Fiber Distributed Data Interface

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FDDI (Fiber Distributed Data Interface )

FDDI standard was developed by ISO and ANSI and concerns over a double ring network, fiber optic, providing a speed of 100Mbs.
Standard recommends the use of optical fibers with dimensions of 62.6 / 125 micrometers but prohibits the use of another type of fiber. Wavelength of radiation must be 1300 nm (infrared) and vary between 1270nm and 1380nm.
The network is designed for a maximum length of 200 km and 1000 optical cable stations. The length of a ring with its ramifications (lobes) may not exceed 100Km
The maximum propagation through each station is 756ns and 200 km of optical fiber
is 1.017 ms.
A modified version for twisted copper wires is called CDDI (Cooper Distributed Data Interface).

FDDI network structure

The network is made of two fiber rings (primary and secondary) through which information flows in opposite directions.
FDDI operates three types of devices:
? SAS (Single Attachment Station)
? DAS (Dual Attachment Unit)
? Concentrator
SAS device type conevteaza only primary ring through a concentrator. The advantage of these devices is that connecting / disconnecting them will have no effect on the ring.
DAS type devices have two ports denoted by A and B. These ports connect the device directly to the two ports. Any malfunction, disconnect or shutdown affects of these stations and the network ring.

How is processed binary signal

Binary signal is modified according 4b/5b code, passed in NRZI (Non Return to Zero Invertet) and then applied modulator (voltage converter-radiation).
4b/5b code is passing each group of 4 bits in a 5 bit so that no more than two successive 0 bits.
In each bit NRZI code of 1 corresponds to a transition at the beginning of a bit interval. Bits 0 does not correspond to any transition.

How to send data over fiber

Fiber optic data transmission is done by modulating the intensity of basic banda illuminated ON-OFF mode. A bit of 1 corresponds to an emission of radiation and a bit of 0 nu .. (LASER issued only when the transmitted bit is 1).

Access to transmission medium

Access to the transmission medium is controlled by chip type unaddressed. This means that a system can be issued only if it is in possession of the token, token that is passed from one computer to his successor. The chip does not contain the address of the destination computer (as with Token Bus) as successor is physically and logically.
The chip is a sequence of bits identified as "chip" sequence which entitles the holder to issue the ring.

How data flows in ring

A Framework issued by the computer which has token ring through all either found or not found destination. If the target computer is in the ring, he copies the data and then changes the context in which they are included so that the source computer knows that the message has reached its target and it was copied.
After completing the ring, the reajunge the computer that issued it, the computer is not repeated (him out of the ring). Actually part of the frame (the beginning to the source address) will circulate in the ring until reach the next computer with a chip, computer propiile issuing repeated frames and not the rest of the frame.

What distinguishes FDDI network to a Token Ring network
What distinguishes one network FDDI Token Ring token is issued by the computer when it holds and ring presence of several frames from different systems.
If Token Ring network, a computer chip hold until he has nothing to transmit until it expires or the allotted time. The chip is released only after setting utimul delivered back to the issuing ? computer at a time running a single ring frame.
On the source computer FDDI token issued after the issued without waiting for the return of the ring frame. Reach the computer chip can emit before the success of this first computer data to pass through the ring. And this computer do the same ? at a time in the ring can move multiple frames from different computers.

What distinguishes FDDI network to a Token Bus Network

Token Token Bus network operates addressed in a logical ring (BUS physical topology and logical topology ring)
FDDI network operates unaddressed chip in a physical ring (ring physical topology and logical topology ring)

First issue of chip

When starting the network, the ring is initialized. One of the steps is to establish, by auction, the maximum time allowed for completing the token ring (Target Token Rotation Time-TTRT). TTRT consists of time allocated to each system for synchronous and asynchronous transmissions and a time frame length to pass through the ring. System that won the tender issued for the first chip.

How to detect token loss

Each system has a count of the token rotation time called TRT (Target Rotation Timer) with which to determine the actual time browsing the token ring.

Decide that a token is lost if its numerator TRT reaches zero twice without having received the token.

How to replace a lost token

The system detects token loss initiates the procedure by generating a new token.
System sends frames called "claim token" which suggests a value for TTRT ? triggers for value TTRT auction. If another system issue frames "claim token" with higher values ??for TTRT, the initiator starts to retransmit the frames
Who wins the auction system will generate a new token ring traveling once and then entitles the winner to give.

How to detect a break in the ring

If a system supposed to lose the chip, send frames "claim token". If they can not spread the ring (move twice the maximum time) means that the token is lost because the ring is interrupted.
The emergency procedure system fault detection position by sation (beacon).
If sation system receives frames from one another, it will stop sending frames propiile sation and will repeat those received. In this way the system frameworks issued immediately following the fault goes through all network computers.
Ring system is reconfigured by bridging defect or by using secondary ring.

How to detect ring recovery

If a system issue sation frames received their shots propiile conclude that the ring was recovered and starts the procedure of generating a new token.

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