Foundation Section 1

8/3/2019 Foundation Section 1

1/95

Dr. G. A. Marin

Network Analysis: Foundation 1-1


2/95

1970s1970sBusiness Environment: IBM's SNA

erarc ca, or er y, re a e

Sysdef for Topology and Paths

Connection-oriented and guaranteed service at link layer

a a on y

Low-speed (unreliable) leased lines

Research Environment: TCP/IP

Distributed, disorderly, less reliable

Next hop vs fixed paths

Connectionless (data only)

Phone Companies (European): X.25 StandardConnection-oriented

SVC and PVC with Flow Control


Assumes noisy lines


3/95

NSFNet Backbone to Interconnect Super Computers

o c a protoco o rpanet n 1983

56 kbps backbone for 6 super computer centers

Thousands of researchers connect their subnets

Version 2 (IBM RS6000s and Fiber) ran at 448 kbps

By 1990 1.5 Mbps , ,

IBM Develops Advanced Peer-to-Peer Networking

LAN Growth Explosive

Ethernet (collisions)Token Ring (no collisions)

Novell's Netware (IPX based on XNS)



4/95

Internet

1990: 200,000 computers and 3,000 networks1995: multiple backbones, 100s regional nets, 10so ousan s o , m ons o os s

Doubles yearly!?

non-academics (mosaic)

NREN Gigabit Network funded by ARPA & NSF

Internet Society Founded 1992!Commercially Cisco uses IP to grow to Billions


eve ue


5/95

1990s1990s continuedcontinuedMIT, U Pa, IBM, Bellcore Research in

highspeed paket switchingFrame Relay Forum evolve X.25concepts for higher speed WAN lines

(1.5Mbps)ATM becomes successful in telephonybackbones (multiples of 155mbps)

DARPA Funding Next GenerationInternet

Network Analysis: Foundation 1-5multi-gigabit per second capabilities


6/95

oundationGoals:

Overview:

ay oun a on or

understanding how tobuild a network

Applications

Concepts & Terminology

Required services

Terminology

Network Architecture

Performance TCP/IP & OSI

Protocols and

Channel Multiplexing

Socket Interface

Performance Metrics



7/95

A licationsDistributed rocesses communicatin

across networkExamples: world wide web, email, FTP,

stream ng au o v eo, c atrooms

Use Application Layer Protocolsser agen mp emen s e app ca onlayer protocol: Email mail reader (e.g. outlook) Streaming audio/video media player



8/95

Client-server aradi mTypical network app has two

ieces: clientand serverapplicationtransport

data link

physicalClient: initiates contact with server

request

(speaks first)

typically requests service fromserver

applicationtransportnetwork

Web: client implemented inbrowser; e-mail: in mail reader

rep y

data linkphysical

provides requested service to client

e.g., Web server sends requested Web


page, mail server delivers e-mail


9/95

Internet Protocol Gra



10/95

The Web: the htt rotocol

htt : h ertext transfer

protocol Webs application layer PC running

Ex lorer

client/server model

client:browser that, ,

displays Web objects

server:Web server

Serverrunning

NCSA Webserver

response to requests

http1.0: RFC 1945

Mac runningNavigator


p . :


11/95

T e tt rotocol: orehtt : TCP trans ort htt is stateless

service: client initiates TCP

server maintains noinformation aboutast client re uests

to server, port 80

server accepts TCP Protocols that maintain

aside

http messages (application-layer protocol messages)

past history (state) mustbe maintained

(http client) and Web server(http server)

server c en cras es,their views of state maybe inconsistent, must be



12/95

http exampleSuppose user enters URLwww.someSchool.edu/someDepartment/home.index

(contains text,

references to 10

1a. http client initiates TCPconnection to http server

rocess at1b. http server at host

jpeg images)

www.someSchool.edu. Port 80is default for http server.

www.someSchool.edu waitingfor TCP connection at port 80.

accepts connection, notifying2. http client sends http request

message(containing URL) intoTCP connection socket 3. http server receives request

,messagecontaining requestedobject(someDepartment/home.index),


sen s message into sockettime


13/95

http example (cont.)4. http server closes TCP

5. http client receives responsemessage containing html file,dis la s html. Parsin html

connec on.

