data link layer issues 1

8/2/2019 Data Link Layer Issues 1

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Data Link Layer IssuesDealing with Different Types of Networks


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Types of Networks Network hardware can be categorized into:

Circuit-switched (e.g. telephone)

Prior to communication, the hardware establishes a dedicated

end-to-end connection Since there is a dedicated connection, a continuous stream of bytes

can be sent

Frequency or time-division multiplexing can be used to sharelinks in such a network

Packet-switched (e.g. Ethernet, ATM) Data is divided into packets of limited size, and each is

forwarded through the network to the destination

This can be done by routers or switches


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Types of Networks Disadvantages

Circuit-switched

A dedicated connection that has no transmission means wasted

bandwidth

A connection is time consuming if short, infrequent, or sporadic

communication is to occur

Packet-switched

Forwarding each packet means that each router must decide thenext hop for every packet (even for the same destination)

Routers are typically network slowdowns due to the amount of

processing, as well as input/output buffering


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Types of Networks Circuit-switching is used in a telephone conversation

A connection to the receiver is established by the sender (the caller)

The telephone company reserves a certain bandwidth (64 Kbps for voicecommunication) for this call

If the bandwidth is not used by the callers, it is wasted

Packet-switching is similar to the postal service Each message (envelope) is addressed to the recipient individually,

and the postal service delivers each message to the recipient

The postal service may deliver these envelopes through different cities

and methods of transport (airplane, truck, ) It can be said that these messages can be delivered using different routes


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Circuit-Switching

A BTelephone Company

Switching System

Call: B

Disconnect

TalkTalk


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Circuit-Switching

A BTelephone Company

Switching System


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Packet-Switching

A B

Postal Network

Buffalo, NYWindsor, ON

London, ONKitchener, ON

Toronto, ON

Ottawa, ON

Montreal, QC

Quebec, QC

Niagara Falls, ON


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Types of Packet-Switching Virtual circuit-switching

A virtual circuit is created between source and

destination This VC is used for all subsequent sending of

packets

Datagram

Each packet is routed individually


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Virtual Circuit Packet-SwitchingAdvantages After the first message, routing is faster

A route must only be determined once, for the first message

Once the route has been determined, the path used by the router isreused for all messages

As a result, routing tables are much smaller (and can be searchedmore quickly)

Because a connection is created, the connection identifier can

be used (alone) to address packets Typically, such as with ATM cells, this can reduce the size of a

cell/packets header

Messages do not arrive out of order As a result, receivers do not need to reorder the cells


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Virtual Circuit Packet-SwitchingDisadvantages Connections take some time to create

Routers/switches must intercommunicate in order to create theconnection

Infrequent messaging is not suitable for connection-basedmessaging The connection may be lost after a timeout, and will have to be

recreated again and again

The time delay for creating the connection may outweigh the speedbenefits of using connection-based transport

Routing tables will be dynamic, and routing algorithms aremore complex


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Datagram Packet-SwitchingAdvantages

Connections need not be created

Infrequent messaging is perfect for connectionless messaging

Connectionless messaging can be resumed after any amount of delay,any number of times, without any delays due to the resumption ofcommunication

Routing each message separately allows for load balancing Some messages may be sent through one route, but when that route

becomes saturated, messages may then be sent through a differentroute in order to achieve the most optimal communication possible


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Datagram Packet-Switching

Disadvantages

Each message takes a certain amount of time to transmit

(including transmission, routing, reception, etc.)

