A.S.Tanenbaum, Computer networks, ch4 MAC 1

The Medium Access ControlSublayer

Medium Access Control: a means of controlling access to the medium to promote orderly and efficient use.

A.S.Tanenbaum, Computer networks, ch4 MAC 2

OSI Model and Project 802

A.S.Tanenbaum, Computer networks, ch4 MAC 3

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

A.S.Tanenbaum, Computer networks, ch4 MAC 4

Pure ALOHA

In pure ALOHA, frames are transmitted at completely arbitrary times.

Multiple Access Protocols

A.S.Tanenbaum, Computer networks, ch4 MAC 5

Pure ALOHA (2)

Vulnerable period for the shaded frame.

A.S.Tanenbaum, Computer networks, ch4 MAC 6

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%

Relative formulas for the ALOHA

7

Throughput orChannel Utilization

Probability of collision Probability of success

Pure ALOHASlotted ALOHA

tG GeGeS 22 tGr eecollisionP 22 11)(

tGr eeSuccessP 22)(

tG GeGeS tGr eecollisionP 11)( tG

r eeSuccessP )(

tG sec/: request slotrequestG /:

rate data

size frame

vilocity

distance fp ttt

A.S.Tanenbaum, Computer networks, ch4 MAC 8

Pure ALOHA and Slotted ALOHA

Throughput versus offered traffic for ALOHA systems.

A.S.Tanenbaum, Computer networks, ch4 MAC 9

Persistent and Nonpersistent CSMAa)All stations know that a transmission has

started almost immediatelyb)First listen for clear medium (carrier sense)c)If medium idle, transmit with a probability.d)If two stations start at the same instant,

collisione)Propagation time is much less than

transmission timef) Wait reasonable time (round trip plus ACK

contention)g)No ACK then retransmit

A.S.Tanenbaum, Computer networks, ch4 MAC 10

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various random access protocols.

A.S.Tanenbaum, Computer networks, ch4 MAC 11

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.

A.S.Tanenbaum, Computer networks, ch4 MAC 12

CSMA/CDOperation

A.S.Tanenbaum, Computer networks, ch4 MAC 13

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.

A.S.Tanenbaum, Computer networks, ch4 MAC 14

Token RingOperation

A.S.Tanenbaum, Computer networks, ch4 MAC 15

FDDI MAC ProtocolFiber Distributed Data InterfaceAs 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

A.S.Tanenbaum, Computer networks, ch4 MAC 16

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

13.17

Ethernet evolution through four generations

A.S.Tanenbaum, Computer networks, ch4 MAC 18

Ethernet Cabling

The most common kinds of Ethernet cabling.

13.19

10Base5 implementation

13.20

10Base-T implementation

A.S.Tanenbaum, Computer networks, ch4 MAC 21

Ethernet topology

Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.

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

Baseband Configuration

A.S.Tanenbaum, Computer networks, ch4 MAC 23WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

Ethernet MAC Sublayer Protocol

A.S.Tanenbaum, Computer networks, ch4 MAC 24WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

PDU Format

A.S.Tanenbaum, Computer networks, ch4 MAC 25

Minimum and maximum length

13.26

Example of an Ethernet address in hexadecimal notation

A.S.Tanenbaum, Computer networks, ch4 MAC 27

Ethernet Performance

Efficiency of Ethernet at 10 Mbps with 512-bit slot times.

A.S.Tanenbaum, Computer networks, ch4 MAC 28

Switched Ethernet

A simple example of switched Ethernet.

FAST ETHERNETFAST ETHERNET

Fast Ethernet was designed to compete with LAN Fast Ethernet was designed to compete with LAN protocols such as FDDI or Fiber Channel. IEEE protocols such as FDDI or Fiber Channel. IEEE created Fast Ethernet under the name 802.3u. Fast created Fast Ethernet under the name 802.3u. Fast Ethernet is backward-compatible with Standard Ethernet is backward-compatible with Standard Ethernet, but it can transmit data 10 times faster at a Ethernet, but it can transmit data 10 times faster at a rate of 100 Mbps. rate of 100 Mbps.

Fast Ethernet topology

Fast Ethernet implementations

13.32

GIGABIT ETHERNETGIGABIT ETHERNET

The need for an even higher data rate resulted in the The need for an even higher data rate resulted in the design of the Gigabit Ethernet protocol (1000 Mbps). design of the Gigabit Ethernet protocol (1000 Mbps). The IEEE committee calls the standard 802.3z.The IEEE committee calls the standard 802.3z.

A.S.Tanenbaum, Computer networks, ch4 MAC 33

Gigabit Ethernet

(a) A two-station Ethernet. (b) A multistation Ethernet.

A.S.Tanenbaum, Computer networks, ch4 MAC 34

Gigabit Ethernet (2)

Gigabit Ethernet - DifferencesCarrier extensionAt least 4096 bit-times long (512 for 10/100)Frame bursting extended to 200m.New coding

Summary of Ten-Gigabit Ethernet implementations

IEEE standard for LANs

A.S.Tanenbaum, Computer networks, ch4 MAC 37

IEEE 802.2: Logical Link Control

(a) Position of LLC. (b) Protocol formats.

13.38

Figure 13.2 HDLC frame compared with LLC and MAC frames