Overview of Computer Networkingcs.hac.ac.il/staff/martin/Networks/slide01-1.pdf · Computer Networks — Hadassah College — Fall 2015 Overview Dr. Martin Land 5 Topology Node Host

1Dr. Martin LandOverviewComputer Networks — Hadassah College — Fall 2015

Overview

of

Computer Networking


What is Computer Networking?Logical separation of tasks in digital systems

Data exchange between computation unitsCommunication:

Local operations (ALU, load, store, branch, OS, …)Computation:

Local computationRequest information

Receive informationLocal computation

Accept requestProcess requestLocal computationSend response

communication

communication


What is Computer Networking?

Local computationRequest information

Receive informationLocal computation

Accept requestProcess requestLocal computationSend response

Making this workRules — lots of rules!Special hardwareSpecial software

Logical separation of tasks in a digital system

Data exchange between computation unitsCommunication:

Local operations (ALU, load, store, branch, OS, …)Computation:

communication

communication


Approaches to NetworkingWhat's required

Understanding how people and machines communicate

What's technically possibleNetwork topology (graph theory)Message encoding (information theory)Speed and delay (performance theory)

Historical engineering solutionsDivision of laborHierarchy (top-down)Security


Topology

Node Host node

Network edge — user systemsComputer, workstation, …

Intermediate node Hardware/software systems for data communicationModem, hub, switch, concentrator, multiplexor, router, …

LinkTransmission path between neighboring nodes

HopData transfer between neighboring nodes over one link

ChannelTransmission path between nodesMay include intermediate nodes

Computer network as directed or undirected graph

Link

Channel

HostNode

HostNode

IntermediateNode Host

Node


Network Topologies

Bus

Tree

Star

IrregularCompletely Connected

Ring


How People (and Machines) CommunicateRequirements

LanguageMediumNames Rules of conversation (protocols)

PreferencesKeep it simpleWork with minimum details necessary for specific taskObtain details dynamically as needed

Models Define roles in computation processDefine roles in communication processDefine rules of behavior for each role


Models Typical roles in computation

Application programCalling function / called functionOS serviceClass or object

Typical roles in communication

Example — client/server model

both roles Primary and SecondaryBalanced

swap roles Primary ←→ SecondarySymmetric

responds to requestSecondary

initiates request and accepts responsePrimary

Responds to client request (Secondary)Server

Initiates request to server (Primary)Client

Concurrent application programs / threadsClient and Server


Transaction Model

Transaction → request + response

Send Request

Send ResponseReceive Response

Accept RequestRequest

Response

Primary Secondary

Processing

General model with many casesFamiliar examples

main() calls function(x)Procedural transaction

Browser requests page from websiteClient / Server transaction


Layered SystemsSystem divided into logical layersWithin layer

Subsystems interact tightlyExample

Between layersSubsystems interact through programming interfaceExample

// subsystems: i, a[i], b[i], c[i]for ( i = 0 ; i < 1024 ; i++){

a[i] = b[i] + c[i] ;}

// subsystems: main(), f(x)main(){

y = f(x) ;}f(x){

return y;}

main()

Calling function, Primary f(x)

Called function, Secondary


Standard Agent RelationshipsAgent

Software or hardware entity

Peer relationshipTwo+ independent agents at same layer in layered modelExamples

Independent user application layer programsMicrosoft Word + PowerPointWeb Client (browser) + Web Server (website)

Independent OS layer programsUSB driverWiFi driver

Service relationshipmain() calls function(x)Microsoft Word calls printer driverApplication program opens socket (OS call)


Peer‐to‐Peer TransactionPeer-to-Peer (P2P)

Transaction between agents of equal level or statusUsually CLIENT / SERVER model (not necessarily)

ExampleWeb service

Browser and web server — application programs (equal status)

Request Browser (web client) sends page request to web server

Response Web server sends page content to browser

http://www.domain/page.html

page.html

Primary —web client Secondary —web server


Protocol ExamplesTransaction protocols

Hypertext Transfer Protocol (HTTP)Browser requests web page from web serverWeb server provides page as response

Post Office Protocol version 3 (POP3)Client system requests email messages from email serverEmail server provides messages as a response

Protocols


Service TransactionService

Transaction between agents of unequal level or statusExample

User program makes OS call to open fileUser program is application running above OSOS performs performs low-level services for applications

RequestApplication program issues OS call

ResponseOS opens file and returns file descriptor

Primary — user program

Secondary —OS

open file

filedescriptor


Service Transaction ExampleCalling function → Called function

Request Caller invokes called function with parameter

ResponseCalled function returns with result

user(){local workresponse = provider(parameters)local work

}provider(parameters){

local workreturn response

}

Service transactionService request

+Service response


General Layered Service ModelTask divided into layers

Layer n

Provider to layer n + 1User to layer n – 1

Interface

Boundary between layers

Simple example

Two service transactionsLayer 3 calls layer 2Layer 2 calls layer 1

Layer 2 Provider to layer 3User to layer 1

layer_3(){local workresponse-2 = layer_2(p3-2)local work

}layer_2(p3-2){

local workresponse-1 = layer_1(p2-1)local workreturn response-2

}layer_1(p2-1){

local workreturn response-1

}


ProtocolProtocol

Rules for transaction between peersExamples

SyntaxSemanticsSynchronizationProceduresAlgorithms Naming

Layered communicationCommunication task divided into layers

Protocol stackSpecific peer-to-peer protocol defined at each layer

Protocols


Protocol Stack

Tanenbaum (3rd ed) Figure 1‐9, p. 17


Services and Protocols


Protocol Stack Example

Tanenbaum (3rd ed) Figure 1‐10, p. 19


Layered Protocol ModelLayer n protocol

Performs VIRTUAL COMMUNICATION between layer n peers Exchanges layer n information with layer n peer

