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Fiber Distributed Data Interface (FDDI) Network Architecture

Fiber Distributed Data Interface (FDDI)

Fiber Distributed Data Interface is a media access control protocol with token-ring architecture which has a communication bandwidth of 100 Mbps. It is supported on a fiber network medium and is fast compared to standard token ring and Ethernet.

Fiber Distributed Data Interface (FDDI) Dual-ring Architecture

FDDI uses dual-ring architecture for the flow of traffic on each ring in opposite directions called counter-rotating. The dual rings consist of a primary and a secondary ring. The primary ring is used for data transmission and the secondary ring remains idle. The main purpose of the dual ring is to provide superior reliability and robustness. If the primary ring fails, the secondary ring allows the nodes to continue to operate.

Machines on the ring are classified in A or B groupings. The B stations are only on the primary ring. Thus, if the primary ring fails, all Class B nodes would be broken down. An FDDI concentrator is also called a dual-attachment concentrator (DAC). It is the building block of a Fiber Distributed Data Interface (FDDI) network. It attaches directly to both the primary and the secondary rings and ensures that the failure or the power-down does no break down the ring.

FDDI Dual-ring Architecture
Image: FDDI Dual-ring Architecture

Fiber Distributed Data Interface (FDDI) Specifications

FDDI specifies the physical and media access layers of the OSI reference model. There are four specifications in FDDI. They are;

  1. Media Access Control (MAC)
  2. Physical Layer Protocol (PHY)
  3. Physical Medium Dependent (PMD)
  4. Station Management specifications (SMT)

1. Media Access Control (MAC)

The Media Access Control specification defines the method of accessing the medium with the frame format, token handling, addressing, algorithms for calculating cyclic redundancy check (CRC) value, and error recovery mechanisms.

2. Physical Layer Protocol (PHY)

The Physical Layer Protocol specification defines the data encoding/decoding procedures, clocking requirements, and framing, along with the other functions.

3. Physical Medium Dependent (PMD)

The Physical Medium Dependent specification defines the characteristics of the transmission medium, together with the fiber-optic links, power levels, bit-error rates, optical components, and connectors.

4. Station Management specifications (SMT)

The Station Management specification defines the FDDI station configuration, ring configuration, and ring control features, including station insertion and removal, initialization, fault isolation and recovery, scheduling, and statistics collection.

Fiber Distributed Data Interface (FDDI) Frame Format

The FDDI frame format is similar to that of the Token ring frame. The following frame format shows the extent of similarities between the FDDI and Token ring frame format.

FDDI Frame format
Image: FDDI Frame format

  • Preamble

This field gives a unique sequence that prepares each station for the upcoming frame.

  • Start Delimiter

This field indicates the starting of the frame and consists of the signaling patterns which differentiate it from the other frame.

  • Frame Control

This field indicates the size of the address fields and confirms the frame contains asynchronous or synchronous data among the other control information.

  • Destination Address

The Destination Address field is 6 bytes long and it contains a unicast, multicast or broadcast address.

  • Source Address

The Source Address field is 6 bytes long and the field identifies the station which is sent the frame.

  • Data

It contains either the information destined for an upper-layer protocol or control information.

  • Frame Check Sequence

Frame Check Sequence field is used to check or verify the traversing frame for any bit errors. This field is filled by the source station with a calculated 32-bit cyclic redundancy check value dependent on frame contents. The destined address recalculates the value to determine whether the frame was damaged in transit, otherwise, the frame is discarded.

  • End Delimiter

This field contains unique symbols which indicate the end of the frame.

  • Frame Status

This field allows the source station to determine the error check and identifies whether the frame is reorganized and copied into the memory of the intended receiver.

T-Carrier System

T-carrier system is a series of data transmission formats developed by Bell Telephone. It is used in the telephone network system. The base unit of a T-carrier is DS0, which is 64 Kbps. The T-carrier system uses in-band signaling which is a method that actually robs bits from being used for data and uses them instead for overhead. This reduces the transmission rates used for T-carrier signals The E-carrier system does not perform bit-robbing and as a result has a higher throughput rate.