file, finds 10 referenced jpegobjects

6. Steps 1-5 repeated for eachtimeof 10 jpeg objects



14/95

Electronic Mailoutgoing

Three ma or com onents:

user mailbox

user

user agents mail servers sim le mail transfer

useragent

mailserver

protocol: smtp

User Agent

mailserver user

agentSMTPa. .a. ma rea er

composing, editing, readingmail messages

useragent

mailserver

SMTP

e.g., u ora, u oo , e m,Netscape Messenger outgoing, incoming messages

useragent

user


agent


15/95

Electronic Mail: mail serversMail Servers user

mailbox contains incomingmessages (yet to be read)for user

useragent

mailserver

message queue of outgoing(to be sent) mail messages

mailserver user

agentSMTPservers to send emailmessages

useragent

mailserver

SMTP

server

server: receiving mailuseragent


agent


16/95

Electronic Mail: smt RFC 821 uses tcp to reliably transfer email msg from client to

server, port 25

direct transfer: sending server to receiving server

three hases of transfer

handshaking (greeting)

transfer of messages

command/response interaction

commands: ASCII text

response: status code and phrasemessages must be in 7-bit ASCII



17/95

Sam le smt interactionS: 220 hamburger.edu

C: HELO crepes.fr

S: 250 Hello crepes.fr, pleased to meet you

C: MAIL FROM:

S: 250 [email protected]... Sender ok

: : < o am urger.e u>

S: 250 [email protected] ... Recipient ok

C: DATA" " , .

C: Do you like ketchup?

C: How about pickles?

C: .

S: 250 Message accepted for delivery

C: QUIT

S: 221 hamburger.edu closing connection



18/95

:

telnet servername 25

see 220 reply from server enter HELO, MAIL FROM, RCPT TO, DATA, QUIT

commands

above lets you send email without using email client



19/95

s t : inal words smtp uses persistent Com arison with htt :

connections

smtp requires message(header & body) to be in 7-

http: pull email: push

bit ASCII

certain character strings

not permitted in msg (e.g.,

both have ASCIIcommand/responseinteraction status codes

CRLF.CRLF). Thus msg hasto be encoded (usually intoeither base-64 or quoted

http: each objectencapsulated in its own

pr n a e smtp server uses

CRLF.CRLF to determine

res onse msg smtp: multiple objects sent

in multipart msg


en o message


20/95

Foundation

Goals:

Overview:ay oun a on or


Applications Concepts & Terminology

Required services

Terminology


Network Software TCP/IP & OSI

Protocols and

Performance

Pa ers Socket Interface

Performance Metrics Problems

Network Analysis: Foundation 1-20Network Analysis: Foundation 1-20


21/95

Connectivit router workstationserver

obile

local ISP

connect computers Ma su ort a

regional ISP

strictly limited

number or many suppor sgrowth, it is said

company

.


ne wor


22/95

in s and Nodes, ,

computers..) some of which are directlyconnected by links.

Links (coax cable, twisted pair, fiber) are

either point-to-point or multiple access.Cooperating nodes can provide connectivity

across several direct links.

Directly connected Indirectly connected



23/95

Multi le Access Links and ProtocolsThree types of links: point-to-point (single wire, e.g. PPP, SLIP)

broadcast (shared wire or medium; e.g, Ethernet,, .

switched (e.g., switched Ethernet, ATM etc)



24/95



25/95

RSVP/IS-C

A working network

ISDN

PSTN

ES

ESRSVP/IS-CMSS-C

RSVP/IS-C

MSS-C

MSS-C

LAN

Token Ring

8210MSS-S

ARIS/GSMP

RSVP/IS

VLAN

DDNS-S

DHCP-S

FrameRelay

ATMOC/12, OC/48

8260/8265

HUBS

MSS-SARIS/GSMP

RSVP/IS

VLAN

DHCP

DDNS

RSVP/IS-C

MSS-C

8260/8265

HUBS MSS-S

ARIS/GSMP

RSVP/IS

VLAN

8260/8265

HUBS MSS-S

ARIS/GSMP

RSVP/IS

LAN

Public DataNetwork

X.25, X.21

8260/8265

HUBS MSS-S

ARIS/GSMP

RSVP/IS

VLAN

DHCP

DDNS

ROUTER2210/2216

ARIS

RSVP/IS

DHCP

DDNS

DHCP

DDNS

ROUTER

2210ARIS

/

RSVP/IS-C

MSS-C

Token Ring

ATM

39xx, 374X

OSA, MAE

390TCP Sprayer

HPR/IP

HPR/ATM

-

DHCP-S

Firewall

IP Sec.

