IEEE 802 LANs
• LAN: Local Area Network
• What is a local area network?
– A LAN is a network that resides in a
geographically restricted area
– LANs usually span a building or a campus
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Characteristics of LANs
• Short propagation delays
• Small number of users
• Single shared medium (usually)
• Inexpensive
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Common LANs
• Bus-based LANs
– Ethernet (*)
– Token Bus (*)
• Ring-based LANs
– Token Ring (*)
• Switch-based LANs
– Switched Ethernet
– ATM LANs
(*) IEEE 802 LANs
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IEEE 802 Standards
802.1: Introduction
802.2: Logical Link Control (LLC)
802.3: CSMA/CD (Ethernet)
802.4: Token Bus
802.5: Token Ring
802.6: DQDB
802.11: CSMA/CA (Wireless LAN)
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IEEE 802 Standards (cont’d)
• 802 standards define:
– Physical layer protocol
– Data link layer protocol
• Medium Access (MAC) Sublayer
• Logical Link Control (LLC) Sublayer
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OSI Layers and IEEE 802
OSI layers
Higher Layers
IEEE 802 LAN standards
Higher Layers
802.2 Logical Link Control
Data Link Layer
Physical Layer
802.3 802.4 802.5
Medium Access Control
CSMA/CD Token-passing Token-passing
bus
bus
ring
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IEEE 802 LANs (cont’d)
• Ethernet
• Token Ring
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Ethernet (CSMA/CD)
• IEEE 802.3 defines Ethernet
• Layers specified by 802.3:
– Ethernet Physical Layer
– Ethernet Medium Access (MAC) Sublayer
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Ethernet (cont’d)
• Possible Topologies:
1. Bus
2. Branching non-rooted tree for large
Ethernets
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Ethernet: MAC Layer
• Data encapsulation
– Frame Format
– Addressing
– Error Detection
• Link Management
– CSMA/CD
– Backoff Algorithm
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Ethernet Frame Format
Bytes:
7
1
Preamble
SFD
Preamble SFD
1.
2.
3.
4.
5.
6.
6
6
DA
DA
SA
SA
2
Type
Type
0-1500
Data
Data
0-46
4
Pad
CRC
Pad CRC
Preamble: trains clock-recovery circuits
Start of Frame Delimiter: indicates start of frame
Destination Address: 48-bit globally unique address
assigned by manufacturer.
1b: unicast/multicast
1b: local/global address
Type: Indicates protocol of encapsulated data (e.g. IP = 0x0800)
Pad: Zeroes used to ensure minimum frame length
Cyclic Redundancy Check: check sequence to detect bit errors.
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Ethernet MAC Frame
Address Field
• Destination and Source Addresses:
– 6 bytes each
• Two types of destination addresses
– Physical address: Unique for each user
– Multicast address: Group of users
– First bit of address determines which type of address
is being used
0 = physical address
1 = multicast address
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Ethernet MAC Frame
Other Fields
• Length Field
– 2 bytes in length
– determines length of data payload
• Data Field: between 0 and 1500 bytes
• Pad: Filled when Length < 46
• Frame Check Sequence Field
– 4 bytes
– Cyclic Redundancy Check (CRC-32)
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CSMA/CD
• Recall:
– CSMA/CD is a “carrier sense” protocol.
• If channel is idle, transmit immediately
• If busy, wait until the channel becomes idle
– CSMA/CD can detect collections.
• Abort transmission immediately if there is a
collision
• Try again later according to a backoff algorithm
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CSMA/CD (cont’d)
• Carrier sense reduces the number of
collisions
• Collision detection reduces the impact of
collisions
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CSMA/CD and Ethernet
• Ethernet:
– Short end-to-end propagation delay
– Broadcast channel
• Ethernet access protocol:
– 1-Persistent CSMA/CD
– with Binary Exponential Backoff Algorithm
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Ethernet Backoff Algorithm:
Binary Exponential Backoff
• If collision,
– Choose one slot randomly from 2k slots, where k is
the number of collisions the frame has suffered.
