udlap computer networks spring 2015 pages 191-271

7/24/2019 UDLAP Computer Networks Spring 2015 pages 191-271

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Dr. Vicente Alarcn Aqui no

Copyright 1996-2015 J.F Kurose

and K.W. Ross

1

23

0111

value in arrivingpackets header

routing algorithm

local forwarding table

header value output link

0100010101111001

3221

4.8 Routing algorithms

Interplay between

routing and forwarding

191


Copyright 1996-2015 J.F Kuroseand K.W. Ross

u

yx

wv

z2

2

13

1

1

2

53

5

Graph: G = (N,E)

N = set of routers = { u, v, w, x, y, z }

E = set of links ={ (u,v), (u,x), (u,w), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) }

Graph abstraction

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and K.W. Ross

Graph abstraction: costs

u

yx

wv

z2 21

3

1

1

2

5

3

5 c(x,x) = cost of link (x,x)

- e.g., c(w,z) = 5

cost could always be 1, orinversely related to bandwidth,or inversely related tocongestion

Cost of path (x1, x2, x3,, xp) = c(x1,x2) + c(x2,x3) + + c(xp-1,xp)

Question: Whats the least-cost path between u and z ?

Routing algorithm: algorithm that finds least-cost path

193



Routing Algorithm classification

Global or decentralizedinformation?

Global: all routers have complete

topology, link cost info

link state algorithms

Decentralized: router knows physically-

connected neighbors, linkcosts to neighbors

iterative process ofcomputation, exchange ofinfo with neighbors

distance vector algorithms

Static or dynamic?

Static:

routes change slowlyover time

Dynamic:

routes change more

quickly periodic update in response to link cost

changes

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and K.W. Ross

A Link-State Routing Algorithm

Dijkstras algorithm

net topology, link costsknown to all nodes

accomplished via link

state broadcast all nodes have same info computes least cost paths

from one node (source)to all other nodes

gives forwarding table forthat node

iterative: after k iterations,

know least cost path to kdest.s

Notation: c(x,y): link cost from node x

to y; = if not direct

neighbors D(v): current value of cost of

path from source to dest. v

p(v): predecessor nodealong path from source to v

N': set of nodes whose leastcost path definitively known

195



Dijsktras Algorithm

1 Initialization:2 N' = {u}3 for all nodes v4 if v adjacent to u5 then D(v) = c(u,v)6 else D(v) = 78 Loop

9 find w not in N' such that D(w) is a minimum10 add w to N'11 update D(v) for all v adjacent to w and not in N' :12 D(v) = min( D(v), D(w) + c(w,v) )13 /* new cost to v is either old cost to v or known14 shortest path cost to w plus cost from w to v */15 unti l all nodes in N'

u

yx

wv

z2

2

13

1

1

2

53

5

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and K.W. Ross

Dijkstras algorithm: example

Step01

2345

N'u

ux

uxyuxyv

uxyvwuxyvwz

D(v),p(v)2,u2,u

2,u

D(w),p(w)5,u4,x

3,y3,y

D(x),p(x)1,u

D(y),p(y)

2,x

D(z),p(z)

4,y4,y4,y

u

yx

wv

z2

2

1 3

1

1

2

53

5

197



Dijkstras Algorithm Example

The first 5 steps used in computing the shortest path from A toD. The arrows indicate the working node.

