Chapter 13 and 14 – Multiple Access, Local Area Networks

Spring 2006 Computer Networks 2

A Link and Types of Links

A link is a communication channel established through some kind of physical medium (guided or unguided)

Type of links with concern to line configuration Point-to-point link

Consists of a single sender on one end and a single receiver on the other end.

Broadcast link - Multipoint Multiple sending and receiving nodes all connected to

the same shared broadcast channel When one node transmits, the channel broadcast the

frame and each of the other nodes receives a copy

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Multiple Access Protocols and LANs

In case of a broadcast link, there must be rules for sharing the channel. Otherwise no-one can “hear” no-one.

Media access protocols are protocols that resolve the multiple access problem

Broadcast links and media access protocols are used in computer networks that are local (span a single or several buildings)

LAN (Local Area Network)

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Examples for Multiple Access Channels

Sharing a room(Conversation of many persons)

Sharing the wire

Sharing satellite channel

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Media Access Control Protocols

Three types of sharing a single channel Channel partitioning or using some kind of

multiplexing techniques Multiple access by allowing contention

Collisions are allowed Recover from collision Ethernet is an example

Taking turns in accessing the channel The access is controlled by a circulating token Token ring is an example

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Evolution of Contention Protocols

Developed in 1970 to be used on radio LAN on Hawaiian islands. The access to the channel is random

Improvement to Aloha: Start transmission only at fixed time slots

Carrier Sense Multiple Access: Start transmission only if no transmission is ongoing

CD=Collision Detection: Stop ongoing transmission if collision is detected

Aloha

SlottedAloha

CSMA

CSMA/CD

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CSMA Operation

Carrier sensing the ability to detect idle/busy channel quickly

The logic followed: There is no point of transmitting if the channel is busy (another station is transmitting)

The chances for collisions are reduced The possibility of collision still exists

because of the propagation time The signal sent by a single device is sent

immediately by the closest devices. For the others to sense it, some time is needed

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Persistent vs. Non-persistent CSMA

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Normal Operation and Collisions

A

B

D

Data

C

Address mismatchpacket discarded

Address mismatchpacket discarded

Address matchpacket processed

Send datato node D

Transmitted packet seenby all stations on the LAN

(broadcast medium)The packet carries the

address of the destination

A

B C

D

Collision

Data transmission for A Data transmission for C

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CSMA/CD

Sense for carrier. If carrier present, wait until carrier ends. Send packet and sense for collision. If no collision detected, consider packet

delivered. Otherwise, abort immediately, perform

“exponential back off” and send packet again. CSMA/CD is used in traditional Ethernet LAN

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Exponential Back-off

When a sender detects a collision, it sends a “jam signal”. Jam signal is necessary to make sure that all nodes are

aware of the collision Length of the jam signal 48 bits

When collision is detected, the sender resends the signal after a random time The random time is picked from an interval of 0 to 2N x

maximum propagation time N is the number of attempted retransmission Length of the interval increases with every retransmission

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Local Area Networks (LANs)

A computer network in a limited geographical area, a single building or several close to each other buildings

LANs are privately owned and built by the companies

Generally less expensive than WAN for comparable speed

LAN technologies use multiple access channels

Ethernet is the most common LAN technology

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Traditional Ethernet

Work started back in 1973 by Bob Metcalfe and David Boggs from Xerox Palo Alto Research Center, as an improvement of the ALOHA

Experimental Ethernet implemented in 1975. Cooperative effort between Digital, Intel, and

Xerox produced Ethernet Version 1.0 in 1980. Ethernet was adopted with modifications by the

standards committees IEEE 802.3 and ANSI 8802/3.

Structure of Ethernet frame

(Length)

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Structure of Ethernet Frame Preamble:

7 bytes with pattern 10101010 followed by one byte with pattern 10101011

Used to synchronize receiver, sender clock rates

Addresses: 6 bytes, the frame is received by all adapters on a LAN and dropped if address does not match

Type: 2 bytes, is actually a length field in 802.3 CRC: 4 bytes, checked at receiver, if error is

detected, the frame is simply dropped Data payload: maximum 1500 bytes, minimum

46 bytes. If data is less than 46 bytes, pad with zeros to 46 bytes

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Annimation for Better Understanding

The following link will lead you to several annimations that explain important issues in the area of networking.