DDNS-S

DHCP-S

Firewall

IP Sec.

RSVP/IS-C

MSS-C

BUT COMPLEXITY!AS/400

SP-2

MSS-C

---------ARIS/GSMP

RSVP/IS

IPSEC



26/95

Ter inolo

organized systematically to forward dataas it moves from source to destination.

Circuit Switched: establishes a circuit

from entry point to exit point of network,and analog or digital data moves alongcircuit with strict timing.

ac et w tc e : o es n networ sendiscrete blocks or packets of information


.


27/95

Ter inoloOne ho : acket moves across a network from one

node to a node directly connected. Eachmovement is a hop toward the destination.

ore-an - orwar : ac e s rece ve n sentirety, is stored and processed in an

intermediate node, and is then forwarded to anadjacent node.

Virtual Circuit: A store-and-forward network that

-end-to-end connection that guarantees certainquality of service.



28/95

Host: a node that uses the network (outside thenetwork)

Switch: a node that implements the network (inside thenetwork

Internetwork: A set of independent networks that areinterconnected

networks

Address: a byte string that identifies a node

network based on source and destination addresses.

Unicast and Multicast Addresses: addresses assigned to


.


29/95

oundationGoals:

Overview:

ay oun a on or


ApplicationsConcepts & Terminology

Required services

Terminology



Protocols and

Papers

Problems Socket Interface Performance Metrics



30/95

Networ Arc itecture Is the blue rint that uides the desi n and

implementation of a type of network. The blueprint follows a philosophy that falls

e ween sen an pray an guaran ee .

Generally includes a layer reference model.

protocols and various layers.

May include applets to be used by user-layer

applications. Treats user, designer, and service provider

.


31/95

i erin Pers ectivesA lication ro rammer cares mostl about

the services that the network provides(and the cost for those services). oss o a a Error-free delivery?

Network designer cares that networkresources are used efficiently and

a ocate to users air y.Network provider wants ease of


.


32/95

Networ arc itecture

Connectivity Re uired service levels

Robustness in case of failures

Fairness to users Cost-effective solutions

Again: A network architecture is a

uepr n or a way o u ng a ne worthat will satisfy all its requirements.


, , ,


33/95


34/95

La ers Services ProtocolsLa ers Services ProtocolsLayers: offer services to higher layers

that are requested by parameters beingpassed to service access points (SAPs).

Services: A set of primitives

o erations that a la er can erform forits users

Protocol: is a set of rules overnin the

format and meaning of frames, packets,or ms s exchan ed b eer entities.


Protocols implement the services.


35/95

La er O ver viewPhysical Layer: Transmits bit s over comm channel. Howto r epresent bit s and ensure reception. (Physical mediumlies lo icall below .

Data Link Layer: Takes raw t ransmission facilit y and

transfor ms to line free of undetected t ransmission er ror s.Sends data frames. Retransmits if needed. Flow controlusua y prov e .

N etwork Layer : Gets packets from source to dest inat ionacross a network. Provides rout ing, network congest ioncontr ol call admission net - net i/f erha s account into suppor t net mgt .

Transpor t Layer : Accept data from session layer ,segmentat ion & reassembly if needed, pass to networkayer , assure a p eces arr ve sa e y a o er en . rsend-to-end layer . Mult iplexing, flow control



36/95

Session Layer: Provides enhanced services tocertain applications over those provided bytransport layer. Synchronization as example.

resen a on ayer: rov es a onafunctions such as translation among different

character numerical and bit-strinrepresentations.

Application Layer: Supports protocols needed

y app ca ons suc as ne wor v r ua erm na,file format conversions, electronic mail, remoteob entr etc.



37/95

TCP/IP & Course ReferenceTCP/IP & Course ReferenceModelsModels

Application Layer Application Layer

Transport Layer Transport Layer

Network Layer Internet Layer

Data Link LayerrData Link LayerrData Link Layer

Host-to-network


Physical Layerr


38/95

Networks Interconnected by IP Routers


39/95

TCP/IP Application LayerTCP/IP Application Layer

Contains all higher level protocolsnc u ng:

Telnet virtual terminal protocol

SMTP Simple Mail Transfer Protocol

HTTP World-Wide-Web page trasfer protocol



40/95

TCP/IP Layer OverviewTCP/IP Layer Overview

Internet Layer: Heart of the Internet architecture. Hostsinsert packets into IP network that travel independentlyto destination (over distinct subnets). Implements the IP

(Internet Protocol) "protocol." Routing and networkcon estion avoidance are ke issues.