Layer n serviceReceives request from layer n + 1Passes request to layer n – 1 for communication serviceReceives response from layer n – 1

Layer 1

…

Layer n – 2

Layer n – 1

Layer n

Layer 1

…

Layer n – 2

Layer n – 1

Layer nLayer n protocol

Virtual peer transaction

Layer 1 protocol

Physical peer transaction

ServiceTransactions Layer n – 2 protocol

Virtual peer transaction


Encapsulation — Protocol HeadersLayer n – 1 protocol

Receives service request from layer nRequest = message to layer n peer agent

Adds layer n – 1 HEADER

Header = message to layer n – 1 peer agent

Protocol Data Unit (PDU) at layer n – 1 Message output from layer n – 1 protocolLayer n PDU + layer n – 1 header

Service Data Unit (SDU) at layer n – 1 Layer n PDU = random data for layer n – 1

Layer n – 1

Layer n

Layer n – 1

Layer n

Layer n – 1 SDU = Layer n PDULayer n –1 Header

Layer n PDU

Layer n – 1 PDU


Functional Analysis of CommunicationOpen System Interconnection Model (OSI)

DescriptionFunctionLayer

Physical

Data Link

Network

Transport

Session

Presentation

Application

Data transmission between neighboring hardware agents on physical channels (electrical, optical, radio, …)1

Control of data transmission between neighboring hardware agents (one hop)2

End-to-end data routing between host nodes via multiple hops3

Reliable end-to-end data exchange between host nodesPrevents data loss, errors, repetitions, ordering errors

4

Identification, separation, and continuity of multiple ongoing data transactions between software agents5

Syntax and semantics of exchanged data6

Exchange of data between user applications7


Example of OSI Functional LayersHypothetical OSI web browser

Example FunctionsLayer

Physical

Data Link

Network

Transport

Session

Presentation

Application

Data bits exchanged with next-hop data communication hardware on physical channels

Data bytes exchanged between host computer and next-hop data communication hardware

Find route to web server by network addressFile requests/data exchanged with server by network address

Each request/response checked for errors and completenessEach requested file provided to session layer without errors

Web page includes multiple graphic filesEach file requested and received as separate conversation

Encoding standard for Hebrew (Windows, UTF, ISO, …)

Browser provides GUI — requests web pages by URL


Internet Functional Model

Physical

Data Link

Network

Transport

Session

Presentation

Application

OSI Function CommentInternet

LayerOSI

Layer

Infrastructure

Network

Transport

Application

1

Internet protocols do not discuss physical data transmission

2

End-to-end data routing as in OSI3

4

Internet session management can be:Reliable — with transport serviceUnreliable — without transport service

5

6Application provides presentation service and some session service (transactions)

7

Ref: http://tools.ietf.org/html/rfc4949


Example of Internet Functional LayersTypical web browser

Example FunctionsLayer

Infrastructure

Network

Transport

Application

Network layer messages sent to Internet data communication equipment

File requests/data exchanged with server by network routing (RIP, OSPF, IGRP, BGP)Transfer data across network by network address (IP)

Each file request conversation identified for error control (TCP)Each requested file provided to session layer without errors

Browser provides GUI — requests web pages by URLTranslate (DNS) URL into network address (IP) for web server

Encoding standard for Hebrew (Windows, UTF, ISO, …)Web page includes graphic files

Each file requested/received as separate conversation (HTTP)


Internet PDUsProtocol Data Unit (PDU)

PDUMessageLayer

Signal

Frame

Datagram

Segment

Message

Bits

Header + Trailer

Header

Header

Data

Physical

Data Link

Network

Transport

Application

T-DLApplication DataH-TH-NH-DL

Headers added by layers 2, 3, 4 Trailer

Host-to-host data frame

network datagram

transport segment


Internet EndpointsNetwork Endpoint

Address of SOFTWARE AGENT running in HARDWARE AGENT

Network Address + Port

Physical connection

Identifies hardware device (node) in local network

Identifies computing node in global network

Software address identifies program exchanging data

Associates file descriptor with network endpoint

Communication IDLayerSystem Level

Physical

Data Link

Network

Transport

Application

Attachment

Hardware Address

Network (IP) Address

Port

Socket

Hardware

OperatingSystem

User

Well-known portsStandard services defined on ports 0 – 1023


Data Communication Equipment (DCE)

Physical

Data Link

Network

Layer

Modulator/demodulator (modem)Transmits and receives digital bits over physical medium