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and K.W. Ross

Distance Vector Algorithm

Bellman-Ford Equation (dynamic programming)

Define dx(y) := cost of least-cost path from x to y

dx(y) = min {c(x,v) + dv(y) } for each node yN

where min is taken over all neighbors of x

Basic idea:

Each node periodically sends its own distance vectorestimate to neighbors

When node a node x receives new DV estimate from

neighbor, it updates its own DV using B-F equation:

199



Bellman-Ford example

u

yx

wv

z2

2

13

1

1

2

53

5Clearly, dv(z) = 5, dx(z) = 3, dw(z) = 3

du(z) = min { c(u,v) + dv(z),c(u,x) + d

x(z),

c(u,w) + dw(z) }= min {2 + 5,

1 + 3,5 + 3} = 4

Node that achieves minimum is nexthop in shortest path forwarding table

B-F equation says:

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x y z

xy

z

0 2 7

from

cost to

from

from

x y z

xy

z

0 2 3

from

cost tox y z

xy

z

0 2 3

from

cost to

x y z

xyz

cost tox y z

xyz

0 2 7

from

cost to

x y z

xyz

0 2 3

from

cost to

x y z

xyz

0 2 3

from

cost tox y z

xyz

0 2 7

from

cost to

x y z

xyz

7 1 0

cost to

2 0 1

2 0 1

7 1 0

2 0 17 1 0

2 0 13 1 0

2 0 1

3 1 0

2 0 1

3 1 0

2 0 1

3 1 0

time

x z12

7

y

node x table

node y table

node z table

Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)}

= min{2+0 , 7+1} = 2

Dx(z)= min{c(x,y) + Dy(z), c(x,z) + Dz(z)}

= min{2+1 , 7+0} = 3



and K.W. Ross

201



Comparison

Distance vector algorithm

Very simple to implement May have convergence problems Used in RIP and EIGRP

Link-state algorithm

Much more complex Switches perform independent computations Used in OSPF (Open Shortest Path First Protocol)

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and K.W. Ross

4.9 Hierarchical Routing

scale: with 200 milliondestinations:

cant store all dests inrouting tables!

routing table exchangewould swamp links!

administrative autonomy

Internet = network ofnetworks

each network admin maywant to control routing in itsown network

Our routing study thus far - idealization

all routers identical

network flat

not true in practice

aggregate routers into regions,autonomous systems (AS)

routers in same AS run samerouting protocol

intra-AS routing protocol

routers in different AS canrun different intra-ASrouting protocol

Gateway router: Direct link torouter in another AS

203



Classification Of Internet Routing Protocols

Two broad classes Interior Gateway Protocols (IGPs): aka Intra-AS routing protocols.

Used among routers within autonomous system Destinations lie within IGP RIP: Routing Information Protocol. RIPng for IPv6 OSPF: Open Shortest Path First. OSPFv3 for IPv6 IGRP/EIGRP: Interior Gateway Routing Protocol (Cisco proprietary)

Exterior Gateway Protocols (EGPs) Used among autonomous systems (BGP) Destinations lie throughout Internet

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and K.W. Ross

3b

1d

3a

1c

2a

AS3

AS1

AS21a

2c

2b1b

Intra-ASRoutingalgorithm

Inter-ASRoutingalgorithm

Forwardingtable

3c

Interconnected ASes

Forwarding table isconfigured by both intra-and inter-AS routingalgorithm

Intra-AS sets entries forinternal dests

Inter-AS & Intra-As setsentries for external dests

205



RIP ( Routing Information Protocol)

Distance vector algorithm

Distance metric: # of hops (max = 15 hops)

DC

BA

u v

w

x

yz

destination hopsu 1v 2

w 2x 3y 3z 2

Distance vectors:exchanged amongneighbors every 30 secvia Response Message(also calledadvertisement)

Each advertisement: listof up to 25 destinationnets within AS

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Destination Network Next Router Num. of hops to dest.

w A 2y B 2

z B A 7 5x -- 1. . ....



and K.W. Ross

RIP: Example

w x y

z

A

C

D B

Routing table in D207

Dest Next hopsw - -x - -z C 4. ...

Advertisementfrom A to D



An OSPF AS Consists of Multiple Areas Linked by

Routers

(a) an interconnect of routers andnetworks, and

(b) an equivalent OSPF graph Router corresponds to a node in the

graph

Illustration Of OSPF Graph

Carried in OSPF messages directly over IP(rather than TCP or UDP): protocol number89 for the IP Protocol field

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and K.W. Ross

BGP basics

Pairs of routers (BGP peers) exchange routing info oversemi-permanent TCP conctns: BGP sessions on port 179.