Play annimation 6.1 and 6.2 to understand how Ethernet works.

http://www.netbook.cs.purdue.edu/othrpags/page15.htm

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Network Interface Card (NIC)

Each device on Ethernet network has its own interface card (NIC) to connect to the network

The NIC is usually plugged into the device and has a 6 bytes (48 bits) physical address

The physical address is normally written in hexadecimal notation

02-11-02-2C-4D-1B (example address)

NIC for a desktop

NIC for a laptop

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Ethernet Addressing

Each station recognizes three classes of addresses. Own address Broadcast address (all 1's) Optionally, one or more multicast addresses

Major reason for broadcast is address discovery. Brodcast Ethernet address is all 1s, or in hexadecimal FF : FF : FF : FF : FF :FF

Multicast addresses are used for specialized link

layer functions. Ethernet addresses are unique

First three bytes assigned to manufacturer by IEEE, the other three bytes assigned by the manufacturer

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Classic 10Mbps Ethernet

Four different implementation at the physical layer for the baseband 10Mbps Ethernet Thick Ethernet (10base5) – obsolete

Thick coaxial cable (0.5” diameter) 500meter max length, bus physical topology

Thin Ethernet (10base2 802.3a) - obsolete RG58 coaxial cable 185 meter max length, bus physical topology

Twisted Pair Ethernet (10baseT 802.3i) 4 pair UTP (unshielded twisted pair) cable 100 meter max length, star physical topology

Fiber-link Ethernet (10Base-FL) Fiber cable connected to external transceiver Star topology is used

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Physical Layer of the Ethernet

PLS (Physical Layer Signaling) encodes and decodes data Ethernet uses Manchester encoding

AUI (Attachment Unit Interface) – interface between PLS and medium dependent interface

MAU (Medium Attachment Unit) or transceiver MDI (Medium Dependent Interface) is a piece

of hardware connecting the transceiver to the medium

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10Base5 (Thick Ethernet)

transceiver

Resistor terminator

Resistor terminator Ethernet medium (cable)

Drop cable

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10Base2 (Thin Ethernet)

Terminator

BNC T connector

BNC connector

To next equipment or terminator

Thin coaxial cable

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10BaseT (Twisted Pair Ethernet)

High-Speed Backplane or Interconnection fabric

hubswitch

The central device can be a hub or a switch

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Hub Concept

Separate transmit and receive pair of wires.

The hub retransmits the signal received on any input pair onto all output pairs.

Essentially the hub emulates a broadcast channel with collisions detected by receiving nodes.

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Ethernet Evolution

Introducing bridges Unlike a hub, a bridge is capable of filtering frames Each port of the bridge is connected to a single segment

of LAN Capable of learning which the stations are connected to

which ports Separates collision domains and therefore increases

bandwidth

Introducing switches Similar function as bridges Contain bigger number of ports A single device can be attached to a port

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Bridged vs. Switched Ethernet

Bridge

A B C D E F

Switch

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Fast Ethernet

Go from 10mbit/s to 100mbit/s 3 competing standards:

100Base-TX 100Base-T4 100VG-Anylan

100Base-T4 and 100VG-Anylan are the losers (were not very well accepted).

100Base TX is the winner. It is almost a standard everywhere.

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100Base - TX

100 Mbps over 2 pairs of wire (just like 10base-T)

Requires Category 5 UTP wiring or STP De facto standard today Very small price difference with

10Mbps-only equipment Has clearly won over 100baseT4 and

100VG-Anylan by now

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100Base-FX

Fast Ethernet with fiber optic cables Uses two optical fibers, one for

transmission and one for reception

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Gigabit Ethernet

Provides speeds of 1000 Mbps (i.e., one billion bits per second capacity) for half-duplex and full-duplex operation.

Uses Ethernet frame format and MAC technology CSMA/CD access method Backward compatible with 10Base-

T,100Base-T and 100BaseTX

Can be shared (hub) or switched

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Gigabit Ethernet Implementations

Fiber 1000 Base – SX

Short wavelengths, two fiber-optic cables

1000 Base – LX Long wavelengths, two fiber-optic cables

Copper 1000 Base – CX

Uses shielded twisted pair copper jumpers

1000 Base – TX Uses category 5 twisted pair copper cable

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1000Base - T

Four pairs of Category 5 UTP IEEE 802.3ab ratified in June 1999. Category 5, 6 and 7 copper up to 100

meters Uses encoding scheme 4D-PAM5 Five level of pulse amplitude

modulation are used Complicated technique