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Link Layer Protocols

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Link Layer Services

• Framing and link access: – encapsulate datagram into frame implement channel

access if shared medium, – ‘physical addresses’ are used in frame headers to

identify source and destination of frames on broadcast links

• Reliable Delivery: – seldom used on fiber optic, co-axial cable and some

twisted pairs too due to low bit error rate. – Used on wireless links, where the goal is to reduce

errors thus avoiding end-to-end retransmissions

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Link Layer Services (more)

• Flow Control: – pacing between senders and receivers

• Error Detection: – errors are caused by signal attenuation and noise. – Receiver detects presence of errors: – it signals the sender for retransmission or just drops the

corrupted frame

• Error Correction: – mechanism for the receiver to locate and correct the

error without resorting to retransmission

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Link Layer Protocol Implementation

• Link layer protocol entirely implemented in the adapter (eg,PCMCIA card). Adapter typically includes: RAM, DSP chips, host bus interface, and link interface

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Network Adapters

• Network adapters, sometimes called Network Interface Cards (NIC), plug into the data bus on a computer and into a network cable

• The NIC performs all of the functions required to communicate on a network

• Some computers have a NIC built in

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Selecting a NIC

• Before selecting a NIC for your computer, you need to determine three things:– What type of network are you connecting to?– What type of media (cable) are you using?– What type of bus does your computer have?

• If you have a choice of bus, you will want to consider the performance of each bus and how will affect the NIC

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Error Detection

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Errors

• Transmission errors are a way of life.

• In the digital world an error means that a bit value is flipped.

• An error can be isolated to a single bit.

• Errors on some media come in bursts– Harder to detect and correct than isolated

errors.

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Dealing with Errors

• Error detecting codes– provide enough redundant information to enable the

receiver to deduce that an error occurred

• Error correcting codes– provide enough redundant information to enable the

receiver to deduce that an error occurred AND how to fix it

• So a message consists of m data bits and r redundant or check bits.

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Hamming Distance

• Hamming distance:– the number of bit positions in which two codewords

differ

• Simple to calculate find the XOR• If two codewords are a Hamming distance d apart,

it will require d single-bit errors to convert one into the other.

• The Hamming distance of a code is the minimum Hamming distance between any two codewords.

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Hamming Distance 2 Code

000011101110

• Note that not all of the 8 different bit patterns are included in the code•Any single error will not convert a valid codeword into another valid codeword.

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How Error Detection Works

Invalid Code Words

Validcodeword

Validcodeword

2e

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Parity

• A simple single error detecting code could be constructed by counting bits.– Any codeword with an even number of bits is consider

valid (you could also make it the other way around).

• The minimum distance of this code is 2, so it is capable of detecting single errors.

• This code can be created by adding a parity bit:– chose the parity bit so that the number of ones in the

codeword is even (or odd).

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Parity in ActionData With Parity Bit00 00001 10110 11011 011

Want to send: 10

Data Link Sends: 110 Receive: 111 (ERROR)

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Protecting Blocks

• The probability of detecting a burst error on a block using a single parity bit is 50%.

• This can be improved by viewing the block as a n by k bit matrix.

• A parity bit is then computed for each column.• The check bits are placed in a k-bit row and

affixed to the matrix as the last row.• Bursts of length n can be detected.

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Detecting Burst Errors10010000110000111101101111011011110100101101110111001111010000011100011011011110110010011100101011001001

100100011000011101101110110111010011101110110011101000001100011110111111001001100101

Data

VRC (Vertical Redundancy Check)

LRC (Longitudinal Redundancy Check)

n

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What About Error Correction?

• How do we get error correction?– Must increase the minimum distance of the

code

• The key to error correction is that it must be possible to detect and locate the error.

• The minimum distance must be at least 2e+1

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Error Correction

Validcodeword

Validcodeword

Invalid codewords

The +1 ensures the circles will not overlap

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A Simple Single Error Correcting Code

Data Send Received0 000 000

001010100

1 111 011101110111

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Hamming Codes

• Hamming codes are n-bit codes that can correct single errors.

• The basic idea is to split the codeword into two portions– information or message bits (m)– parity bits (k)

• The result are codewords that consist of m+k bits

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Choosing m and k

• Selecting m is easy, you are usually told what it is.• How do you pick k?• The parity bits are used to generate a k-bit word

that identifies where in the codeword the error, if any, occurred.