Note that BGP sessions do not correspond to physicallinks.

When AS2 advertises a prefix to AS1, AS2 is promising it

will forward any datagrams destined to that prefix towardsthe prefix. AS2 can aggregate prefixes in its advertisement

3b

1d

3a

1c2aAS3

AS1

AS21a

2c

2b

1b

3c

eBGP session

iBGP session

209



5. Link Layer and LANs

Some terminology: hosts and routers are nodes

communication channels thatconnect adjacent nodes alongcommunication path are links

wired links wireless links

LANs layer-2 packet is a frame,

encapsulates datagram

link

data-link layer has responsibility oftransferring datagram from one nodeto adjacent node over a link

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and K.W. Ross

Two types of networks at the data link layer

Broadcast Networks: All stations share a singlecommunication channel

Point-to-Point Networks: Pairs of hosts (or routers) aredirectly connected

Typically, local area networks (LANs) are broadcast andwide area networks (WANs) are point-to-point

Broadcast Network Point-to-Point Network

211



Local Area Networks

Local area networks (LANs) connect computers within a building or a enterprisenetwork

Almost all LANs are broadcast networks

Typical topologies of LANs are bus or star orring. There are also mesh, hybrid,hierarchical star, star-wireless.

We will work with Ethernet LANs. Ethernet has a bus or star topology.

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IEEE 802 Standards

Number Description

802.1 Internetworking

802.2 Logical Link Control

802.3 Ethernet (CSMA/CD)

802.3u Fast Ethernet

802.3z Gigabit Ethernet

802.3ae 10 Gigabit Ethernet

802.4 Token Bus

802.5 Token Ring

802.6 Distributed Queue Dual Bus (MAN)

802.7 Broadband Technology

802.8 Fiber Optic Technology

802.10 LAN Security

802.11a/b/g/n Wireless LAN

802.15 Wireless Personal Area Network (WPAN)

802.15.3a UWB (Ultra-WideBand)

802.16 Wireless MAN WiMAX802.20 Wireless WAN (Mobile Broadband Wireless Access MBWA).

802.22 Wireless RAN (Regional Area Network)



and K.W. Ross

213



5.1 Link layer: context and services

Datagram transferred by different link protocolsover different links:

e.g., Ethernet on first link, frame relay onintermediate links, 802.11 on last link

Framing, link access:

encapsulate datagram into frame, adding header, trailer

channel access if shared medium MAC addresses used in frame headers to identify source,

dest

different from IP address! Reliable delivery between adjacent nodes

seldom used on low bit error link (fiber, some twisted pair) wireless links: high error rates

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and K.W. Ross

Link Layer Services (more)

Flow Control: pacing between adjacent sending and receiving nodes

Error Detection: errors caused by signal attenuation, noise. receiver detects presence of errors:

signals sender for retransmission or drops frame

Error Correction: receiver identifies and corrects bit error(s) without

resorting to retransmission

Half-duplex and full-duplex

with half duplex, nodes at both ends of link cantransmit, but not at same time

215

Adaptors Communicating

link layer implemented inadaptor (aka NIC)

Ethernet card, 802.11 card sending side:

encapsulates datagram in aframe

adds error checking bits, rdt(reliable data transfer), flowcontrol, etc.

receiving side

looks for errors, rdt, flowcontrol, etc

extracts datagram, passes torcving node

adapter is semi-autonomous

link & physical layers

sendingnode

frame

rcvingnode

datagram

frame

adapter adapter

link layer protocol



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and K.W. Ross

5.2 Error Detection

EDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking, may include header fields