– One contention slot length = 2 x end-to-end
propagation delay
This algorithm can adapt to
changes in network load.
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Binary Exponential Backoff (cont’d)
slot length = 2 x end-to-end delay = 15 µs
A
t=0µs:
t=30µs:
t=45µs:
t=75µs:
B
Assume A and B collide (kA = kB = 1)
A, B choose randomly from 21 slots: [0,1]
Assume A chooses 1, B chooses 1
A and B collide (kA = kB = 2)
A, B choose randomly from 22 slots: [0,3]
Assume A chooses 2, B chooses 0
B transmits successfully
A transmits successfully
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Binary Exponential Backoff (cont’d)
• In Ethernet,
– Binary exponential backoff will allow a
maximum of 15 retransmission attempts
– If 16 backoffs occur, the transmission of the
frame is considered a failure.
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Ethernet Performance
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Ethernet Features and Advantages
1. Passive interface: No active element
2. Broadcast: All users can listen
3. Distributed control: Each user makes own
decision
Simple
Reliable
Easy to reconfigure
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Ethernet Disadvantages
• Lack of priority levels
• Cannot perform real-time communication
• Security issues
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Ethernet Switching
• Recent development: Connect many
Ethernet segments or subnets through an
“Ethernet switch”
to segment 4
to segment 1
to segment 3
to segment 2
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Why Ethernet switching?
• LANs may grow very large
– The switch has a very fast backplane
– It can forward frames very quickly from one
segment to another
• Cheaper than upgrading all host interfaces
to use a faster network
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Token Ring
• IEEE 802.5 Standard
• Layers specified by 802.5:
– Token Ring Physical Layer
– Token Ring MAC Sublayer
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Token Ring (cont’d)
• Token Ring, unlike Ethernet, requires an
active interface
Ring
interface
Host
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Token Ring Configuration
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Token Ring Configuration
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Token Ring MAC Sublayer
• Token passing protocol
• Frame format
• Token format
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Token Passing Protocol
• A token (8 bit pattern) circulates around the ring
• Token state:
– Busy: 11111111
– Idle: 11111110
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Token Passing Protocol (cont’d)
• General Procedure:
– Sending host waits for and captures an idle token
– Sending host changes the token to a frame and
circulates it
– Receiving host accepts the frame and continues to
circulate it
– Sending host receives its frame, removes it from the
ring, and generates an idle token which it then
circulates on the ring
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Token Ring Frame and Token
Formats
Bytes
1
1
1
SD AC ED
Token Format
1
1
1
SD AC FC
2/6
2/6
Destination
Address
Source
Address
unlimited
Data
4
Checksum
1
1
ED FS
Frame Format
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Token Ring Delimiters
SD AC ED
SD AC FC
Destination
Address
Source
Address
Data
Checksum
ED FS
• SD = Starting Delimiter
• ED = Ending Delimiter
• They contains invalid differential Manchester
codes
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Token Ring Access Control
Field
SD AC ED
(Note: The AC field
is also used in frames)
PPPTMRRR
• P = Priority bits
– provides up to 8 levels of priority when accessing the
ring
• T = Token bit
– T=0: Token
– T=1: Frame
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Token Ring Access Control Field
(cont’d)
SD AC ED
PPPTMRRR
• M = Monitor Bit
–
–
–
–
–
Prevents tokens and frames from circulating indefinitely
All frames and tokens are issued with M=0
On passing through the “monitor station,” M is set to 1
All other stations repeat this bit as set
A token or frame that reaches the monitor station with M=1 is
considered invalid and is purged
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Token Ring Access Control Fields
(cont’d)
SD AC ED
PPPTMRRR
• R = Reservation Bits
– Allows stations with high priority data to request (in
frames and tokens as they are repeated) that the next
token be issued at the requested priority
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Token Ring Frame Control Field
SD AC FC
Destination
Address
Source
Address
Data
Checksum
ED FS
• FC = Frame Control Field
– Defines the type of frame being sent
– Frames may be either data frames or some type of
control frame. Example control frames:
• Beacon: Used to locate breaks in the ring
• Duplicate address test: Used to test if two stations have the
same address
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Token Ring Address & Data Fields
SD AC FC
Destination
Address
Source
Address
Data
Checksum
ED FS
• Address Fields:
– Indicate the source and destination hosts
– Broadcast:
• Set all destination address bits to 1s.