Nodes communicating large amounts of information in a short timewill:

Use a lot of bandwidth for things such as header information

Waste a lot of time routing messages to the same destination

Messages may arrive out of order

Messages must be reordered by the recipient


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Multiple Access StrategiesSchemes for Sharing a CommunicationMedium


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Multiple Access

Most networks are shared medium

This means that a single medium (e.g. radio frequency) is

shared by all of a networks hosts

We need a scheme to allow the hosts to share the

medium, without collisions

Collisions occur when two (or more) messages are

transmitted at the same time The result is constructive and destructive interference in

the carrier wave

This causes the messages to be combined and scrambled


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Contention

In contention networks, any node that has a

packet to send, merely sends the packet

It is clear that this type of network frequentlyexperiences collisions

The more nodes trying to communicate, the

higher the chance of collisions Thus, contention networks are severely limited in

the number of hosts possible


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Contention

Transmit


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Contention


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Contention

Transmit


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Contention


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Contention: Collisions

Transmit

Transmit


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Contention: CollisionsScrambled

Signal


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Contention

No collision avoidance is present

Messages are just sent

When collisions occur, the messages are simplyresent after some random (or pseudo-random)

amount of time

Collisions can occur anytime


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Carrier Sensing

Test the medium

for a signal


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Carrier Sensing

Test the medium for a signal: Available

Transmit


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Carrier Sensing

Test the medium

for a signal


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Carrier Sensing

Test the medium

for a signal: In use


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Carrier Sensing

Transmission

Complete


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Carrier Sensing

Test the medium

for a signal


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Carrier Sensing

Test the medium

for a signal: Available


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Carrier Sensing: Collisions

Test the medium


Test the medium



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Transmit

Transmit


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Scrambled data


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Detect collision

Detect collisionTransmit

Transmit


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Carrier Sensing (CSMA)

To reduce the number of collisions, the medium istested for a signal before each transmission

If a signal exists, the node waits

Signal testing can be anything from detection of anelectrical signal, to testing for photons

Collisions can still occur (although less often)

If a node tests for a signal before a transmission from

another node, and transmits after, a collision occurs


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Carrier Sensing Hardware

Signal

Detector

Transmitter ReceiverIf the message is

broadcast or the

address is this

stations address,

the message is

forwarded to the

receiver

When a signal is

detected,

transmissions are

blocked by the

signal detector


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CSMA/CA

CSMA/CA networks (such as wireless 802.11g) also

use carrier sensing and collision detect

However, detecting collisions in wireless networks is

significantly more complicated

Also, after detecting carrier and determining there is

no signal, a CSMA/CA network transmits a Do not

broadcast message If this message is sent without a collision, the host can

assume it is safe to transmit


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Carrier Sensing Networks

Advantages

No tokens

Simple hardware

No need for token transmission

Disadvantages

Collisions

Wasted bandwidth for re-transmits

Require complicated re-collision avoidance schemes


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Token Passing

T

TransmitTransmitTransmitTransmit


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Token Passing

T

Transfer

Token


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Token Passing

T


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Token Passing

T

Transmit


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Token Passing

A small packet (the token) is passed fromnode to node

When a node has the token, it has sole use of thenetwork medium

There are no collisions

The nodes must have the token in order to

transmit The network hardware ensures that there is only

one token at any given time


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Token-Based Networks

Advantages

No collisions, so no bandwidth is wasted bycollisions and re-transmits

No need for re-collision avoidance schemes

Disadvantages

Token transmission uses bandwidth

More complicated hardware Hardware must be built to use tokens, dynamically

determine token sequence, etc.


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Local Area Networks

Networks which span a small geographic area

They typically represent high bandwidth,short delays, few errors

They commonly support features such asbroadcasting, multicasting

They are typically limited to hundreds of

network nodes (maximum)


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Typical Local Area Networks

A collection of computers in the same room

e.g. The basement of the computer centre

All computers within an office building e.g. The computers in the offices of the professors

and staff in Lambton tower


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Local Area Network

TopologiesStructures of LANs


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Token Bus Networks

The token is passed in a specific sequence

Nodes must know the address if the next node in thesequence

The token sequence is not necessarily in the same order asthe physical order of nodes on the communicationmedium

When a node has completed transmission, itforwards the token, addressed to the next node in thetoken sequence

