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Standard FDDI

jako protokół dostępu

do medium światłowodowego

w szkieletowej sieci pierścieniowej

Technologies

for LAN and MAN

(ISO)

FDDI

802.3 802.4 802.5

FDDI

802.2 Logical Link Control

ISO 8802.2

802.6

NETWORK

DATA LINK

PHYSICAL

LLC

MAC

CSMA/CD TOKEN

BUS

TOKEN

RING

DQDB

ISO

8802

.3

ISO

8802

.4

ISO

8802

.5

ISO

8802

.6

ISO

9314

FDDI (Fiber Distributed Data Interface)

Standard processed by:

- ANSI (ANSI X3T9.5)
- ISO (ISO 9314)
Standard specify ring architecture with fibre optical medium.

Characteristic:

 ring topology with double loop
 throughtput 100 Mbps
 optical fibre medium ( 200 km - single-mode fibre),
  = 1300 nm,
 coding – 4B/5B,
 number station - max. 1000.



FDDI is an ANSI (American National Standards Institute) standard



Initial proposals for MAC and PHY (June 1983)

- February 1986 MAC (Rev. 10)
- August 1985 PHY (Rev. 11), problem with specification of elasticity buffer discovered
- August 1987 PHY (Rev. 15)
- June 1988 PMD (Rev. 8)

ANSI X3.139-1987 FDDI Token Ring Media Access Control (MAC)
ANSI X3.148-1988 FDDI Token Ring Physical Layer Protocol (PHY)
ANSI X3.184-1993 FDDI Single-Mode Fiber Physical Layer Medium Dependent (SMF-PMD)
ANSI X3.231-1994 FDDI Token Ring Physical Layer Protocol (PHY-2)
ANSI X3.239-1994 FDDI Token Ring Media Access Control-2 (MAC-2)
ANSI X3.229-1994 FDDI Station Management (SMT)

FDDI - History

ANSI X3T9.5 defines four mayor sections:
– Physical Medium Dependent (PMD) specifies optical fibre link
and related optical components
– Physical Layer Protocol (PHY) specifies the encode/decode,

clocking, and data framing

– Media Access Control (MAC) defines the access to the medium,

addressing,

data checking, and frame generation/reception
– Station Management (SMT) specifies FDDI station configurations,

ring configurations,

and control for proper ring operations (Network Management

Concept)

FDDI RM

FDDI station architecture

FDDI is based on IEEE 802.5 Token

Ring

– FDDI adopts as much of 802.5 as

possible

– FDDI makes changes where necessary



optical fiber



100 Mbps



NRZI-4B/5B code



explicit reliability
specification



distributed clocking



timed token rotation



new token after transmit



FDDI frame structure



4500-octet maximum
frame size



16- and/or 48-bit addresses



distributed recovery



shielded twisted pair



1 and 4 Mbps



differential Manchester code



no explicit reliability specification



centralized clocking



priority and reservation bits



new token after frame receive



802.5 frame structure



no maximum frame size



16 or 48 bit addresses



active monitor

FDDI

IEEE 802.5 Token Ring

FDDI Characteristic

FDDI Optical Fiber Characteristics

•

Two contra rotating rings with 100 Mbit/s data

rate each:

–

primary ring

used for data transmission

–

secondary ring

is for reliability only

• Nodes can be connected to both rings for

reliability

• In case of link failures, stations reconfigure

using primary

and secondary ring (bypassing of failed link)

FDDI Topology

FDDI defines two classes of stations:
– DAS:

Dual-Attachment-Station

(connected to both

rings)

– SAS:

Single-Attachment-Station

(connected only to

the primary ring)

FDDI Station Types

FDDI Station Types

FDDI defines two classes of concentrators:
– DAC:

Dual-Attachment-Concentrator

(connected to both

rings)

– SAC:

Single-Attachment-Concentrator

(connected to DAC

or other SAS)

 Point-to-Point Topology
 Star
 Tree Topology
 Dual Ring
 Dual Ring of Trees

FDDI Network Topologies

 Point-to-Point

– possible between two Single-Attachment-

Stations only

FDDI Network Topologies (cont.)

 Star Topology

– only the

primary ring

is used

– failure isolation is

not possible

FDDI Network Topologies (cont.)

 Tree Topology

– only the

primary ring

is

used

– failure isolation is

not

possible

FDDI Network Topologies (cont.)

 Dual Ring

FDDI Network Topologies (cont.)

 Dual Ring of Tree

Standard FDDI use enhancing techniques

(dual ring topology, concentrators and station bypass)

to ring reconfiguration in case of link errors or node failures.