• Consequently, k must satisfy the following:

12 kmk

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Constructing the Codeword

• The codeword consists of m+k bits.

• The location of each of the m+k bits is assigned a decimal value, 1 is assigned to the MSB, and m+k to the LSB.

• Parity bits go in positions 1, 2, 4, …, 2k-1

1 2 43 5 876 ... m+k9

p0 p1 m0 p2 m1 m2m3 p3 m4 ... mm+k

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Parity Checks

• The parity checks must be specified so that when an error occurs, the position number will take on the the value assigned to to location of the error

Error Position Position Number0 (no error) 0001 0012 0103 0114 1005 1016 1107 111

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Putting It TogetherPosition 1

p02p1

3m0

4p2

5m1

6m2

7m3

Original Message 0 1 0 0

Parity check in positions1,3,5,7 requires p0=1

1 0 1 0 0

Parity check in positions2,3,6,7 requires p1=0

1 0 0 1 0 0

Parity check in positions4,5,6,7 requires p2=1

1 0 0 1 1 0 0

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ExampleSend the message 0010 using a hamming code

Step 1: Find k. Here k=3

Step 2: Determine where things go

Step 3: Figure out the parity bitsp1 will cover 1,3,5,7,9,11,…p2 will cover 2,3,6,7,10,11,…p3 will cover 4,5,6,7,12,13,14,15,...

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Correcting Burst Errors

• Hamming codes can be used to correct burst errors• A sequence of s consecutive codewords are

arranged as a matrix, one codeword per row.• Transmit data one column (s bits) at a time.• The matrix is reconstructed by the receiver one

column at a time.• If a burst error of size s occurs, only a single

column will be affected.

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Correcting Burst Errors

001100100001011100100111101010101111010101010110101100101101010110011110011111001100000011111000011101010111111111100110000111000101

100100011000011101101110110111010011101110110011101000001100011110111111001001100101

HammIng

code

Character ASCII Check Bits

s

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Correcting vs. Detecting

• Most often error detection followed by retransmission is more efficient.

• Consider a channel with an error rate is 10-6 per bit (one error per million bits sent)– Block size 1000 == 10 check bits ( k == 10 )– For parity one check bit will suffice

• Overhead for sending 1MB– Hamming == 10,000 bits– Parity == 2001 bits (since 1 block will be

retransmitted)

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Cyclic Redundancy Check

• The transmitter generates a bit pattern called a Frame Check Sequence (FCS), based on the contents of the frame.

• The combined contents of the frame and the FCS are exactly divisible by some predetermined polynomial.

• If the contents of the frame are damaged during transmission, the receiver’s division will yield a non-zero remainder.

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Cyclic Redundancy Check

• CRC detects all of the following errors:– All single bit errors– All double bit errors if the divisor is at least

three terms– Any odd number of errors, if the divisor

contains a factor (x+1)– Any error in which the length of the error (an

error burst) is less than the length of the FCS– Most errors with larger bursts

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Polynomials

• CRC is based upon treating bit strings as representations of polynomials with coefficients of 0 and 1 only.

– 11001 == x5+ x4+ x0

• A k-bit frame is regarded as the coefficient list for a polynomial with k terms, ranging from xk-1 to x0

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The Generator Polynomial

• Both the sender and the receiver must agree upon a generator polynomial, G(x).

• Both the high and low order bits of the generator must be 1.

• The length of the generator is one bit longer than the FCS.

• Finally the frame must be longer than the generator.

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The Algorithm

• The frame contents are appended by a set of 0s equal in number to the length of the FCS.

• The resulting value is divided modulo 2 by the generator polynomial

• Each division step is carried out in the conventional manner, except that the subtraction is done modulo 2.

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Modulo 2 Subtraction

1 1 0 0-1 -0 -1 -0-- -- -- -- 0 1 1 0

1 1 0 0+1 +0 +1 +0-- -- -- -- 0 1 1 0

• Subtraction and addition are identical to XOR (no borrows or carries).

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The Algorithm (continued)

• The division produces a quotient which is discarded.

• The remainder replaces the 0s appended to the frame (subtracted from the frame modulo 2).

• The resulting frame is now evenly divisible by the generator polynomial.