The token sequence forms a logical ring


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Common Token Bus Networks

IEEE 802.4 networks

Nodes are share a communication medium

similar to that of Ethernet (IEEE 802.3)

Coaxial cable connection


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Token Bus Operation

A

B

C

D

Token sequence: C,A,D,B

Transmit


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Token Bus Operation

A

B

C

D


Transmit

Token


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Token Bus Operation

A

B

C

D


Receive

Token


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Token Bus Operation

A

B

C

D


Transmit


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Token Bus Operation

A

B

C

D


Transmit

Token


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Token Bus Operation

A

B

C

D


Receive

Token


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Token Bus Operation

A

B

C

D


Transmit


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Token Ring Networks

The token is passed to each node, in the

physical order on the network

The physical medium must be a closed loop tomeet this network category

So the token can keep going around the network


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Common Token Ring Networks

IEEE 802.5 networks

Nodes are share a coaxial communication medium similarto that of Ethernet (IEEE 802.3)

FDDI networks (fibre distributed data interface) Nodes use 2 fibre optic rings as the communication

medium

CDDI networks (copper dist. data interface)

Based on FDDI technology, but uses copper wiringsimilar to 802.4

However, CDDI uses 2 rings like FDDI


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Token Ring Operation

D

C

B

A

Transmit


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D

C

B

A

Transmit

Token


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D

C

B

A

Receive

Token


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D

C

B

A

Transmit


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D

C

B

A

Transmit

Token


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D

C

B

A

ReceiveToken


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D

C

B

A

Transmit


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Bus and Ring Networks

Advantages

Less wiring is necessary

Disadvantages Node failure can mean partial (or complete)

LAN failure

This can mean locating network problems is alsomore difficult


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Star Topology

Star networks send all messages through a

central hub

Each node on the network is wired separatelyto the hub

Star networks are not a shared bus

technology, but a private bus technology However, nodes still share the hub


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Common Star Networks

Twister pair Ethernet (logical star):

All nodes connect to a central hub (an Ethernethub) via Cat5 cables

The hub forwards messages to all wires, and thedestination node keeps the message

Other nodes ignore the message

An Ethernet switch (similar to an ATM switch)forwards only in the one correct direction (or not,if appropriate)


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Star Network Operation

A B

C D

Hub

Transmit


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A B

C D

Hub

Receive


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A B

C D

Hub

Transmit


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A B

C D

Hub

Receive


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Twisted Pair Ethernet

Physically, all Ethernet types are bus

networks

However, the actual layout of the cables intwisted pair Ethernet forms a star topology

Twisted pair is called a logical star topology,

while still a physical bus topology


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Twisted Pair Ethernet as a Bus

Hub

B C

F G

A D

E H

Long Private Lines

Short Shared Bus


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Traditional Ethernet as a Bus

B C

F G

A D

E H

Long Shared Bus

Short Private Lines


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Star TopologyAdvantages

Simple installation and wiring

Node failures do not affect the rest of the system

Disadvantages All traffic passes through same hub, so network bandwidth is

limited by hub speed This can be reduced with buffers inside hubs which store messages

that come in when the hub is busy

Hub failure = LAN failure More wiring

Duplication of messages


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LAN Service Models

In general, most LANs implement (in some sense)the OSI reference model

The IEEE committee on LAN technology (IEEE

802) chose to subdivide the Data Link Layer into 2sub-layers:

1. MAC (Medium Access Control): Deals with issuesspecific to each type of LAN Such as token passing, collision detection, error detection, etc.