FDDI Reliability

Station or Link Failure

a) Station failure

b) Link failure

Opitacal bypass station

 FDDI uses 4b/5b NRZI (Non-Return to Zero Invert)
with 125 Mbps baud rate to achieve 100 Mbps data rate
 Ethernet uses Manchester encoding with 20 Mbps baud rate (20 MBd)
to achieve 10 Mbps data rate

FDDI Signal Encoding

NRZ

- used by PC bus (1 = high, 0 = low)

NRZI

- used in combination with 4b/5b

(1 = transition, 0 = no transition)

Manchester

- used by Ethernet

(1 = high-low transition,
0 = low-high transition)

• FDDI uses NRZI - 4B/5B line code
• 4B/5B = 4 bits are encoded as 5 bit pattern (as

symbols)

• Bandwidth efficiency of this code is 80 %
• 100 Mbitps data rate is achieved by a signalling

rate of 125 Mbaud

• 16 data symbols and 8 control symbols

FDDI Data Encoding

Starting and ending delimiter patterns

FDDI uses a distributed clocking scheme:

• Each station has its own autonomous transmit clock:
– frequency variance between clocks: 0,01 % → 4,5 bits
– frequency accuracy: ± 0,0005 %

• Each station has an Elasticity Buffer (EB) for

compensating clock differences receive and transmit
clock:

– data is clocked into the EB at the incoming clock rate
– data is clocked out the EB at the stations own clock

rate

– EB is similar to FIFO memory
– minimum EB elasticity is 4,5 bits

• IDLE symbols between frames (preamble) are used for

synchronisation

– remove IDLE symbols if incoming clock is faster
– add IDLE symbols if incoming clock is slowe

• Smoothing function removes/adds IDLE symbols from/to

the preamble longer/ shorter than 14 IDLE symbols

FDDI Clocking

FDDI Clocking (cont.)

Two frame formats are used by

MAC

– Token
– Frame

FDDI MAC Frame Format

PA: 16 or more IDLE symbols for frame

synchronisation

SD: Start Delimiter (2 symbols “JK”)
FC: Frame Control (2 symbols)
ED: End Delimiter (2 symbols “TT”)

FDDI Token

> 16

2

2

2

FDDI MAC Frame Format (cont.)

FDDI Frame

PA: 16 or more IDLE symbols for frame synchronisation
SD: Start Delimiter (2 symbols “JK”)
FC: Frame Control (2 symbols)

C = class bit (0=asynchronous frame, 1=synchronous frame)
L = address length bit (0=16 bit, 1=48 bit)
FF = format bits (LLC or MAC control frame)
ZZZZ = control bits (type of frame control)

DA: 16 or 48 bit Destination Address
SA: 16 or 48 bit Source Address
FCS: Frame Check Sequence (32 bit CRC)
ED: End Delimiter (1 symbol “T”)
FS: Frame Status (3 or more symbols)
-

error detected (E), address recognized and frame copied indicators

> 16

2

2

1

4/12

4/12

3

9000

8

• Point-to-point, multicast, and broadcast capability

• FDDI employs IEEE 802.5 address structure:

FDDI Addressing

15 bits

• Station wishing to transmit frames removes token

• Station transmits one or more frames of data

• After transmission station issues a new token

• Transmitted frames are regenerated and repeated

by active stations

• Destination station copies frame(s)

FDDI Token Ring Protocol

FDDI Token Release on Multiple Frame Transmission

• FDDI uses a Timed Token Rotation Protocol controlling the

access to the medium

• The amount of bandwidth a station can use for data

transmission

is limited by the time of token possession

•

Target Token Rotation Time (TTRT)

– defines the max. value for the circulation time of the token in

the ring

– definition of TTRT at ring initialisation time
– example: TTRT = 50 ms (30 km ring length, 75 stations)

•

Token Rotation Time (TRT)

is measured by each station for each

token circulation

• When the token arrives at the station:
– TRT is saved in the Token Holding Timer (THT)
– TRT is set to zero and starts running again

•

The station may transmit asynchronous data only as long as THT

< TTRT

FDDI Timed Token Rotation Protocol

FDDI Timed Token Rotation Protocol

TRT is measure of load:

• IF TRT < TTRT (early token)
→

little load

, station can send

synchronous and

asynchronous data

• IF TRT > TTRT (late token)
→

heavy load,

station can only send

synchronous

data

• Average TRT value is on the order of TTRT or less
• Maximum TRT value is 2 x TTRT

• Ring monitoring is distributed among all stations in the ring
• High value of TRT (> 2 x TTRT) is an indicator for errors
• Three processes are involved in initialisation and recovery:

– Beacon Process

– Claim Token Process
– Initialisation Process

Beacon Process:

• This process is used to isolate ring failures
• Each station sends beacon frames
• A station receiving beacon frames from an other station stops

issuing own frames

and forwards the received beacon frames
• The station receiving its own beacon frames initiates the claim

token process

Claim Token Process:

• This process is used to negotiate the TTRT value and to resolve

contention among

the stations for ring initialisation
• Each station generates a claim frame containing a TTRT bid
• A station stops generating claim frames when receiving a lower

TTRT bid

and forwards the other frames
• The MAC with the lowest TTRT bid that had the highest address

will win the process

Initialisation Process:

• The station which won the CLT is responsible for the initialisation

of the ring

FDDI Ring Initialisation and Recovery

FDDI Ring Initialisation and Recovery (cont.)