Transport Layer: End-to-end peer protocol as in OSI.Protocol (TCP) and User Datagram Protocol (UDP).TCP is reliable and connection-oriented. Supports

'

as flow control. UDP is unreliable and connectionless.Used with aps that do their own thing or voice or video


...


41/95

Hybrid Model

this course.



42/95

Two major models for layer

serv ces Connection-oriented service

Like the telephone network. End-to-end agreements made before user

ra c ows. Greater complexity and greater reliability

Like the post office network. Each letter (packet) moves independently

roug e ne wor . Simpler and reliability can be improved with

hi her-la er services like re istered mail.



43/95

Internet trans ort rotocols servicesTCP service: UDP service: connection-oriented:setup

required between client,server

unreliable data transferbetween sending andreceivin rocess

re ia e transportbetweensending and receiving process

flow control:sender wont

does not provide:connection setup,

reliabilit , flow control,

congestion control:throttlesender when networkoverloaded

congestion control, timing,or bandwidth guarantee

does not provide:timing,minimum bandwidthguarantees

Q: why bother? Why isthere a UDP?



44/95

Example NetworksExample Networks

System Network Architecture (SNA)

IBM's networking architecture used by 10,000+business networks

Mainframe model of computing...hierarchical- -

(APPN) architecture

Novell NetwarePredominant PC-based networkarchitecture...client/server model...downsizing

Pro rietar rotocol stack derived from

XNS...similarities to IPIPX uses 10-byte vs 4-byte (IP) addresses


e wor ore ro oco orconnection-oriented transport


45/95

Exam le Networks continuedExam le Networks continued

ARPANET

Funded b Advanced Research Pro ects A enc(ARPA) in early 60s as command/control network

Introduction of packet-switching (vs-

Hosts connected to subnet of IMPs (Interface Msg

ProcessorsIncredible growth

Led to invention of TCP/IP model in 1974 (Vinter o a n

Protocols integrated into Berkeley UNIX (by BBN)and rovided free with UNIX license...ex losive!



46/95

Designed by National Science Foundation as highspeedsuccessor to ARPANET for university access

rs connec e sx supercompu er cen ers: an ego,Boulder, Champaign, Pittsburgh, Ithaca, Princeton

First TCP/IP WANEventually 20 regional networks connected to backbone

In 1990 ANS (Advanced Networks and Services) took overand renamed ANSNET...45 mbps links & first step towards

commercializationIn 1995 ANSNET sold to AOL and regional networks wentto commercial service



47/95

In mid-80s this collection of IP-based networks" "

Growth exponential

Defn: Machine on Internet if runs TCP/IP, has IP@, and sends IP packets to other internet hosts

Internet Society founded in Jan 1992

-

millions of non-academic users to Internet in early90s (web pages, links, ubiguitous URLs)



48/95

X .25 NetworksX .25 Networks

A standard developed by CCITT (now ISO) in 1970s

Physical layer is X.21 - mostly analog RS-232 used instead

Network layer handles addressing, flow-control, delivery

User establishes a "virtual circuit" and sends max 128byte packets reliably and in order.

,

Flow control ensures fast sender does not swampreceiver

- .

is used (PAD)Terminal - X.28 - PAD - X.29 standards also defined.


.


49/95

Frame RelaFrame Rela

Also connection-oriented standaard for moving bitsgenerally across a public network.

Takes advantage of fact that leased lines are now fast,

digital, reliable --> simple protocols.

.

Frames can be up to 1600 bytes and carry a number (dlci)identifying the virtual circuit.

ontract w t carr er or an average serv ce

burst allowed as in SMDS

Generally operates at T1 (1.5mbps) or above

FR determines start and end of frame, detects sometransmission errors, discards bad frames.


.


50/95

BISDN Architecture - ATMBISDN Architecture - ATM

- envisioned for universal networking...