2. LLC (Logical Link Control): Deals with issues commonto all LAN types Such as data transmission, etc.


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Data Link Addressing

The data link layer is represents the network

e.g. Ethernet

Addressing, then, is specific to the network hardware

MAC addresses are typically used for this purpose

These addresses are not used in routing

They are only used on a single network

Thus, they are used for hop to hop delivery End-to-end delivery is the domain of the Network layer


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MAC Addresses

Officially the IEEE 802 committee standardizedaddresses to be 16bit, 48bit, and even 60bit

48bit addresses (in use by most LANs covered by the 802

committee) allow for globally unique identifiers (GUIDs)to be assigned to each network card by the manufacturer

As a result, each NIC can be uniquely identified on any network

These are called MAC addresses, due to the DataLink sub-layer that deals with them

e.g. 8D-F0-A6-75-9C-13


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Data Link Flow Control

Flow control is limiting the packet rate so that

both the source or destination can keep up

At the data link layer, source and destinationare on the same LAN

Thus, limiting the packet rate is relatively easy


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Data Link Reliability Reliability:

Best effort: The network takes no steps to ensure packetsarrive The majority of packets should be received without problems

Reliable: The network uses acknowledgements to ensurepackets arrive When packets are lost (for whatever reason), they are handled

appropriately

Error handling: Corrupt packets should be re-sent

Reliability at the Data Link layer is usuallyunnecessary, since the Transport layer will typicallybe able to do it more efficiently


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Error Control

Error control is achieved using one of thefollowing methods:

Checksum: An n-bit sum is taken of thebinary stream

In other words, a checksum counts the ones

What if one 0 became a 1 and a 1 became a 0??

Cyclical redundancy check: Should generate different CRC values, despite the

same number of 0s and 1s


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EthernetAn Early Incarnation of LANs


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What Started It All

Robert Metcalfe (from Xerox PARC)
http://www.ots.utexas.edu/ethernet/metcalfe-drawing.html


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Ethernet History

In 1973, Xerox PARC developed a packet-switchedLAN, called Ethernet

In 1978, IEEE created a standard (802.3) based on

the research of Xerox, Intel, and DEC IEEE: Institute of Electrical and Electronics Engineers

802.3 Ethernet uses a coaxial cable to connect nodes(called 10Base5 or ThickNet)

Since then, several new forms of Ethernet haveevolved


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ThickNet (10Base5)

Outer Insulating Jacket

Braided

Metal

Shield

(Ground)

Inner

Insulating

Layer

Transmission

Wire

Inch Diameter

10Base5

5 => 0.5


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ThickNet (10Base5)

Transceiver

Each network node uses

a transceiverA transceiver taps into

the wire through holes

Maximum throughput is

10 million bits per second

(10 Mbps)

10Base5

10 => 10 Mbps


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ThinNet (10Base2)

Create as an inexpensive alternative to ThickNet (or

10Base2)

Called thin-wire Ethernet, because it uses a thin

cable with less shielding

Less shielding means more interference, so cable

placement is important

10Base2 does not use transceivers, which areexpensive, which further reduces cost


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ThinNet (10Base2)

Node A Node DNode B Node C

The signal passes through each node

The network interface card (NIC) retransmitsthe signal, so transceivers are not required

Maximum throughput is 10 million bits per

second (10 Mbps)

10Base2

2 => 0.2

10Base2

10 => 10 Mbps


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Twisted Pair Ethernet (10BaseT)

Uses 4 pairs of twisted wires inside an

unshielded cable

The twisting of the wires reduces interference The absence of shielding makes the cable

flexible and inexpensive

The cable is capable of 10Mbps


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Connectors on twisted pair Ethernet (RJ45) looksimilar to telephone wire connectors (RJ11)

This kind of Ethernet uses unshielded twisted pair

(UTP) UTP cable has various categories:

Cat3: Can only be used for 10BaseT

Cat5: Can be used for 10BaseT, 100BaseT

Cat5e, Cat6: Can be used for up to 1000BaseT


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ThinNet Ethernet

011100110011100110


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acceptmessage

ignoreignore

ignore

011100110
http://www.circuitcity.com/ccd/%20%20/ssm/Netgear-5-Port-Copper-Ethernet-Switch-GS105NA-/sem/rpsm/oid/97241/rpem/ccd/productDetail.do