T-1 carrier: T-1 is a digital line made up of 24 channels which consist the rate of 1.544Mbps used to connect corporate networks and Internet Service Providers. It is also called as DSO or DS1. TI often uses DS1 signaling standards and are referred to as DS1 lines. T-1 carrier is mainly associated with the phone connection supporting data rates of 1.544Mbps per second.

T-3 carrier: T-3 carrier is also associated with the phone connection supporting data rates of 43Mbps. A T3 line usually consists of 672 individual channels. It is also called as DS3 lines. Each channel supports 64Kbps. These lines are mainly used by the Internet Service Providers connecting to the internet backbone.

E1: E1 is the European format for digital transmission. E1 carries signals at 2Mbps, 32 channels at 64 Kbps with 2 channels reserved for signaling and controlling with the T1, which carries signals at 1.544Mbps, 24 channels at 64Kbps. E1 and T1 lines can be interconnected for international purposes.

X.25: X.25 is a set of protocols incorporated in a packet switching network made up of switching services. It uses packet switching and virtual circuits. It provides robust error-checking features, which makes it a good option for older networks. Because of its extensive error checking, it works on the older networks that are more susceptible to physical interference, and also is a widely accepted protocol that is used in many parts of the world.

The main purpose of switching services was to connect the remote terminals to mainframe host systems. It provides shared, a variable capacity that may be either switched or permanent. X.25 is expensive because tariffs are based on the amount of data delivered at any rate up to connection capacity. In addition, the data packets are subjected to the delays of the shared networks.

The network consists of interconnected nodes to which user equipment can connect. Data Terminal Equipment (DTE) is the user end. Data Circuit-terminating Equipment (DCE) is the carrier’s equipment. Most of the packet switching technology does not use a dedicated physical or virtual circuit and is generally connectionless in nature, therefore; X.25 establishes virtual circuits that allow it to be connection-oriented. The connection is established, the data is transferred, and then the connection is terminated.

The maximum packet size of x.25 varies from 64 bytes to 4096 bytes, with 128 bytes being a default on most networks. It is optimized for low-speed lines of 100kbps and below. Above 100 kbps the effects of latency, small packet sizes, and small window sizes are such that the bandwidth cannot be efficiently utilized. X.25 has been the basis of the development of other packet-switched protocols like TCP/IP and ATM. The major drawback is the inherent delay caused by the store-and-forward mechanism, which in turn restricts the useful data transmission rate.

OCx Optical Carrier: Optical Carrier levels describe a range of digital signals. These digital signals can be carried on SONET fiber-optic network. The number in the Optical Carrier level is directly proportional to the data rate of the bit-stream carried by the digital signal. Calculating the speed of Optical Carrier lines is when a specification is given as OC-n, that the speed will equal n x 51.8 Mbit/s. some specifications of an Optical Carrier are

  • OC-1 is a SONET line with transmission speeds of up to 51.84 Mbit/s.
  • The transmission speed of OC-3 is up to 155.52 Mbit/s.
  • OC-12 is a network line with transmission speeds of up to 622.08 Mbit/s (payload: 601.344 Mbit/s; over-head: 20.736 Mbit/s). These are commonly used by ISPS as WAN connections.
  • Transmission speed of OC-48 network line is up to 2488.32 Mbit/s (payload: 2405.376 Mbit/s; overhead: 82.944 Mbit/s). These connections are used as the backbones of many regional ISPS.
  • OC-96 transmission speed is up to 4976.64 Mbit/s (payload: 4810.752 Mbit/s; overhead: 165.888 Mbit/s).
  • OC-192 transmission is up to 9953.28 Mbit/s (payload: 9621.504 Mbit/s; overhead: 331.776 Mbit/s). Test Yourself


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