Transcript 4). Medium Access Control

The Medium Access Control
Sublayer
Medium Access Control: a means of
controlling access to the medium to
promote orderly and efficient use.
A.S.Tanenbaum, Computer networks,
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OSI Model and Project 802
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The Channel Allocation Problem
•
•
Static Channel Allocation in LANs and MANs
FDM: small and constant users, heavy load of
traffic of each.
TDM:same problem. Poor performance.
None of the static channel allocation methods work
well with bursty traffic.
Dynamic Channel Allocation in LANs and MANs
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Multiple Access Protocols
Pure ALOHA
In pure ALOHA, frames are transmitted at completely arbitrary times.
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Pure ALOHA (2)
Vulnerable period for the shaded frame.
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Slotted ALOHA
a) Time in uniform slots equal to frame
transmission time
b) Need central clock (or other sync
mechanism)
c) Transmission begins at slot boundary
d) Frames either miss or overlap totally
e) Max utilization 36.8%
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Relative formulas for the ALOHA
Throughput or
Channel
Utilization
Probability of collision
Probability of
success
Pure
2G
2t
2G
2t
P
(
Success
)

e

e
2G
2t
r
Pr (collision)  1  e
 1 e
ALO S  Ge  Ge
HA
Slott
ed
G
 t P (collision)  1  e G  1  e t
r
Pr (Success)  eG  et
S  Ge  Ge
ALO
HA
G  t
 : request/ sec
G : request/ slot
distance framesize
t  tp  tf 

vilocity data rate
7
Pure ALOHA and Slotted ALOHA
Throughput versus offered traffic for ALOHA systems.
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Persistent and Nonpersistent CSMA
a)All stations know that a transmission has
started almost immediately
b)First listen for clear medium (carrier sense)
c)If medium idle, transmit with a probability.
d)If two stations start at the same instant,
collision
e)Propagation time is much less than
transmission time
f) Wait reasonable time (round trip plus ACK
contention)
g)No ACK then retransmit
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Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various
random
access
protocols.
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Computer
networks,
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CSMA/CD (with collision detection)
a) If collision detected, jam then cease
transmission rather than finish transmitting
their frame
b) After jam, wait random time then start again
c) Half-duplex system
d) Save time and bandwidth.
e) Basis of Ethernet LAN.
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CSMA/CD
Operation
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Token Ring (802.5)
MAC protocol
–
–
–
–
–
–
–
–
Small frame (token) circulates when idle
Station waits for token
Changes one bit in token to make it SOF for data frame
Append rest of data frame
Frame makes round trip and is absorbed by transmitting
station
Station then inserts new token when transmission has
finished and leading edge of returning frame arrives
Under light loads, some inefficiency
Under heavy loads, round robin makes efficiency and
fair.
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Token Ring
Operation
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FDDI MAC Protocol
Fiber Distributed Data Interface
As for 802.5 except:
–
Station seizes token by aborting token
transmission
–
Once token captured, one or more data frames
transmitted
–
New token released as soon as transmission
finished
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Ethernet
•
•
•
•
•
•
•
•
•
Ethernet Cabling
Manchester Encoding
The Ethernet MAC Sublayer Protocol
The Binary Exponential Backoff Algorithm
Ethernet Performance
Switched Ethernet
Fast Ethernet
Gigabit Ethernet
IEEE 802.2: Logical Link Control
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Ethernet evolution through four generations
13.17
Ethernet Cabling
The most common
kinds
of networks,
Ethernet cabling.
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Computer
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10Base5 implementation
13.19
10Base-T implementation
13.20
Ethernet topology
Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.
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Baseband Configuration
a) The size limitation is
usually solved by using
repeaters to divide the
medium into smaller
segments
b) Repeaters relay digital
signals in both
directions, making the
segments appear like
one medium
c) As repeaters recover the
digital signal, they
remove any attenuation
Ethernet MAC Sublayer Protocol
WCB/McGraw-Hill
A.S.Tanenbaum, Computer networks,
 The McGraw-Hill Companies, Inc., 1998
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PDU Format
WCB/McGraw-Hill
A.S.Tanenbaum, Computer networks,
 The McGraw-Hill Companies, Inc., 1998
ch4 MAC
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Minimum and maximum length
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Example of an Ethernet address in hexadecimal notation
13.26
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times.
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Switched Ethernet
A simple example of switched Ethernet.
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FAST ETHERNET
Fast Ethernet was designed to compete with LAN
protocols such as FDDI or Fiber Channel. IEEE
created Fast Ethernet under the name 802.3u. Fast
Ethernet is backward-compatible with Standard
Ethernet, but it can transmit data 10 times faster at a
rate of 100 Mbps.
Fast Ethernet topology
Fast Ethernet implementations
GIGABIT ETHERNET
The need for an even higher data rate resulted in the
design of the Gigabit Ethernet protocol (1000 Mbps).
The IEEE committee calls the standard 802.3z.
13.32
Gigabit Ethernet
(a) A two-station Ethernet. (b) A multistation Ethernet.
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Gigabit Ethernet (2)
Gigabit Ethernet - Differences
Carrier extension
At least 4096 bit-times long (512 for 10/100)
Frame bursting extended to 200m.
New coding
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Summary of Ten-Gigabit Ethernet implementations
IEEE standard for LANs
IEEE 802.2: Logical Link Control
(a) Position of LLC. (b) Protocol formats.
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Figure 13.2 HDLC frame compared with LLC and MAC frames
13.38