Error detection not 100% reliable! protocol may miss some errors, but rarely

larger EDC field yields better detection and correction

217



Parity Checking

Single Bit Parity:Detect single bit errors

Two Dimensional Bit Parity:Detect and correct single bit errors

0 0

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and K.W. Ross

Checksumming: Cyclic Redundancy Check

view data bits, D, as a binary number

choose r+1 bit pattern (generator), G

goal: choose r CRC bits, R, such that

exactly divisible by G (modulo 2) receiver knows G, divides by G. If non-zero

remainder: error detected!

can detect all burst errors less than r+1 bits widely used in practice

219



CRC Example

[D.2rXOR R]/G = Q

[D.2r]/G = Q XOR R/G

if we divide D.2rbyG, want remainder R

R = remainder[ ]D.2r

G

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Polynomials



and K.W. Ross

221



5.3 Multiple Access Links and Protocols

Two types of links:

point-to-point

PPP for dial-up access point-to-point link between Ethernet switch and host

broadcast (shared wire or medium)

traditional Ethernet

upstream HFC 802.11 wireless LAN

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and K.W. Ross

Multiple Access protocols

single shared broadcast channel

two or more simultaneous transmissions by nodes:interference collision if node receives two or more signals at the same time

multiple access protocol

distributed algorithm that determines how nodes sharechannel, i.e., determine when node can transmit

communication about channel sharing must usechannel itself!

223



Ideal Multiple Access Protocol

Broadcast channel of rate R bps

1. When one node wants to transmit, it can sendat rate R.

2. When M nodes want to transmit, each cansend at average rate R/M

3. Fully decentralized: no special node to coordinate transmissions no synchronization of clocks, slots

4. Simple

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and K.W. Ross

MAC Protocols: a taxonomy

Three broad classes:

Channel Partitioning divide channel into smaller pieces (time slots,

frequency, code) allocate piece to node for exclusive use

Random Access channel not divided, allow collisions recover from collisions

Taking turns

Nodes take turns, but nodes with more to send cantake longer turns

225



Channel Partitioning MAC protocols: TDMA

TDMA: time division multiple access

access to channel in "rounds"

each station gets fixed length slot (length = pkt transtime) in each round

unused slots go idle

example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle

TDM (Time Division Multiplexing): channel divided into N time slots, oneper user; inefficient with low duty cycle users and at light load.FDM (Frequency Division Multiplexing): frequency subdivided.

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and K.W. Ross

Channel Partitioning MAC protocols: FDMA

FDMA: frequency division multiple access channel spectrum divided into frequency bands

each station assigned fixed frequency band

unused transmission time in frequency bands go idle

example: 6-station LAN, 1,3,4 have pkt, frequency bands2,5,6 idle

frequencybands

227



Channel Partitioning MAC protocols: CDMA

Code Division Multiple Access (CDMA)

used in several wireless broadcast channels (cellular, satellite, etc)standards

unique code assigned to each user; i.e., code set partitioning

all users share same frequency, but each user has own chippingsequence (i.e., code) to encode data

encoded signal = (original data) X (chipping sequence)

decoding: inner-product of encoded signal and chipping sequence

allows multiple users to coexist and transmit simultaneously with

minimal interference (if codes are orthogonal)

228

Hadamard Matrix (Walsh Codes)

A Hadamard matrix of ordern is a matrix with elements 1 or1 such that

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and K.W. Ross

CDMA Encode/Decode Example

slot 1 slot 0

d1 = -1

1 1 1 11- 1- 1- 1-

Zi,m= di.cm

d0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

slot 0channeloutput

slot 1channeloutput

channel output Zi,m

sender

code

databits

slot 1 slot 0

d1 = -1

d0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

slot 0

channeloutput

slot 1

channeloutputreceiver

code

receivedinput

Di = Zi,m.cm

m=1

M

M

229



CDMA: two-sender interference

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and K.W. Ross

Random Access Protocols

When node has packet to send

transmit at full channel data rate R. no a priori coordination among nodes

two or more transmitting nodes

collision, random access MAC protocol specifies:

how to detect collisions how to recover from collisions (e.g., via delayed

retransmissions)

