Multimedia Traffic. ATM’s greatest advantage is its ability to specify QOS by applications. This allows ATM switches to efficiently allocate network resources among applications with very different needs. For example, LAN data traffic may be able to tolerate delay but no loss, and desktop video can drop frames but tolerates no delay. Choices of quality and the class of applications that can exploit them are shown in Exhibit 2.

ATM also has several other advantages, including:6

  Network resources given on demand (i.e., statistical multiplexing).
  Easy transition from the existing network.
  AAL-specific adaptability.
  Easy network management.
  Reduced error checking means that it works only on low rate links.

The problems associated with ATM technology include:

  The need for traffic parameters to be stated on start-up.
  The need to resolve interoperability issues. The PNNI standard has just been defined and approved by the ATM Forum (a standards group for ATM).
  The need for switching architectures capable of supporting the high data rates of broadband applications.

LAN Emulation

As seen in Exhibit 3, ATMs and LANs differ in their basic nature; ATMs are inherently connection-oriented while LANs are connectionless. The aim of LANE is to make the ATM switch invisible to legacy LANs. It also provides a way to protect existing investments in bus and Token Ring networks.7

Exhibit 3. Comparison of ATMs and LANs
ATM LANs

Connection oriented Connectionless
Leads to inefficient use of network resources when broadcast and multicasting is used Broadcast and multicast achieved through shared media
Hierarchical addresses Address format based on serial number in adapter card
User setup needs speed and traffic characteristics for QOS Because it is impossible to specify traffic characteristics, there is no guarantee of service


Exhibit 4.  LAN Emulation.


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