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10 Mbps Ethernet Overview

10Base2 and 10Base5 both used coaxial cable

which joined each node in a line

10BaseT uses UTP cabling, where each nodeis directly connected with the hub

The hub receives messages and forwards them to

all nodes

The one that is connected to the recipient node


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Fast Ethernet

Using the same Cat5 cabling used for 10BaseT, an

Ethernet-based LAN that operates at 100 Mbps

(100BaseT) is possible

Standard: IEEE 802.3u

While using the same cable, network hubs and

network interface cards (NICs) must be upgraded to

transmit messages at 100 Mbps


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Fast Ethernet

While very few computers can handle 100 Mbpsthroughput (bus speeds of computers are oftenslower than this), multiple computers can share this

bandwidth 10/100 Ethernet (or 10/100 switched Ethernet)

allows you to use the same NICs and hubs for both10BaseT and 100BaseT

If a NIC and hub can both handle 100BaseT, that speed isused, otherwise 10BaseT is used

10/100 Ethernet allows you to slowly upgrade yournetwork with minimal downtime

Gi bi E h


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Gigabit Ethernet

Gigabit Ethernet allows for 1000 Mbps throughput

Gigabit Ethernet (Gig-E) can use Cat5 cabling(1000BaseT) or shielded Cat5E cabling (1000BaseTX) Standard: IEEE 802.3ab

Gig-E pushes the limits of the speed capable with Cat5cabling, due to interference with the electrical signal,Cat5E cabling results in better performance

Gigabit Ethernet is so fast, that it is sometimes used as

a backbone for a Wide Area Network (WAN) insteadof more expensive optical networks e.g. One of the backbones of the network here at the U


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Ethernet Future

Another form of Gigabit Ethernet which uses fibre

optic cabling has been proposed (802.3z)

Using multimode (multiple channel1000BaseSX), orsingle mode (1000BaseLH, 1000BaseZX)

Research groups are in the process of developing

10 Gigabit Ethernet (802.3ae)

This research is managed by the 10 GigabitEthernet Alliance

http://www.10gea.org


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LAN Service Models

LLC (Logical Link Control), for LANs, canbe one of two types:

Type 1: A straight datagram scheme

The packet is delivered using best-effort service

No acknowledgements are used to ensure packetarrival

Type 2: A reliable scheme Packets are numbered

Packets are acknowledged as they are received


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IEEE 802 Committees

Five 802 committees were developed to researchvarious technologies associated with LANs:

802.1: Issues common to all LANs e.g. addressing, management, bridges

802.2: Issues related to the LLC sub-layer e.g. reliability schemes, packet transmission

802.3: Issues related to CSMA/CD category LANs e.g. Ethernet

802.4: Issues related to token bus category LANs

802.5: Issues related to token ring category LANs


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LAN Addresses

The 48 bit addresses (often called MAC

addresses) are the ones used by Ethernet

LANs

e.g. 02-60-8C-08-E1-0C

All Ethernet cards contain a globally unique

MAC address


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Ethernet Overview

Ethernet is not a reliable service

There are no acknowledgements for packet receipt

Ethernet uses best-effort delivery

Most Ethernet networks use broadcasting to achievemessaging

Each message is received by each node

Ethernet is one network in a category of networks

known as shared bus networks Each node shares a single communication medium


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Ethernet Overview

Ethernet is a carrier-sensing network

Carrier-sensing networks use distributed accesscontrol methods

Each station determines whether it can access thecommunication medium

Each station senses whether or not thetransmission medium (wire) is charged

If not, an attempt at transmission is made If so, the node will wait and sense again


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Ethernet Overview

Sometimes, more than one station will attempt totransmit at roughly the same time

This is called a collision

Due to the finite speed of electrons traversing a wire 70% of the speed of light

Or due to the finite speed of photons moving throughglass The speed of light

The two (or more) messages collide or interfere with oneanother, creating scrambled data packets