Examples of random access MAC protocols: slotted ALOHA ALOHA

CSMA, CSMA/CD (IEEE 802.3), CSMA/CA (IEEE 802.11)

231



The ALOHA Protocol: Pure (unslotted) ALOHA

Multiple users share a single broadcast channel Users do not test the channel before they transmit. They

transmit fixed-size frames at arbitrary times

If a collision occurs (i.e., by overlapping transmissions), thecolliding frames get garbed and thus discarded Retransmission by upper-layer protocols

Maximum throughput is ~18.4%

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and K.W. Ross

Slotted ALOHA

Pure Aloha performance can be substantially improved if werequire frame transmissions to occur on time slot boundaries

Time slots are the same as the frame size Assume that all stations are globally synchronized The frame will be scheduled for transmission at the start of the

next time slot User sends the frame without testing the state of the channel If a collision occurs, the colliding frames discarded

Retransmission by upper-layer protocols Maximum throughput is ~37%

233



CSMA (Carrier Sense Multiple Access)

CSMA: listen before transmit:If channel sensed idle: transmit entire frame If channel sensed busy, defer transmission

Multiple Access (MA) Multiple computers attach to shared media Each uses same access algorithm

Carrier Sense (CS) Wait until medium idle Begin to transmit frame

Simultaneous transmission possible

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and K.W. Ross

CSMA collisions

collisions can still occur:propagation delay meanstwo nodes may not heareach others transmission

collision:entire packet transmissiontime wasted

spatial layout of nodes

note:role of distance & propagationdelay in determining collision

probability

235



CSMA/CD (Collision Detection)

CSMA/CD: carrier sensing,deferral as in CSMA collisions detected within short

time

colliding transmissionsaborted, reducing channelwastage

collision detection: easy in wired LANs: measuresignal strengths, comparetransmitted, received signals

difficult in wireless LANs:receiver shut off whiletransmitting

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The CSMA Protocols 1-persistent CSMA:

listens to the channel prior to its transmission. If it is idle then it sends its frame. If a collision occurs the station waits for a random time and tries again. If it is busy it waits for the ongoing transmission to finish.

Non-persistent CSMA Waits for a random period before it sends the frame if the channel is in use

P - Persistent:

1. If medium idle, transmit with probability p, and delay one time unit with probability (1 p) Time unit typically maximum propagation delay2. If medium busy, listen until idle and repeat step 13. If transmission is delayed one time unit, repeat step



and K.W. Ross

237

Taking Turns MAC protocolsPolling: master node invites slave

nodes to transmit in turn

concerns:

polling overhead latency single point of failure

(master)

Token passing: control token passed from one

node to next sequentially; tokenmessage; concerns (token overhead,latency, single point of failure (token)).



master

slaves

poll

data

data

T

data

(nothingto send)

T

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and K.W. Ross

5.4 MAC Addresses and ARP

32-bit IP address: network-layeraddress used to get datagram to destination IP subnet

MAC (or LAN or physical or Ethernet) address:

used to get datagram from one interface to another physically-connected interface (same network) 48 bit MAC address (for most LANs)

burned in the adapter ROM

RARP

Ethernet MAC

address

(48 bit)

ARPIP address(32 bit)

IGMP Internet Group Management ProtocolICMP Internet Control Message ProtocolNDP Neighbor Discovery Protocol for IPv6 and ICMPv6

239



LAN Addresses and ARP

Each adapter on LAN has unique LAN address

Broadcast address =FF-FF-FF-FF-FF-FF

= adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN(wired orwireless)

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and K.W. Ross

ARP: Address Resolution Protocol

Each IP node (Host,Router) on LAN hasARP table

ARP Table: IP/MACaddress mappings forsome LAN nodes< IP address; MAC address; TTL>

TTL (Time To Live): timeafter which addressmapping will be forgotten(typically 10 min)

Question: how to determineMAC address of Bknowing Bs IP address?