- integrated networking* voice, video, data, and image in the same

- scaleable in distance* LAN, MAN, WAN

- scaleable in bandwidth* 1.5 Mbps to several Gbps



51/95

MotivationsMotivations

Why connection oriented?- prov ng serv ce guaran ees requ re

resourcesto be reserved in the network

- simpler network management

Why fixed size cells?- efficient switching- hardware switching

Why small size cell?- mainly for voice



52/95

Connection-Oriented vsConnection-Oriented vsConnectionlessConnectionless

Connection-Oriented

Path created (fixed) from transmission site to

Intermediate nodes set aside resources for the

new connectionConnection may be allocated a given transmissionrate (r bps).

connection (if needed for connection type)If no path can be found having r bps on all links,


ca s reec e .

C ti lC ti l


53/95

ConnectionlessConnectionless

Session setup without reservingw w .

First-come, first-served and noper ormance guaran ees c angng now .

No call-admission decision. All usersaccep e connec e .

Packet waits in queue if needed to be

transm tte on next n .Advantage is that data can be sent


without any connection-setup protocols.

E x am p le P ro toc o l G rap h


54/95

E x am p le P ro toc o l G rap h

F T P H T T P V id eo T F T P

IP

A T MEtherne t O the r N e tArch


N ote : O ne a rch itec tu re m ayr ide on top o f an other .


55/95

Internet a s: a lication, trans ort rotocols

Application Underlying

e-mail

smtp [RFC 821]

TCP

Webfile transfer

streamin multimedia

http [RFC 2068]ftp [RFC 959]

ro rietar

TCPTCP

TCP or UDP

remote file serverInternet telephony

(e.g. RealNetworks)NFSproprietary

TCP or UDPtypically UDP

(e.g., Vocaltec)



56/95

oundationGoals:

Overview:

ay oun a on or


Applications


Required services

Terminology



Protocols and


Socket Interface Performance Metrics



57/95

Per or ance bandwidtBandwidth . .

3000Hz) Bandwidth of a communication link means the

- - . .Ethernet is 100 million-bits-per-sec)

Throu h ut Usually implies achievable performance for the

data unit of interest (user data,packet,etc.)., , .

Depends on input rate to the channel Influenced by protocol and protocol data unit


over ea .


58/95

Per or ance dela

the start and end of an event of interest. Retrievin data from a disk Sending a frame on an ethernet link

Sending an IP datagram across the Internet.

Measured in time (e.g. 24 milliseconds tosend a packet from US east to west coast.)

May be interested in one-way or in RTT(round-trip-time).


L k D l T P


59/95

Link Dela = ueue+Transmit+Pro a ate time

ueue time is time a acket/bit waits tobegin transmission from a node

Transmit time

Propagation time

Size (bits)/Bandwidth (bits per sec)

Propagation speed

stance m, m, eet, m es ropagat on pee

8 sec3.33

in a cable in fiber

82.3 10 m/sec82 10 m/sec

m

( )sec4.35 kn

( )sec5 km



60/95

i le Per or ance xa leHow lon does it take to send a 1000 b te

frame over a 1000km 100 Mbps link fromthe time transmission begins to the timeframe is received (no queueing delay)?

Ans. Bandwidth should be bps so we mustconvert ytes to its. en we ivi e ybandwidth and add propagation delay.

bitsbyte 61000 bytes 8

ransm ss on me sPropagation time is

6 bitssec

sec sec.100 10

= =

33

8 meterssec

1000 10 meters4.348 10 sec=4.348 ms

2.3 10

=

ota time is . ms.


Problem


61/95

Problem

Calculate the latency (from first bit sent to last bit received) of the following:

10-Mbps Ethernet with a single store-and-forward switch in the path and a packet

size of 5,000 bits. Assume that each link introduces a propagation delay of 10 s

and that the switch begins retransmitting immediately after it has finished receivingthe packet.

10-Mbps

10x10-6 sec 10x10-6 sec

65,000 bits1. Transmit: 500 10 sec.=

6

6

6

bits10 10

sec2. Propagate last bit on first link 10 10 sec.

5,000 bits3. Transmit: 500 10 sec.

=

=

Total 1.02 ms.

=


s10 10sec

4. Propagate last bit on second link 10 1

= 60 sec.


62/95

ProbleTwo nodes, A and B, are separated by 2000km and are connectedby a fiber 10 Mbps point-to-point link. Suppose that node A transmits

1,000 packets per second to node B and that each packet is 1,000 bytes

long. Also assume that each packet is delayed for 1 ms in node A.