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Collision Detection in Ethernet

When scrambled messages are read by thetransmitting stations, it is determined to be acollision

Both (or all) of the stations involved will detectthe collision

This type of network is known as CSMA/CD

Carrier-sensing, multiple access with collision

detection

Each station must retransmit their packets


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Collision Avoidance in Ethernet

After a collision occurs, if both stations tried

to transmit after the same period of time,

another collision would occur

To combat this, Ethernet uses a binary

exponential back-off policy

Each subsequent collision would cause the station

to wait double the amount of time before

reattempting transmission


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Ethernet Packets (Frames)

Size: 64 octets1518 octets

An octet is another term for an 8-bit byte

The frame contains more than just data The source and destination addresses

An identifier, signifying that the frame is in fact

an Ethernet frame

A Cyclical Redundancy Check (CRC) to ensure

data integrity upon arrival


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Ethernet Frames

Preamble

Dest Address

Source Address

Frame Type

DataCRC

Sequence of 01010101 used tosynchronize the receiving station

The MAC address of thedestination node

The MAC address of the sendernode

The identifier used to identify theframe as an Ethernet frame

The data to be sent to thedestination

A cyclical redundancy check (CRC)used to determine if data hasbeen corrupted

8 octets

6 octets

6 octets

2 octets

46-1500

4 octets


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Ethernet Distance Limitations

Coaxial Ethernet cables have a maximum length

Due to signal deterioration

This length could be extended using repeaters

Machines that read signals through a port and recreatethem (at full strength) out another port

The use of more than 2 repeaters between any 2 stationswould interfere with times used in CSMA/CD schemes As a result, a maximum of 2 repeaters can be placed between any

2 nodes


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Ethernet Distance Limitations

Ethernet LAN sizes could also be increased by using

Bridges to connect separate LANs into a single LAN

Bridges filter out erroneous frames, as well as line noise

Some bridges (adaptive bridges) are even intelligentenough to know when a frame must be forwarded or not

e.g. If the destination node is not on the other side of a Bridge,

the frame need not be forwarded


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FDDI

Fiber Distributed Data Interconnect


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FDDI

Use optical fibre cabling as a shared communicationmedium

Optical fibre cables are made of glass

Because they are so thin, they are fairly flexible Capable of 100 Mbps

Light is used to transmit data

Light is not susceptible to electrical interference

Optical cabling can span longer distances Optical cabling does not need to be shielded near devices which

generate electromagnetic interference

Light waves (photons) travel faster than electrons


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FDDI

Is a token-ring category network

A token is passed from station to station

When a station receives the token, it may transmit data

If a station has no data, it allows the token to pass to the next

station

FDDI uses 2 rings of cabling, moving in oppositedirections

The second ring is used to allow twice the flow of data

The purpose of the second ring is to allow data to reach itsdestination, even when one station has failed (and cannotforward messages)


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FDDI Ring Technology


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FDDI With Node Failure


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FDDI Token Passing

11

12

10 9 8 7

6

5

1 2 3 4

T

S:12D:07

S:12D:07

S:12D:07

S:12D:07

S:12D:07

S:12D:07

S:12D:07

S:12D:07

S:12D:07

S:12D:07

S:12D:07

S:12D:07

S:12D:07

S:12D:07


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FDDI Token Passing

11

12

10 9 8 7

6

5

1 2 3 4T

T


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FDDI Frames

Preamble

Start Delimiter

Frame Control

Dest AddressSource Address

Routing Info

Data

FCSEnd Delimiter

Frame Status

Data Used to Synchronize Stations

Indicates Start of Frame

Identifies the Type of Frame

Address of the Destination NodeAddress of the Source Node

Routing Information

Frame Data

Frame Check SequenceIndicates End of Frame

Status of Frame

octets: 2+

1

1

2 or 62 or 6

0-30

0-4500

40.5

1.5+


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Wireless Networks

Radio-Based LANs


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Wireless LANs

Contrary to ones initial guess, wireless LANs

are very similar to wired LANs

Wireless LANs are a shared media network,

just like Ethernet

However, in a wireless LAN, the shared medium

is not the air, but something called a base station

or wireless access point


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Wireless LANs (WLANs)