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

237.196.7.23

237.196.7.78

237.196.7.14

237.196.7.88

241



Address Translation with ARP

ARP Request:Argon broadcasts an ARP request to all stations on thenetwork: What is the hardware address of Router137?

Argon128.143.137.144

00:a0:24:71:e4:44

Router137128.143.137.1

00:e0:f9:23:a8:20

ARP Request:

What is the MAC address

of 128.143.137.1?

ARP Request from Argon:

Source hardware address: 00:a0:24:71:e4:44Source protocol address: 128.143.137.144Target hardware address: FF:FF:FF:FF:FF:FFTarget protocol address: 128.143.137.1

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and K.W. Ross

Address Translation with ARP

ARP Reply:Router 137 responds with an ARP Reply which containsthe hardware address

Argon128.143.137.144

00:a0:24:71:e4:44

Router137128.143.137.1

00:e0:f9:23:a8:20

ARP Reply:

The MAC address of 128.143.137.1

is 00:e0:f9:23:a8:20

ARP Reply from Router137:

Source hardware address: 00:e0:f9:23:a8:20Source protocol address: 128.143.137.1

Target hardware address: 00:a0:24:71:e4:44Target protocol address: 128.143.137.144

243

Internet Architecture & Internet Application

Example



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5.5 High-Speed LANs: Ethernet, Token Ring

dominant wired LAN technology:

cheap $20 for 100Mbs!

first widely used LAN technology

Simpler, cheaper than token LANs and ATM Kept up with speed race: 10 Mbps 10 Gbps

Robert Metcalfes (1946)(MIT, 1973) EthernetSketch.



and K.W. Ross

245

Ethernet Frame Structure

Sending adapter encapsulates IP datagram (or other network layerprotocol packet) in Ethernet frame

Preamble: 7 bytes with pattern 10101010 followed by one byte withpattern 10101011. Used to synchronize receiver, sender clock rates

Addresses: 6 bytes

if adapter receives frame with matching destination address, or withbroadcast address (e.g., ARP packet), it passes data in frame to net-layer protocol; otherwise, adapter discards frame

Type: indicates the higher layer protocol (mostly IP)

CRC: checked at receiver, if error is detected, the frame is simply dropped



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Ethernet CSMA/CD algorithm

1. Adaptor receives datagramfrom net layer & createsframe

2. If adapter senses channelidle, it starts to transmitframe. If it senses channelbusy, waits until channel idleand then transmits

3. If adapter transmits entireframe without detectinganother transmission, theadapter is done with frame !

4. If adapter detects anothertransmission whiletransmitting, aborts andsends jam signal

5. After aborting, adapterenters exponentialbackoff: after the mthcollision, adapter choosesa K at random from{0,1,2,,2m-1}. Adapterwaits K512 bit times andreturns to Step 2



and K.W. Ross

247

Ethernets CSMA/CD (more)

Jam Signal: make sure all othertransmitters are aware ofcollision; 48 bits or 32bits.