• Data
– No fixed limit on length
– Caveat: Hosts may only hold the token for a limited
amount of time (10 msec)
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Token Ring Checksum and
Frame Status
SD AC FC
Destination
Address
Source
Address
Data
Checksum
ED FS
• Checksum: 32-bit CRC
• FS = Frame Status
– Contains two bits, A and C
– When the message arrives at the destination, it sets
A=1
– When the destination copies the data in the message,
it sets C=1
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The Token Ring Monitor Station
• One station on the ring is designated as the
“monitor station”
• The monitor station:
– marks the M bit in frames and tokens
– removes marked frames and tokens from the ring
– watches for missing tokens and generates new ones
after a timeout period
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Using Priority in Token Ring
• If a host wants to send data of priority n, it may only grab
a token with priority value n or lower.
• A host may reserve a token of priority n by marking
setting the reservation bits in the AC field of a passing
token or frame
– Caveat: The host may not make the reservation if the token or
frame’s AC field already indicates a higher priority reservation
• The next token generated will have a priority equal to the
reserved priority
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• When a new token is generated (i.e., when
a sender finishes sending and releases an
idle token), or when a sender sends a data
frame, RRR is set to the lowest priority.
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Priority Transmission: Example
A
B
C
D
Host B has 1 frame of priority 3 to send to A
Host C has 1 frame of priority 2 to send to A
Host D has 1 frame of priority 4 to send to A
Token starts at host A with priority 0 and circulates
clockwise
Host C is the monitor station
(priority 0: lowest priority in this example)
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Example (cont’d)
Event
Token/Frame AC Field
A generates a token
P=0, M=0, T=0, R=0
B grabs the token and sets the
message destination to A
P=3, M=0, T=1, R=0
Frame arrives at C, and C reserves
priority level 2. Monitor bit set.
P=3, M=1, T=1, R=2
Frame arrives at D, and
D attempts to reserve priority level 4:
P=3, M=1, T=1, R=4
Frame arrives at A, and A
copies it
P=3, M=1, T=1, R=4
Frame returns to B, so B removes
it, and generates a new token
P=4, M=0, T=0, R=0
Token arrives at C, but its priority is
too high. C reserves priority 2. M bit. P=4, M=1, T=0, R=2
HMG/HUT MAC Protocols (802.x)
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Example (cont’d)
Event
Token/Frame AC Field
Token arrives at D, and D grabs
it, sending a message to A
P=4, M=0, T=1, R=0
Frame arrives at A, and A
copies it
P=4, M=0, T=1, R=0
Frame arrives at B, which does
nothing to it
P=4, M=0, T=1, R=0
Frame arrives at C, which sets the
monitor bit
P=4, M=1, T=1, R=2
Frame returns to D, so D removes
it and generates a new token with P=2 P=2, M=0, T=0, R=0
etc… Attempt to complete this scenario on your own.
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TOKEN RING Performance
• Ring Topology
• A bit pattern token (1111 1111) floats on the ring
• Station captures token, converts to connector
(11 11 1110), transmits frame
• Intermediate stations relay message/token.
• Token is released when
(a) Leading edge of frame is received, and
(b) Frame is transmitted.