FDDI Station Management (SMT)



SMT provides FDDI specific network management



Each FDDI station has one SMT entity



SMT contains three parts:

1. Connection Management (CMT)

• Establishing and maintaining the physical and logical

topology of the ring

• Manages PHY components and their interconnections
• Manages configuration of MAC/PHY entities
• Link error monitoring

2. Ring Management (RMT

)

• Establishing and maintaining the correct logical ring

operation (usable token)

• Managing if MAC layer resources of a FDDI station

3. Operational Management

• Management of the FDDI network in the operational

state

• OSI layer management protocol
• Obtain and modify management attributes by

management agents

FDDI Station Management (SMT)



Connection Management (CMT)



Ring Management (RMT)



Operational Management

Architektura sieci FDDI -

przykład

• FDDI efficiency problem  token propagation time
• With increasing speed and increasing station distances
FDDI becomes less efficient

• Throughput versus access delay

• TTRT is an important factor

• FDDI throughput = (TTRT - Ring Latency) / TTRT
– ring latency = 0,25 ms (30 km ring length with 75

stations)

– TTRT = 50 ms → Utilisation = 99,5 %

• FDDI access delay is less then 2 x TTRT

FDDI Efficiency

FDDI - II (Fiber Distributed Data Interface ver. II)

Extension FDDI standard

- asynchronous/synchronous +

isochronous services

Protocol

– type Slotted Ring

Multiframe (every 125 s)

- 16 channels type WBC (ang. Wide Band Channel) with 6,144 Mbps

Frame organisation:



Preamble + header (116 bits)

0,928 Mbps



N isochronous channels (N*768 bits)

N x 6,144 Mbps



Asynchr./synchr. Transmission (96 +(16-N)*768 bits) 0,768 + (16 -N) x 6,144 Mbps

Data bandwith in FDDI II:



8 / 16 / 32 kbps - voice or data



64 kbps

- channel type B ISDN



384 kbps

- 6 channels type B = channel H0 ISDN



1536 kbps

- 24 channels type B = channel H11 ISDN



1920 kbps

- 30 channels type B = channel H12 ISDN



1544 / 20468 kbps

- system T1 / E1



6144 kbps

- WBC channel

• FDDI-II requires an additional standard between PHY and

MAC

called Hybrid Ring Control (HRC) sublayer
• HRC defines two additional entities to support isochronous

data transmission

– Hybrid Multiplexer (H-MUX)
– Isochronous MAC (I-MAC)
• HRC multiplexes data between the Packet MAC
and the new Isochronous MAC (I-MAC) used for FDDI-II

FDDI - II Standard

•

Additional one station functions as Cycle Master (CM)

• CM generates a cycle structure every 125 μs on to

ring (slotted ring)

• Cycle comprises fixed number of timeslots
• Each cycle consists of:
– minimum packet data channel of 768 kbps
– 16 Wideband Channels (WBCs) each of 6,144 Mbps
→ total bandwidth of 98 304 Mbps
• Total bandwidth will be shared between CS and PS

FDDI-II - Operation

Cycle structure consists of 4 parts:
• PA – Preamble (5 symbols for cycle synchronisation)
• CH - Cycle Header (24 symbols for cycle management)
• DPG0 - DPG11 - Dedicated Data Packet Group:
– 12 bytes minimum guaranteed packet data bandwidth

(=768 kbps)

– byte interleaved among cyclic group
• CCG0 - CG95 - Cyclic Groups:
– 96 cyclic groups of 16 bytes each
– each byte in a CG represents one dedicated WBC
– 96 CGs = 96 bytes * 8000 1/s = 6,144 Mbps

FDDI-II - Cycle Structure

• 16 wideband channels are available
• WBC bandwidth is 6,144 Mbps
• bandwidth of a WBC can be divided into subchannels of 8,

16, 32 or 64 kbps,

plus any multiples of 64 kbps
• 1 WBC = 3 * 2,048 Mbps or 4 * 1,535 Mbps (DS1)
• WBCs are byte interleaved across 96 cyclic groups
• WBC “n” comprises the nth byte from each cyclic group

FDDI-II - Wideband Channels

FDDI-II - Wideband Channels (cont.)

• FDDI-II bandwidth is shred between

CS and PS

• Dynamic CS bandwidth allocation via

SMT

FDDI-II - Operation

Packet switching

• PS bandwidth = total bandwidth - CS bandwidth
• WBCs not used for CS are available for PS
• Minimum PS bandwidth = DPG
• MAC frames will be distributed on cycles by H-MUX

Circuit switching

• Negotiation of maximum CS bandwidth at ring initialisation
• Dynamically allocation of WBCs or subchannels via packet

channel (SMT)

at cycle master (captures token for interrupting PS, releases

token, etc.)

• Each WBC or subchannel represents a dedicated position in the

cyclic structure

• Subchannel consists of a fixed number of adjacent bits of one

WBC

Example: 64 kbps is one byte of a WBC