• The receiver performs the same division, a non-zero remainder indicates that an error occurred.

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CRC Example (transmit)

Frame contents: 111011Polynomial: 11101 (x4+ x3+x2 + x0)Frame with 0s: 1110110000

10000111101 1110110000 11101 ----- 10000 11101 ----- 1101

Frame to send: 1110111101

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CRC Example (receive)

Frame contents: 1110111101Polynomial: 11101 (x4+ x3+x2 + x0)

10000111101 1110111101 11101 ----- 11101 11101 ----- 0

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Standard Polynomials

• CRC-12 (x12+x11+x3+x2+x1+1)– used when the character length is 6

• CRC-16 (x16+x15+x2+1)• CRC-CCITT (x16+x12+x5+1)

– used for 8 bit characters

– catches all single and double errors

– all errors of an odd length

– all bursts of 16-bits or less, 99.997% of 17-bits, and 99.998% of 18-bits and longer.

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Doing the Calculation Quickly

• The CRC calculations are actually performed in a cyclic shift register that uses XOR gates.

x5 x12 x16

MSB LSB

InputCRC-CCITT

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Warning

• All the analysis that has been done on CRC techniques assume the frames contain random bits.

• Real data, however, is not all that random.• As a consequence, under some

circumstances, undetected errors are much more common than had been previously thought.

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Multiple Access Links and Protocols

Three types of links:

(a) Point-to-point (single wire)

(b) Broadcast (shared medium)

(c) Switched

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Multiple Access Control (MAC) Protocols

• MAC protocol: coordinates transmissions from different stations in order to minimize/avoid collisions– Channel Partitioning MAC protocols– Random Access MAC protocols– “Taking turns” MAC protocols

• Goal: efficient, fair, simple, decentralized

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Channel Partitioning MAC protocols

• TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load.

• FDM (Frequency Division Multiplexing)

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Cocktail Party Analogy

• TDM– Everyone stands in the middle of the room,

taking turns talking

• FDM– People group into widely separated clumps,

each clumping holding a conversation at the same time, but still independent of the others.

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Code Division Multiple Access

• CDMA– Used widely in wireless technologies– Allows each station to transmit using the same

frequency

• Cocktail party example– Everyone is talking at the same time, but in

different languages

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Spread Spectrum

• The bandwidth of a signal is increased by artificially increasing the bit data rate

• This is done by breaking each bit into a number of sub-bits called "chips“– Assuming this number is 10, each bit of the original

signal would be divided up into 10 separate bits, or "chips." This results in an increase in the data rate by 10. By increasing the data rate by 10, we also increase the bandwidth by 10.

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Chipping Sequence

• All users share the same frequency– A Unique N-bit “code” assigned to each user– Each bit is multiplied by the code and

transmitted

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Channel Partitioning (CDMA)

• Chipping sequence like a mask: used to encode the signal– encoded signal = (original signal) X (chipping

sequence)– decoding: innerproduct of encoded signal and chipping

sequence (note, the innerproduct is the sum of the component-by-component products)

• To make CDMA work, chipping sequences must be chosen orthogonal to each other (i.e., innerproduct = 0)

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CDMA (cont’d)

CDMA Properties:• protects users from interference and jamming

(used in WW II)

• protects users from radio multipath fading

• allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)

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Random Access protocols

• A node transmits at random at full channel data rate R.

• If two or more nodes “collide”, they retransmit• The random access MAC protocol specifies how

to detect collisions and how to recover from them• Examples of random access MAC protocols:

– SLOTTED ALOHA– ALOHA– CSMA and CSMA/CD

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Slotted Aloha

• Time is divided into equal size slots (= full packet size)

• a newly arriving station transmits a the beginning of the next slot

• if collision occurs the source retransmits the packet at each slot with probability P

• S-ALOHA is channel utilization efficient; it is fully decentralized.