Bit time: 0.1 microsec for 10Mbps Ethernet ;for K=1023, wait time is

about 50 msec

Exponential Backoff: Goal: adapt retransmission

attempts to estimated currentload

heavy load: random wait willbe longer

first collision: choose K from

{0,1}; delay is K

512 bittransmission times

after second collision: choose Kfrom {0,1,2,3}

after ten collisions, choose Kfrom {0,1,2,3,4,,1023}



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and K.W. Ross

Ethernet Technologies: LAN Wiring

Ethernet Technologies Three schemes correspond to three generations

(a) 10Base5, (b) 10Base2, (c) 10Base-T

All run same link speed of 10Mbps

All use same frame format

249



Ethernet Wiring In An Office

10Base5

10Base2

10BaseT

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and K.W. Ross

10BaseT and 100BaseT

10/100 Mbps rate; latter called fast ethernet

T stands for Twisted Pair

Nodes connect to a hub: star topology; 100m

max distance between nodes and hub

twisted pair

hub

251



Repeater

Hardware device

Connects two LAN segments

One repeater can effectively double the length of an LANsegment

Copies signal from one segment to the other

Connection can be extended with Fiber Optic Intra-Repeater Link

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and K.W. Ross

Limits On Repeaters

Can't extend Ethernet with repeaters indefinitely CSMA/CD requires low delay; if medium is too long, CSMA/CD

won't work Ethernet standard includes limit of 4 repeaters between any two

Ethernet stations Very easy to use - just plug in

Repeaters simply re-transmit analog signals Collisions affect entire network Transient problems - noise propagates throughout network

253



Fast (802.3u) and Gbit Ethernet (802.3z)

uses standard Ethernet frame format allows for point-to-point links and shared broadcast channels

in shared mode, CSMA/CD is used; short distances between nodesrequired for efficiency

uses hubs, called here Buffered Distributors or Full Duplex Repeater orBuffered Repeater

Full-Duplex at 1 Gbps for point-to-point links

10 Gbps now !

T4: 1 pair is used to receive while3 pairs are used to transmit.

FX, TX: it uses 1 pair to receiveand 1 pair to transmit

In 1000Base-T, each of four wirepairs transmits 250 Mbps inboth directions.

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and K.W. Ross

CSMA/CD efficiency

Tprop = max prop between 2 nodes in LAN

ttrans = time to transmit max-size frame

N

Efficiency goes to 1 as tprop goes to 0

Goes to 1 as ttrans goes to infinity

Much better than ALOHA, but still decentralized, simple, andcheap

transprop tt /3.441

1efficiency

255



IEEE 802.5 Token Ring

Second most popular LAN topology

Bits flow in single direction

Several technologies exist

Used with ring topology

Guarantees fair access: IEEE 802.5 standards

Token

Special (reserved) message

Small (a few bits) Frame formats:

token:SD AC FC

packet: SD AC FC dest addr src addr data checksum ED FS

8 8 8 48 48 variable 32 8 8 bits

SD, ED: mark start, end of frame

FC: Frame Control: used for monitoringand maintenance, distinguish controlframes and information frames.

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and K.W. Ross

Frame Formats

AC: Access Control Byte:

T - token bit: value 1 means token can be seized, value 0 means data

packet to follow access control byte.

PPP - priority bits: priority level of this packet.

RRR - reservation bits : station can write these bits to prevent stations withlower priority packet from seizing the token next time it becomes free.

M - is the monitor bit which is set by the Active Monitor (AM) station whenit sees this frame.

Example: Priority-Based Ring

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Frame Formats (contd)

Source and destination address: as in Ethernetdata: packet from network layer

checksum: error detection

FS: Frame Status: set by destination, read by receiver

set to indicate that destination is up, packet copied OK from ring

Destination station nonexistent or not active (A=0, C=0)Destination station exists but frame not copied (A=1, C=0)Frame received (A=1, C=1)

A C 0 0 A C 0 0

1 bit 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit 1 bit

Example:At a propagation speed of 200m/s, what is the effective length Ladded to a ring by a bit delay at each repeater? Now suppose a token holdingtime of 10ms, what is the frame length for each case?.a) At 5Mbpsb) At 40Mbpsc) At 1Mbps

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and K.W. Ross

5.6 Interconnecting with hubs and switches

Backbone hub interconnectsLAN segments

Extends max distance betweennodes

But individual segment collisiondomains become one largecollision domain

Cant interconnect 10BaseT &100BaseT

hub hub hub

hub

Data link layer technologies (switched networks): Switched Ethernet ATM (Asynchronous Transfer Mode) Frame Relay Multi-Protocol Label Switching (MPLS)