. at s t e e ay t at eac pac et exper ences rom arr va at to

arrival at B?

.

The ratio of these two quantities determines link "utilization" usually

defined as:

, where is utilization, is throughput (bps) andc =


. .


63/95

olutionThe link delay is given by:

8

sec

8,000 2,0001 10 2.0 10 m

bits kmd qt tt pd ms Mbps= + + = + +

2 6

6 8

80 10 2 100.001sec sec sec 0.0118sec 11.8 .

10 10 2 10

ms

= + + = =

The bandwidth of the link is 10 .Mbps

Node A transmits 1000 pps with each packet 8,000 bits 8.0Mbps; thus,the throughput is 8Mbps. The utilization is clearly 0.8 (notice NO dimension).



64/95

at causes ueue dela at A

pointers from entry port to exit port onthe switch.

Some delays are possible due to the

communication protocol being used. Also, there may be a waiting line formingbecause the transmitter is busy at the

exact moment t at a pac et s rea y totransmit. (Well consider this later.)


Bit Length (previous example)


65/95

Bit Length (previous example)

Propagation Speed:

Note: One bit has length just as one packet does.

82.0 10 / secm

6Bandwidth:

8

6.BitLength 20

BandWidth 10 10meters

bitbps

= = =

2,000,000mThis implies that 100,000 bits can be20m/bit =


held or "stored" on this link at one time.

ela Bandwidt Product


66/95

ela -Bandwidt Product

,

length 20m fitting on the 2,000 km fiber.This can be directl com uted usin

( )

66

8

2 1010 10

metersdelay bandwidth bps

=

-100,000 .bits=

many bits sender must transmit beforefirst bit arrives at receiver.


Per or ance Ter inolo


67/95

Per or ance Ter inolo

B byte

6

b bit

MB and Mb mega-byte and mega-bit.The "mega" commonly means 10 for network bandwidth (Mbps).

20The "mega" commonly means 2 for computer file sizes.

KB and Kb kilo-b3

yte and kilo-bit

The "kilo" commonly means 10 for network bandwidth (Kbps).

10

3

The "kilo" commonly means 2 for computer file sizes.

BUT in this class we will always use 10 for kilo an 6d 10 for mega. It makes arithmetic easier.


Per or ance Ter continued


68/95

Per or ance Ter continued

round-trip time and equals 2 times total one-way delay (latency).RTT

Latency means delay and, in this class, we will always specify delay of something in particular.

for example, latency between first bit sent and last bit received for a file transfer.

Throughput refers to the rate at which data (bits) in which we are interested move from sender

to receiver. We may be interested in all bits. We may be interested only in data bits (minus headers).

We ma or ma not enalize throu h ut for retransmissions in case of errors. Such conditions mustbe specified in any given problem. (Or must be clear because no mention is made of headers or

retransmisions...)


File Transfer and the Train Model


69/95

File Transfer and the Train Model

Suppose that a file is to be transferred from A to B over a km fiber link of bps. The

total size of the file is bytes. HOWEVER, the file cannot be transmitted in a single packet;it must be di

d bw

fsvided into packet that have a maximum size of bytes. How long does it take tops

transfer the entire file? (Here we have no concerns about errors in transmission.)

Example: Suppose a file is to be transferred over a 3000 km link of 155 Mbps. File size

.


Train Model Solution


70/95

Train Model Solution

A Bn begin1

12n + 1 transfer time

n 2 1 + 1 prop delay

First packet arrives after transfer time + propagation delay (1st train car).

1 packets (cars) remain and one will arrive for each new interval of transfer time.

TOTAL transfer time will be one propag

n

ation delay plus ( ).n transfertime


ile Trans er or ula


71/95

ile Trans er or ula

File transfer time where:tf pd n tt = +

number of packetsfs

nps

=

bits

(bytes) 8byte

transfer time

ps

tt

=

sec

m

bw

8kmpropagation delay m

2 10pd =


ile Trans er xa le


72/95

ile Trans er xa leWe were given a link length (distance) of 3000 km, a bandwidth of 155 Mbps, a file size

6

.

