Direct Link Networks: Multiaccess Protocols (2.7) CS/ECpE 5516: - - PowerPoint PPT Presentation

Direct Link Networks: Multiaccess Protocols (2.7)

CS/ECpE 5516: Computer Networks

Originally by Scott F. Midkiff (ECpE) Modified by Marc Abrams (CS) Virginia Tech courses.cs.vt.edu/~cs5516

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 2

Lecture Topics

I Multiaccess control I IEEE 802.5 Token Ring and FDDI

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 3

Multiaccess Communication (1)

I Previous discussion considered

point-to-point links

G Received signal is transmitted signal (plus noise)

I Many networks are such that received signal at

• ne node depends on transmitted signal at two
• r more other nodes

G Satellite systems G Radio networks G Multi-tap bus systems

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 4

Multiaccess Communication (2)

I Multiaccess media are communication media

where received signal is sum of attenuated transmitted signals plus effects of delay, distortion, and noise

I Examples:

Multitap bus (Ethernet) Radio (wireless) network

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 5

Medium Access Control -- MAC (1)

I With multiaccess media, protocol is needed to

coordinate sharing of media

I Medium access control (MAC) protocol performs

this function

I MAC is sublayer between data link control (DLC)

layer and physical layer (usually grouped with DLC)

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 6

LLC MAC physical Data Link

Medium Access Control -- MAC (2)

I LLC provides “link” to adjacent node I MAC coordinates access to shared media I Physical provides hardware interface

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 7

Medium Access Control -- MAC (3)

I Separation of layer functions in multiaccess

networks is not as well-defined as in networks with point-to-point links

G Feedback about errors is part of ARQ strategy of

DLC, but may depend on how media is shared

G Flow and congestion control needed to provide fair,

efficient access to shared media

G Broadcast nature of shared media implements some

routing functions

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 8

Token Ring Networks

Token ring networks are common form of LAN & MAN

G IEEE 802.5 (Token Ring): 4 Mbps or 16 Mbps G Fiber Distributed Data Interface (FDDI): 100 Mbps

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 9

Nodes Arranged in Ring Topology

1 4 3 7 8 2 5 6 Point-to-point links between stations

G Node… G receives bit stream

from last node

G relays bit stream to

next node

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 10

Nodes Arranged in Ring Topology

interface logic Node can repeat

• r replace each

bit 1 4 3 7 8 2 5 6 At least 1 bit delay at each node:

N Propagation N Processing N Regeneration & transmission

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 11

Token (1)

I To transmit its own data, node must discard

input & output its data

I But we can't discard data until it has reached its

destination

G Token is used to coordinate use of ring G Ring is shared medium, so network is multiaccess

system interface logic

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 12

Token (2)

I Conceptually, token is passed from node to

node

G Only send your data when you've got token G Pass token when data reaches destination or you've

got no data to send

I So what is a token anyway?

G Special pattern -- distinguished from data G Similar to framing flags

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 13

Token (3)

I Token can be in 2 states

G Free token (or idle token): ring available

N Discard bits following free token

G Busy token: ring in use

N Follow busy token with data

I Token indicates

G upcoming data (if busy), as well as G permission to transmit (if free)

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 14

Basic Token Ring Operation (1)

I When node with data to transmit receives free

token, it marks token as busy and appends its

• wn data

I Subsequent nodes forward data since token is

marked busy

G Destination node both forwards and stores data G Destination node may mark data as received, but

token is still busy

I Data returns to originating node where it is

discarded

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 15

Basic Token Ring Operation (2)

I After node finishes transmission, it

G marks token as free G forwards token next node G follows token with idle fill

(until it sees busy or idle token again)

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 16

Ring Example (1)

1 3 2 4 1 3 2 4 Node 1 receives free token Node 1 transmits busy token followed by data for node 3 data busy

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 17

Ring Example (2)

1 3 2 4 Busy token followed by data continues around ring, node 3 stores data 1 3 2 4 Busy token completes round trip and is stripped at node 1

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 18

Ring Example (3)

1 3 2 4 Node 1 strips old data from ring and transmits new data until finished 1 3 2 4 When finished, node 1 puts free token on ring, followed by idle fill free token idle

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 19

Ring Example (4)

1 3 2 4 Node 2 forwards free token (no data to send), node 3 still storing data 1 3 2 4 Node 3 receives all of data from 1, forwards free token (no data to send)

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 20

Ring Example (5)

1 3 2 4 Node 4 receives free token, transmits busy token followed by data 1 3 2 4 Node 1 forwards bits (busy token) after its last data bit arrives

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 21

Additional Details of Ring Operation

I Propagation delay around ring must be long

enough to “store” complete token

G Otherwise first part of free token would be discarded

to transmit last part

I Error detection

G Receiving node can check CRC and put an ACK or

NAK in packet trailer on its way back to sender

G Sending node can also check CRC since it sees all

transmitted data

I Numerous variations are possible in ring

• peration

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 22

Token Holding Time

I How long can node hold free token? I Option 1: Transmit only 1 packet

G Lets token rotates at maximum rate G Minimizes latency

I Option 2: Transmit all waiting packets

G Reduces token transmission overhead G Maximizes throughput

I 3: Transmit waiting packets up to time limit

G Best of both worlds

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 23

Retransmission Schemes (1)

I 2 options in handling retransmissions…

G Selfless operation (FDDI):

Give up token when done transmitting; if error detected, reacquire token & retransmit

G Selfish (IEEE 802.5):

Hold token for round trip time, to be sure receiver got data correctly

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 24

Retransmission Schemes (2)

I Pros/Cons

G Selfless (FDDI):

N Penalty: higher latency on error

G Selfish (802.5):

N Ring transmits idle fill until sender gets ack N Penalty: lower throughput for low error rates N Advantage: Lower latency for retransmissions

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 25

Type of Token Failures (1)

I Lost token -- no node can transmit!