Some switched networks are intended for enterprise networks (SwitchedEthernet), wide area networks (MPLS, Frame Relay), or both (ATM)

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Hubs

Hubs are essentially physical-layer repeaters:

bits coming from one link go out all other links at the same rate no frame buffering no CSMA/CD at hub: adapters detect collisions

twisted pair

hub

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and K.W. Ross

Bridge

Hardware device

Connects two LAN segments

handles complete frames Uses NIC like any other station Performs some processing on frame

Does not forward noise or collisions

Learns addresses and filters

Allows independent transmission

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Filtering Example

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Break Cycles--Spanning Tree Algorithm

Bridges must cooperate to broadcast frames exactly onceon each segment

Used by all bridges to

Discover one another

Break cycle(s)

Known as Distributed Spanning Tree (DST)-- resulting in aunique path from each source to each destination

As each bridge joins the network, it communicates withother bridges on special hardware (typically multicast)

address

Learns network topology Performs spanning tree computation Determines if bridge will form a cycle



and K.W. Ross

263

Spanning Tree Algorithm

Step-1: Take the least weighing edge of the graph

and add it to the minimum Spanning Tree.

Step-2: Find the adjacent edges for the vertices in the

spanning tree.

Step-3: Find the least among all those edges.

Step-4: Add that edge to the spanning tree if its notforming a loop in the tree and that its not already

there in the tree.

Step-5: Stop if all vertices in the Graph have been

covered by the Tree else go to step-2.

Dr. Vicente Alarcn Aquino


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Spanning Tree Example

AB - 2 Least weighing

AD - 1 Already in t he tree

DB - 2

Least weighing

DE - 3

> value

DA - 1

Already in t he tree

BA - 2 Alr eady i n the tree

BD - 2 > value

BC - 1

Least weighing

BE - 3 > value

DB - 2 > value

DE - 3 > value

CB - 1 Alr eady in the tr ee

CE - 4 > value

CF - 2 Least weighing

BD - 2 Forming a loop

BE - 3 > value

DB - 2 Forming a loop

DE - 3 > value

FC - 2

Alr eady in the t ree

FE - 3 least weighing

CE - 4

> value

BE - 3

least weighing

DE - 3

least weighing

EB - 3 Alr eady i n the tree

EC - 4 Forming a loop

ED - 3

Forming a loop

EF - 3 Forming a loop

FE - 3

Forming a loop

CE - 4

Forming a loop

DE - 3 Forming a loop

The resultant Spanning Tree with no loops giving you the shortest distancebetween any two vertices.



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Switch

Link layer device

stores and forwards Ethernet frames examines frame header and selectively

forwards frame based on MAC dest address

when frame is to be forwarded on segment,uses CSMA/CD to access segment

transparent

hosts are unaware of presence of switches plug-and-play, self-learning

switches do not need to be configured

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and K.W. Ross

Forwarding

How do determine onto which LAN segment to

forward frame? Looks like a routing problem...

hub hubhub

switch1

2 3

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Switch example

Suppose C sends frame to D

Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table, switch forwards frame into

interfaces 2 and 3

frame received by D

hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEG

1123

12 3

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and K.W. Ross

Switch example

Suppose D replies back with frame to C.

Switch receives frame from from D notes in bridge table that D is on interface 2

because C is in table, switch forwards frame only to interface 1 frame received by C

hub hub hub

switch

A

B CD

EF

G H

I

address interface

ABEGC

11231

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Switches vs. Routers

both store-and-forward devices

routers: network layer devices (examine network layerheaders)

switches are link layer devices routers maintain routing tables, implement routing

algorithms

switches maintain switch tables, implement filtering,learning algorithms

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Summary comparison

hubs routers switches

traffic

isolation

no yes yes

plug & play yes no yes

optimal

routing

no yes no

cut

through

yes no yes

271

udlap computer networks spring 2015 pages 191-271

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