Total file transfer time is as follows:

100 10 b tes

1500 byten = , 7 pkts

s

bits1500 bytes 8

byte

=

= =6

6

. .bits155 10

sec

3 10 meters15 ms.

meterspd

= =

sec

66,667 0.077 15 5.148 seconds.tf

= + =


oundation


73/95

oundation

Goals:

Overview:ay oun a on or


Applications


Required services

Terminology



Protocols and


Socket Interface

Performance Metrics

Network Analysis: Foundation 1-73Network Analysis: Foundation 1-73Network Analysis: Foundation 1-73

Abstract Com m unication Channel


74/95

Server

Client


ulti le Access rotocols


75/95

ulti le Access rotocols

single channel shared by multiple nodes (two or more)

on y one no e can en ucce u y a a me

multiple access protocol: distributed al orithm that determines how stations share

channel, i.e., determine when station can transmit communication about channel sharing must use channel itself!

what to look for in multiple access protocols: synchronous or asynchronous information needed about other stations robustness e. ., to channel errors

performance


Multi le Access Protocols: a taxonom


76/95

Three broad classes:

divide channel into smaller pieces (time slots,

frequency)

a oca e p ece o no e or exc us ve use

Random Access

recover from collisions

Takin turns

tightly coordinate shared access to avoid collisions


oa : e c en , a r, s mp e, ecen ra ze

Channel Partitionin TDMA


77/95

access to channel in "rounds" each station gets fixed length slot (length = pkt trans

time) 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

,

cycle users and at light load. FDM (Frequency Division Multiplexing): frequencysubdivided.


Channel Partitionin FDMA


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FDMA: frequency division multiple access

each station assigned fixed frequency band

unused transmission time in frequency bands goe

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

cy

bands

frequ

en


Rando Access rotocols


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When node has acket to send transmit at full channel data rate R.

no a prioricoordination among nodes

wo or more rasnm ng no es -> co s on ,

random access MAC protocol specifies:

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

retransmissions)

slotted ALOHA ALOHA


CSMA and CSMA/CD

lotted Alo a


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time is divided into e ual size slots = kt trans. time

node with new arriving pkt: transmit at beginning ofnext slot

if collision: retransmit pkt in future slots withprobability p, until successful.


Success (S), Collision (C), Empty (E) slots

What channel service does an a need?


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Data loss

Bandwidth. .,

tolerate some loss other apps (e.g., file

some apps (e.g.,

multimedia) requireminimum amount otrans er, te net require100% reliable data

transfer

bandwidth to beeffective

Timing some apps (e.g.,

o er apps e as capps) make use ofwhatever bandwidth

,

interactive games)require low delay to be

they get


e ec ve


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file transfer-

no loss elastic

no

Web documentsreal-time audio/video

loss-tolerantloss-tolerant

elasticaudio: 5Kb-1Mb

video:10Kb-5Mb

noyes, 100s msec

stored audio/videointeractive games

financial apps

loss-tolerantloss-tolerantno loss

same as abovefew Kbps upelastic

yes, few secsyes, 100s msecyes and no


Co on ailures


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Burst error

Link failure


ailure xa le:


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Suppose that two nodes, A and B, are connected by a communication link and

.

the probability of an error in the packet is .e In this case, transmission fails

and the packet must be retransmitted. What is the expected number of transmissions

per packet?

Let 1 2 ... to re resent the number of the first successful transmissiX = on.

[ ] [ ]

(What do we call ?) What is the probability that 1? (Write this as

1 .) What is 2 ? (Recall the idea of independent events.)

X X

P X P X

=

= =

[ ]What is , for any positive integer ?

: Return to thi

P X k k

NOTE

=

s after review of introductory probability.


olution


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( ) ( )1The random variable has a geometric distribution and 1 ,

for 1,2,

k

X X p k e e

k

=

=

( )1 21

1 1The expected number of transmissions is 1 .

11

k

k

eke e

ee

=

= =


Expected Number of Hops in Linear Network

Suppose that the nodes 1,2, , are connect in a straight line asn


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1 2 3 1 .

Also suppose that a randomly chosen packet is equally likely to

n n

belong to the communication of any of the possible pairs.

What is the expected number of hops for each packet transmitted?

To begin with we consider the case of 3 nodes (2 nodes is trivial).

( )

We define the random variable for 1,2 if a random

packet travels hops from source to destination.