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Throughput : Simple Analysis
Time required by a bit to traverse the whole ring
a = ---------------------------------------------------Frame transmission time
Number of active stations
:
N
Average time to pass token to the next station: a/N
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t0
t0+a
t0+1
t0+a+1
(a)
(b)
(c)
(d)
Fig for case 1
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Case 1 ( a<1)
(a) Frame transmission begins
(b) Leading edge received.
© Total frame is transmitted and token is released.
(d) Total frame is received.
Average time to transmit frame
S =
--------------------------------------------------(Time elapsed between a token is transmitted + Average
Token Passing Time)
1
S=
a
1+
N
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t0
t0+1
t0+a
(a)
(b)
(c)
t0+a+1
(d)
Fig for case 2 ( a > 1)
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Case 2
(a > 1)
(a) Frame transmission begins
(b) Frame transmission completed.
© Leading edge received and token is released.
(d) Total frame is received.
S =
1
a+
a
N
for a > 1
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Delay and stability
No. of stations (equally spaced)
=
Mean time for token to travel round the ring
=
Mean Token Cycle Time
=
Mean Packet Transmission Time
=
During T
• All N queues are served.
• Mean number of packets are transmitted = Q
N
R
T
X
• Token rotates (with mean value R)
T=
R+QX
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For stable system
Departures = Arrivals
Q =NλT
ρ
=
λ
X
(for one station)
T (1 − Nρ ) = R
- Token is free with probability (1-Nρ)
- Token is in use with probability Nρ
• Nρ<1
• ρ<1/N
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• Average number of packets transmitted from a queue in
T=Q/N
• In limited service (IEEE 802.5 has THT) λT < m
– m packets served per token visit
• Tagged job methodology and residual service time
analysis gives
Average waiting delay (excluding service delay),W as
[ ]
(
NρΕ x 2
1 + ρ )R
W=
+
2 + (1 − Nρ − λR ) 2(1 − Nρ − λR )
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Flavor #1: Release After
Reception (RAR)
• Computer captures token, transmits data, waits for data to
successfully travel around ring, then releases token again.
• Allows computer to detect errored frames and retransmit them.
Example time evolution in which host 1 and host 3 have packets to transmit:
TRANSP
PROP
TRANST
Data
Token
l1/c l2/c
Token arrives
at host 1
TRANST
lN/c
Data
Token
l1/c
TRANSP
l2/c
Token arrives
Token departs
at host 3
from host 1
Token arrives
at host 2
HMG/HUT MAC Protocols (802.x)
June 2004
l3/c
time
55
Efficiency of RAR
Recall: Efficiency, η, is the fraction of time spent sending useful data.
Define: Ti,j to be the time from when the token arrives at host i until
it next arrives at host j.
T1, 2 ≤ TRANSP + PROP + TRANST + l1 / c
∴T1,1 ≤ N (TRANSP + PROP + TRANST ) + ∑i li / c
= N (TRANSP + PROP + TRANST ) + PROP
N (TRANSP )
N (TRANSP + PROP + TRANST ) + PROP
1
PROP
≈
,
a=
, TRANSP >> TRANST
1+ a
TRANSP
∴η RAR ≤
HMG/HUT MAC Protocols (802.x)
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Flavor #2: Release After
Transmission (RAT)
• Computer captures token, transmits data, then releases
token again.
Example time evolution in which host 1 and host 3 have packets to transmit:
TRANSP
Data
TRANST
TRANST
Token
Token arrives
at host 1
Data
Token
l1/c
TRANSP
l2/c
Token arrives
Token departs
at host 3
from host 1
Token arrives
at host 2
HMG/HUT MAC Protocols (802.x)
June 2004
Token
time
57
Efficiency of RAT
T1, 2 ≤ TRANSP + TRANST + l1 / c
∴T1,1 ≤ N (TRANSP + TRANST ) + ∑i li / c
= N (TRANSP + TRANST ) + PROP
N (TRANSP )
N (TRANSP + TRANST ) + PROP
1
PROP
≈
,
a=
, TRANSP >> TRANST
1+ a / N
TRANSP
∴η RAT ≤
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