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Slotted Aloha

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Pure (unslotted) ALOHA

• Slotted ALOHA requires slot synchronization• A simpler version, pure ALOHA, does not require

slots• A node transmits without waiting for the

beginning of a slot• Collision probability increases (packet can collide

with other packets which are transmitted within a window twice as large as in S-Aloha)

• Throughput is reduced by one half

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Pure Aloha

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Carrier Sense Multiple Access

• CSMA: listen before transmit. If channel is sensed busy, defer transmission – Persistent CSMA: retry immediately when channel

becomes idle (this may cause instability)

– Non persistent CSMA: retry after random interval

• Collisions may still exist, since two stations may sense the channel idle at the same time

• In case of collision, the entire packet transmission time is wasted

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“Taking Turns” MAC protocols

• So far we have seen that channel partitioning MAC protocols (TDM, FDM and CDMA) can share the channel fairly; but a single station cannot use it all

• Random access MAC protocols allow a single user full channel rate; but cannot share the channel fairly (in fact, capture is often observed)

• Also there are “taking turns” protocols...

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“Taking Turns” MAC protocols

• Taking Turns MAC protocols achieve both fairness and full rate, at the expense of some extra control overhead– Polling: a Master station on a LAN in turn “invites”

the slave stations to transmit their packets. Problems: Request to Send/Clear to Send overhead, latency, single point of failure (Master)

– Token passing: the control token is passed from one node to the next sequentially. Can alleviate the latency and improve fault tolerance Still, elaborate procedures to recover from lost token, etc.

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IEEE Standard 802

• A collection of standards for LANsDescription

802.1 Architecture & Overview802.2 Logical Link Control802.3 CSMA/CD802.4 Token Bus802.5 Token Ring802.6 DQDB802.7 Broadband LANs802.8 Fiber Optic LANs & MANs802.9 Integrated Services LAN Interface for MAC & PHY Layers

802.10 Secure Data Exchange802.11 Wireless LAN MAC & PHY specifications802.12 Demand Priority Access Method

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802.3 - CSMA/CD

• The 802.3 standard encompasses both the MAC layer and the physical layer.

• This standard is for a 1-persistent CSMA/CD LAN.

• Xerox developed a CSMA/CD system to connect personal workstations. This system was called Ethernet.

• Ethernet is often used to refer to all CSMA/CD protocols.

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802.3 Architecture

LLC - Logical Link Control

MAC - Media Access Control

PLS - Physical Signaling

Physical Medium Attachment

Medium

Data Link

Physical

DTE

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Cabling

Name/Rate Topology Media MaximumSegment

Rate(Mbps)

Ethernet Bus 50 ohm thickcoax

500 10

10Base5 Bus 50 ohm thin coax 500 1010Base2 Bus 50 ohm thin coax 200 1010BaseT Star UTP 100 1010Broad36 Bus 75 ohm coax 1800 101Base5 Star UTP 250 110BaseF Star Fiber 2000 10

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802.3 Addresses

• Every machine on a 802.3 network is assigned a unique physical address– the address is typically defined in hardware and

cannot be changed

• This address is often referred to as a MAC address.

• MAC addresses can vary in size– 48 bits is a common length for a MAC address

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Performance

• As more and more stations are added to an 802.3 LAN, the traffic will go up, and eventually the LAN will saturate.

• What do you do?– Faster LAN?– Segment your LAN

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Switched 802.3 LANs

• The main component of this system is a box containing a high-speed backplane and room for plug in adapter cards.

• A station does what it always does.• When the switch sees the packet

– if destined for a machine on the same card send it to the appropriate station

– else ship it using the high-speed backplane to the card that handles that station

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Switch

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Collisions

• How are collisions handled?– One possibility is to wire each card so that it

forms a local on-card LAN.• Only one transmission per card is possible, but all

the cards can transmit in parallel.

– Buffer each input port, so incoming frames are stored in RAM as they arrive.

• Collisions do not occur.

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Topology Issues

• The original Ethernet specification was for a bus topology– only one channel– if the cable breaks the whole network is down

• Today most organizations install 10BaseT– star topology– machines connect to hubs

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Concerns About 802.3

• During the development of the 802 standard, people doing networking on factory floors had serious reservations about it.

• Due to the probabilistic MAC protocol, a station may have to wait arbitrarily long to send a frame.

• In a real time system time, timing is critical.• A simple solution is to form a ring, in which

stations take turns sending frames. No frame will have to wait more than nT seconds.

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802.4 Token Bus

• Physically, the token bus is a linear or tree-shaped cable onto which stations are attached.