H k k

k

= =

Ther

( ) ( ) ( )

e are 3 ways to choose two nodes (source & destination) for

communication - 1,2 , 1,3 , 2,3 . These give us two routes that

are 1 hop 1 2, 2 3 and one route that is two hops 1 3 .

Because a random packet is eq

( ) ( )

ually likely to be traveling any of

2 1the 3 routes, we have 1 and 2 .

3 3 p H p H = = = =

Network Analysis: Foundation1-86

( )Answer: 1 2 .3 3 3

E H = + =i i

E(hops) in a Linear Network General Case

In general there are routes of length for 1,2,..., 1. (HW: proven k k k n =


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this assertion by induction on the number of nodes 2.) We can check

this quickly by noting that

n

1 1 1n n n

number of routes of length( ) ( )1 1 1

. The2

last expression equals , which is the total ways to pick pairs from nodes.2

k k k

k n k k

n n

= = =

= = =

From this it follows that the probability that a random packet is on a route

( )( )

( )( )

of

2length is .

1 1

n kn kk p H k

n n n n

= = =

2

( ) ( )1

1

We can now find the expected number of hops asn

k

E H k p H k

=

= = i

( )

( ) ( ) ( ) ( ) ( )( ) ( ) ( )

1 1 1 12

1 1 1 1

2 2 2 2

1 1 1 1

1 1 2 12 2 2 1 1

n n n n

k k k k

n k nk k n k k k

n n n n n n n n

n n n n nn n n

= = = =

= = =

+

i


( ) ( ).

1 2 1 6 3 3n n n n

Multi lexin exam le continues


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place of node A and are sharing the10Mbps link to B. Actual traffic is 8 Mbps.

A2

A10


Recall TDMA and FDMA


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FDMAExample:

timeTDMA

frequency

Network Analysis: Foundation 1-89time

Consider node A1


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-

node A1 is assigned the first time slot from each second. If incoming

traffic is evenl distributed amon all in ut nodes, then each

node will use 80% of its assigned 1 Mbps channel and total

channel utilization will remain at 0.8.

If all traffic is originating at node A1, then no more than

1 Mbps of total traffic can be sent to B even though link

utilization would only be 0.08.


Recall lotted Alo a


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time is divided into e ual size slots = kt trans. time

node with new arriving pkt: transmit at beginning of

next slot if collision: retransmit pkt in future slots with

probability p, until successful.


Success (S), Collision (C), Empty (E) slots

Slotted Aloha Throu h ut Anal sis


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Recall throughput implies the number of packets per unit time

that successfully cross the network. To make tractable, we

assume that each workstartion transmits the same length packet" "every me an we a e e s o me o e equa o pac e

transmission time. We also take this to be our "unit time."

of all arrivals at these stations has a Poisson distribution with

mean arrival rate of per unit time (or slot-time length). We

make the simplifying assumption that the sum of all arrivals plusretransmissions (due to "collisions") has a Poisson distribution


with mean arrival rate of per unit time.

lotted Alo a Continues

Suppose any terminal begins transmitting a packet What is the


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Suppose any terminal begins transmitting a packet. What is the

probability that the packet transmits successfully? In slotted Aloha

success occurs if no other terminal got a packet ready for transmission

during the previous slot, that is, if there are no other arrivals

during unit time. Let be a random variable (RV) that gives

the number of arrivals plus retransmissions in unit time. has

X

X

[ ]k

,

= . The probability of no "other" arrivals during!

eP X k

k

=

[ ]the slot prior to our given transmission is 0 = . Because

the rate of transmission attempts is p

P X e

=

er unit time and the probability


of success (for a given attempt) is , then the throughput must

be .

e

e

ax rouput n ure o aax roupu n ure o a


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ax rouput n ure o aax roupu n ure o a


From Tanenbaum Com uter NetworksA large population of ALOHA users manages to generate 50 requests/sec


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A large population of ALOHA users manages to generate 50 requests/sec

(including orginals plus retransmissions). Time is slotted in units of 40 ms.

(a) What is the probability of success in the first attempt?

(b) What is the probability of exactly collisions and then a success?k

1000For a : There are 25 slots er second. The load is 2 frames/slot.=

40

Success o 2

2 2

ccurs if no other transmission so that prob of success is .

k

s e

=

=

2

For (c): The throughput is 2e 0.27 frames per slot (assuming slottedALOHA).

=


Foundation Section 1

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