department of computer science, university of peshawar multiple access

Department of Computer Science, University of Peshawar

MultipleAccess


Multiple-access protocols


Random AccessRandom Access

MA

CSMA

CSMA/CD

CSMA/CA


Evolution of random-access methods


ALOHA

The earliest random-access method Developed at the University of Hawaii in

early 1970’s Used on a wireless LAN with a data rate

9600 bits/sec A base station is the central controller The base station receives the frame and

relays it to the intended destination The uploading transmission uses carrier

frequency of 407 MHz The downloading uses carrier freq of 413

MHz


ALOHA network


Procedure for ALOHA protocol


Back off time

The station waits an amount of time b/w 0 and 2N *(maximum_propagation_time)

For the first time between: 0 and 21 *(max_propg_time)

For the second time between: 0 and 22 (max_propag_time), and so on

N is the number of attempted transmissions


CSMA

Carrier Sense Multiple Access To minimize the chance of collision Based on the principle “sense before

transmit” Can reduce the possibility of

collision, but can not eliminate it Collision because of the propagation

delay


Collision in CSMA


Persistent Strategy

The procedure for a station that senses a busy medium

Two substrategies Nonpersistent Persistent


Persistence strategies


Persistence strategies cont… In 1-persistent, the station sends its frame

immediately, when finds the line idle It increase the chance of collision due to

same time sending In P-persistent, the station may or may not

send, when finds the line idle The system generates a random number

b/w 1 and 100 If the number is less than 20, then sends,

otherwise not


CSMA/CD

Carrier Sense Multiple Access with collision detection

The station monitors the medium after sending a frame

If successful, the station is finished If collision, the frame is resent Used in Ethernet


CSMA/CD procedure


CSMA/CA

Carrier Sense Multiple Access with collision avoidance

No chance of collision Used in Wireless LANs


CSMA/CA procedure


Control AccessControl Access

Reservation

Polling

Token Passing


Reservation access method


Polling

One primary station, other devices are secondary stations

All data transmission must be made through the primary device

The secondary devices follow its instructions

In case of receiving the data, the primary device does polling

In case of sending the data, the primary device does selection


Select


Poll


Token passing

A station is authorized to send data when it receives a special frame called token

When no data are being sent, a token is circulating around the ring.

If a station needs to send data, it waits for the token


Token-passing network


Token-passing procedure


Local Area Networks:Ethernet


History of Ethernet

Developed by Xerox’s Palo Alto Research Center (PARC) in 1976

IEEE 802.3 standard


TraditionalTraditional EthernetEthernet

Designed to operate at 10 Mbps

Access Method CSMA/CD

Ethernet frame contains 7 fields


802.3 MAC frame


802.3 MAC frame cont…

Preamble 7 bytes (56 bits) alternating 0’s and 1’s alert the receiver to the coming frame

SFD 1 byte (10101011) Shows beginning of the frame Last 2 bits 11 alert the receiver that the next field is the DA

DA 6 bytes Contains the physical address of the receiver

SA 6 bytes Contains the physical address of the sender


802.3 MAC frame cont…

Length/type Defines the length of the Data field

Data Caries the data, min of 46 and max

1500 bytes CRC

Contains error detection information


Minimum and maximum length


Ethernet addresses in hexadecimal notation


Categories of traditional Ethernet


Physical layer


PLS


Fast EthernetFast Ethernet

Operates at 100 MbpsPhysical Layer Implementation

RS (Reconciliation Sublayer) responsible for passing of 4 bit data to MII

MII (Medium Independent Interface) AUI was replaced with MII

PHY (Transceiver) Responsible for encoding and decoding


Fast Ethernet implementations


Fast Ethernet physical layer


Gigabit EthernetGigabit Ethernet

Operates at 1000 Mbps

Physical Layer Implementation RS

responsible for passing of 8 bit data to GMII

GMII Connects RS to the PHY

PHY (Transceiver) Encodes and decodes the data


Gigabit Ethernet implementations


Physical layer in Gigabit Ethernet


Thank you!

department of computer science, university of peshawar multiple access

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