The wireless access point, which is similar to a hub,is the shared medium

Despite the fact that radio waves using the samefrequency will cause mutual interference, the air is not

generally considered a shared medium

Technically speaking, twisted pair Ethernet is similarto WLANs

The cables themselves are just point-to-point connectors

and are not shared

The hub/switch, however, is shared


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Wireless LANs (WLAN) Wireless Access Point (WAP): A base station that

coordinates transmission between one or more wireless hosts Analogous to a cell tower in a mobile phone network

Wireless hosts must be a certain distance away from a WAP to

participate on a WLAN The communicable area of all of the WAPs in a WLAN, define the

coverage area for the WLAN

Some WLANs do without a WAP, but pass messages directlyto one another

These are typically small (2-3 hosts) networks, and are called ad hocnetworks


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802.11 Operation 802.11 networks (such as 802.11g) use CSMA/CA multiple

access scheme Hosts try to detect carrier before sending (CS)

This is not adequate, since there could be hidden hosts

These are hosts out of range of this host, but in range of the same basestation:


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802.11 Operation

To avoid collisions with hidden hosts:

The host will send a request to send (RTS) frame beforetransmitting

The base station will respond with a clear to send (CTS)frame if the channel is clear Once a base station sends a CTS, it will reject any further RTS

requests until the data is received by the host who sent the firstRTS

This is called collision avoidance (CA)

Frames are acknowledged at the data link layer in802.11 networks


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802.11 Frame Format

Flags

MAC Address of sending host

MAC Address of receiving host

MAC Address of sender base stationFragment number, sequence number

MAC Address of receiver base station

Frame data

CRC for frame header and data

Frame Control (2 octets)

Source Address (6)

Destination Address (6)

Receiving Station Address (6)

Transmitting Station Address (6)

Sequence Control (2)

Data (0-2312)

Frame Check Sequence (2)


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802.11 Frame Header: Frame Control

Flags

Management, control or data frame

Type of management or control frame

Sent to an access point?Sent by an access point?

Protocol Version (2 bits)

Type (2)

Subtype (4)

To AP (1)

More Fragments (1)

From AP (1)

Order (1)

Retry (1)

Power Management (1)

More Data (1)

WEP (1)

Are there more fragments from this frame?

Is this a retransmission of a previous frame?

Power state of sender after transmissionIs there more data to come?

Has WEP encryption been applied to frame?

Are the packets strictly ordered?


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Wireless Access Points

WAP1

WAP2

WAP3


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Ad Hoc Networks

In ad hoc networks, stations directly transmit

to one another

Hosts are responsible for routing, addressing,

name translation, security, etc.

Two ad hoc networks using the same

frequency, within range of one another will

cause conflicts Thus, different frequencies should be used


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Handoffs in WAPs

For WLANs with WAPs, roaming hosts must beconsidered

If a host moves into the range of another WAP, then outof range of their current WAP, a handoff takes place

A handoff is when one WAP gives the responsibility for aparticular host to one of its neighbouring WAPs The two WAPs must communicate for this to happen, and thus

neighbouring WAPs must be within each others transmissionrange


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Wireless LAN Standards

Some of the main standardized WLANs:

802.11a, 802.11g: 54Mbps, comparable with 100BaseT

Ethernet, under 100M range

802.11b: 11Mbps, comparable to 10BaseT Ethernet,under 100M range

These technologies are intended for LANs within the same small

to medium-sized building

BlueTooth/802.15: 721 kbps, under 10M range

This technology is intended for communicate within one room or

vehicle

data link layer issues 1

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