G Corrupted by noise (bit errors alter token code) G Node holding token fails

I Token is permanently marked busy -- no node

can transmit

G Idle token corrupted by noise (is marked busy)

I Multiple tokens created -- conflicts for access

G Non-token corrupted by noise to become token G Node failure

I Ring protocol recognizes token failures &

recovers (e.g. generating new token)

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 26

Fiber Distributed Data I nterface -- FDDI

I 100 Mbps timed token ring network based on

fiber optics

I Developed under auspices of ANSI committee

X3T9 formed in 1982

I Limited popularity

G Lack of high BW apps in 1980's G High cost of NIC's ($5000) and concentrators ($$) G Was popular for backbones & switching fabric

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 27

FDDI Standard

LLC Logical Link Control MAC Media Access Control PHY Physical PMD Physical Media Dependent SMT Station Management LLC MAC PHY PMD SMT Data Link Physical

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 28

FDDI Versus Token Ring

I Token Ring: sender waits until all of transmitted

data goes round ring before releasing token

I FDDI: sending node releases token after

sending last bit of data

G Busy token not sent G Data frame header recognized as “busy token” G Improves FDDI’s throughput

I FDDI supports low-priority (asynchronous) and

high-priority (synchronous) packets

G Guarantees throughput and latency G Suitable for digitized voice, real-time control, etc.

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 29

Capacity Allocation in FDDI (1)

I Capacity allocation for high-priority data is

provided by timed token scheme

G Each node measures times between token arrivals G Low-priority traffic can be sent only if intertoken time

is sufficiently small

G High-priority traffic can be sent anytime token arrives

I Limited amount of high priority traffic can be

sent for each token arrival (token holding time is limited)

G Guaranteed transmit time αi is allocated to node i

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 30

Capacity Allocation in FDDI (2)

I Target token rotation time (TTRT), τ, is

established when FDDI ring is initialized

G Used to determine when to send low-priority traffic

I It can be shown that:

G TTRT, τ, is upper bound on time-average intertoken

arrival time

G 2τ is worst-case intertoken arrival time

I Transmission time for node i, αi, i = 0, 1, ..., m-1

(m-node network), allocated such that

α α α τ

1 1

+ + + ≤

−

...

m

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 31

Capacity Allocation in FDDI (3)

I To be more exact, other factors must be

considered in setting TTRT, τ

G Maximum propagation time around ring, TP,max G Time to transmit maximum length frame (4500 bytes),

TF,max

G Token transmission time, TT

I So, allocations αi must be set such that I Can group other factors into values for αi

T T T

P,max F,max T i i m

+ + + ≤

= −

∑ α

τ

1

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 32

Capacity Allocation in FDDI (4)

I Let t0, t1,..., tm-1, be times at which token arrives

at nodes 0, 1,..., m-1, for some given cycle

I Assume that node k = (i mod m) receives token

at time ti, i ≥ 0; node measures intertoken arrival time, ti - ti-m

I If ti - ti-m < τ, node can send low-priority traffic

for τ - (ti - ti-m) seconds and can send high-priority traffic for its allocated time of αk seconds

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 33

Capacity Allocation in FDDI (5)

I If ti - ti-m ≥ τ, node cannot send

low-priority traffic, but it can still send high-priority traffic for αk seconds

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 34

Capacity Allocation Algorithm (1)

I All stations know same value for TTRT (τ) and

each has its own value αi

I Each node maintains two timers and counter

G Token rotation timer (TRT): Times intertoken arrival

time (using LC)

G Token holding timer (THT): Times token holding time

at node

G Late counter (LC): Counter for number of times that

TRT expires

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 35

Capacity Allocation Algorithm (2)

I TRT is initialized to TTRT (τ) and counts down,

LC is 0

I If token is received before TRT expires, TRT is

reset to TTRT

I If TRT expires before token is received, LC is

incremented to 1 and TRT is reinitialized to TTRT

I If TRT expires second time, LC is incremented to

2 and token is considered lost

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 36

Capacity Allocation Algorithm (3)

I If token is received before TRT expires once

(early token)

G THT is set to TRT (τ - [ti - ti-m]) G TRT is reset to TTRT and started G Station transmits high-priority frames until all are

transmitted, but for at most αi seconds

G Station starts THT and transmits low-priority frames

until done or THT expires

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 37

Capacity Allocation Algorithm (4)

I If TRT expires before token is received (late

token)

G LC reset to 0, TRT resets (“rolls over”) G Station can transmit high-priority frames for at most

αi seconds (cannot transmit low-priority frames)

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 38

Latency in FDDI

I For high-priority stream-type traffic, delay is

bounded by τ + T, where T is allocated traffic

G Delay is loosely bounded by 2τ

I Short TTRT decreases delay, but at expense of

efficiency (throughput)

T

i i m

=

= −

∑

α

1

ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 39

You should now be able to … (1)

I Describe IEEE 802.5 LAN protocol I Describe operation of Token Rings and FDDI I Compare operation of FDDI to IEEE 802.5 Token

Ring

I Analyze allocated capacity and latency in an

FDDI network