• Logically, the stations are arranged in a ring.• Each station has an address and knows the address

of the machine to its “left” and “right”.• When the ring starts, the highest numbered

machine sends.• Permission to send is then sent around the ring.

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Tokens

• A control frame known as a token regulates the right of access.

• The token frame contains a destination address.• The station receiving the token is granted control

of the medium for up to some maximum time.• The token must be passed when:

– The station has no (more) data to send

– The station’s time expires

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FDDI

• FDDI was developed to support high-capacity LANs (operates at 100Mbps).

• Is typically used as a backbone to connect copper LANs.

• Uses multimode optical fibers and LEDs (as opposed to laser).

• The cabling consists of two fiber rings, one transmitting clockwise and the other counter-clockwise.

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FDDI Cabling

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Hubs

• Hubs provide a single connection between each workstation and the hub

• The connection is typically made over twisted pair cabling

• Hubs typically have a fixed number of ports. Common sizes are 8 and 16 port hubs.

• Hubs can be daisy chained (up to a limit) to provide more ports for connections

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Hubs

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Working with Hubs

• There is a limit to the number of hubs that can be connected together to extend a network (a typical maximum is 4)

• When possible, connect each hub directly to the backbone or a server

• Label the connections on the hub• The more hubs data passes through, the

slower the connection

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Passive Hubs

• A passive hub simply combines the signals of network segments

• The passive hub absorbs some of the signal and reduces in half the maximum cabling distance permitted

• Passive hubs pass received signals onto to all the computers connected to the hub

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Active Hubs

• Active hubs have electronic components that regenerate or amplify signals

• Distances supported by active hubs are greater than those of passive hubs

• The main drawback is that they amplify noise as well as the signal

• Because active hubs function as repeaters, they are sometimes called multiport repeaters

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Intelligent Hubs

• In addition to signal regeneration, intelligent hubs perform some network management and intelligent path selection

• A switching hub chooses only the port of the device where the signal needs to go, rather than sending the signal along all paths

• Many switching hubs can choose which alternative path will be the quickest and send the signal that way

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Making LANs Bigger

• Usually it does not take too long until an organization needs to expand a LAN.

• This can be done in a number of ways– physically make the cable longer

– use repeaters

– use bridges

• The primary difference between a repeater and a bridge is the OSI level that they operate at.

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Repeaters

• All transmission media attenuate (weaken) the signals that travel through them

• Attenuation is one of the things that limit the distance any medium can carry data

• Adding a device that amplifies the signal can allow it to travel further, increasing the size of the network

• Devices that amplify signals are called repeaters

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Repeaters

• A repeater is an electronic device that receives an electronic signal from a cable and sends it out on another cable segment.

• Repeaters are simple devices because they do not need to understand what they receive. Signals are simply retransmitted.

• A repeater is used when the physical length of the cable has reached its maximum.

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Repeaters

• Repeaters fall into two categories– amplifiers simply amplify the entire incoming signal

(which includes noise)

– signal regenerating repeaters create an exact duplicate of incoming data by identifying the signal amidst the noise, reconstructs it, and retransmits only the signal

• Since repeaters simply deal with the actual signals on a network, they operate at the physical layer of the OSI model

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Bridges

• Bridges connect network segments• Unlike a repeater a bridge understands the data

link protocols and using addressing information to determine the appropriate segment to send a frame to

• Like repeaters, bridges regenerate signals. Since they actually understand frame formats, the are considered to operate at the data link layer of the OSI model

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Bridges

BridgeHubs

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Bridges

• How a bridge handles traffic– receives all signals from segments A and B

– the bridge filters all packets from segment A addressed to segment A (and likewise for segment B)

– signals from A to B are retransmitted to segment B

– signals form B to A are retransmitted to segment A

• Through address filtering, bridges can divide busy networks into segments and reduce traffic

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Problems that Bridges Deal With

• Each LAN uses a different frame format– any copying between different LANs requires

reformatting

• All LANs do not necessarily run at the same rate.• All three 802 LANs have a different maximum

frame length.– Frames that are too large to be forwarded must be

discarded.

• Many problems going from 802.x to 802.y

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Routers

• Routers are devices that connect two or more networks

• They consist of hardware and software– the hardware can be a network server, a PC, or a special

black box

– The software components consist of an operating system and routing software

• Routers operate at the network layer of the OSI reference model

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Routing