Manages physical transmission layerExchanges Frames among neighboring hardware agents

Receives Network Datagrams in Data Link FramesSends Datagrams in Data Link Frames to next hop on path to destination

Function DCE

Network Interface

Card

Switch(Hub)

Router

Ethernet Hub

WiFi Hub

Internet Router

Internet Core


Internet Hops

Host nodesApplication data (message) sent to Transport for reliable exchangeTransport segment sent to Network for addressing and routing

Intermediate nodesExamine Network datagrams for addressing and routingTreat Transport segment as meaningless data

Physical

Data Link

Network

Transport

Application

Physical

Data Link

Network

Transport

Application

Physical

Data Link

Network

Physical

Data Link

Network

Host Node

Host Node

Intermediate Nodes

hop hop hop


Network Zoo

Wide Area Networks (WAN)Public Switched Telephone Network (PSTN)

Local loop, backbone, PDH/SDH, ESS, ISDNPublic Switched Data Network (PSDN) — X.25

Broadband Integrated NetworkATM, B-ISDN, Frame Relay

Cellular 2.5G (GPRS/EDGE), 3G (UMTS, CDMA2000), 4G (WCDMA)

Local Area Networks (LAN < 2 km)Ethernet, WiFi, VLAN, token ring, token bus, FDDI, …

Personal Area Network (PAN < 20 m)Bluetooth, ZigBee, IrDA, …

Commercial network protocol stacksSNA, DECnet, Windows Networking, AppleNet, Netware, …

Many network types with specific protocol stacks


So, what is 'The Internet'?Internet = Inter-Networking

Protocols for connecting heterogeneous networks

Autonomous System (AS)Any network running its own protocol stack

Internet Gateway Runs network-specific protocol stack on ASRuns Internet protocols on connection to Internet core

Internet coreBackbone network of Internet routersConnected by dedicated links

Typical implementationHosts run network-specific protocols on internal ASHosts use Internet protocols for external messagesNo difference at infrastructure level

Gateway

Gateway

Internet Core

AS

AS


Intranet?Intranet

Using internet protocols in ASPure intranet

Internet protocols above Ethernet/WiFi LANWindows network

Uses Internet protocols for transport and addressingUses Microsoft protocols for message syntax, node location, …

Gateway

Gateway

Internet Core

Intranet AS

AS

Internet protocolsover Ethernet


Hey! Hey! You! You! Get Off of My Cloud

Cloud ≠ Internet ≠ NetworkNetwork

Collection of agents with single defined protocol stack

Internet Collection of agents using inter-networking protocols at layers 3 & 4

Cloud Business modelOrganization A rents computing service from provider COrganization A offers service to user B via provider C network

words and music: Mick Jagger and Keith Richards

ProviderC

Massive Computing

Infrastructure

OrganizationA

No Computing Infrastructure

UserB

Client Computing

Infrastructure

BusinessContract

ServiceOffer

ServiceUse

ServiceConfiguration


Why Cloud Computing?Outsourcing service model

User gets computing services from service providerService Level Agreement (SLA) guarantees customer serviceProvider handles operations+administration+maintenance (OAM)

Business advantages to organization Economies of scale — large provider can do it cheaperCuts labor/capital costs from balance sheet → happy investors

Based on standard technologiesCloud service organized from conventional resources

Hardware + software + networkProvider offers menu of services

Not a fundamentally different computing technologyUnique technological issues

Service reliability — provider committed to SLAOptimization of provider-side resource configurationOptimization of user-side resource configuration


Service Configuration in Cloud ComputingInfrastructure as a service (IaaS)

Organization sees virtual hardware environment Real hardware or hypervisor / system virtual machine

Organization installs OS → installs software → user runs jobs

Platform as a service (PaaS)Organization sees virtual OS environment

OS on single hardware platform or virtual OS

Organization installs software → user runs jobs

Software as a service (SaaS)Organization sees virtual application software environment

Applications running on private OS or "sandboxed" on shared OSSandbox — private execution environment per application instance

User runs jobsStorage as a service (STaaS)

User sees virtual mounted storage device


Centralize → Decentralize → Centralize → ?1950s — 60s

Centralized mainframe computer + multiple OS instances over hypervisorTimesharing OS serves multiple usersUser sees OS environment via dumb terminal (thin client)

1970s User applications offloaded to minicomputers + timesharing servicesUser sees timeshared OS environment via dumb terminal

1980sUser applications offloaded to personal workstations (PC)User sees single-user OS environment running locally

1990sNetwork single user workstations User sees single-user OS environment running locally

2000sCentralized control of local OS environment by IT departments

2010sCloud + netbook / tablet / smart phone — dumb terminal with high-res GUI


Issues in Cloud ComputingCost

Provider issuesEconomies of scale ⇒ lower cost per compute job

Organization issuesCapital + OAM costs → operating costsLower start-up costs ⇒ operating debt

Reliability Provider issues

Redundant infrastructure → continuity + disaster recoveryCentralized management of OAM, security, performanceVirtualization → serve multiple users on physical serverMultitenancy → provide multiple sandboxed application instances on OS

User sees guaranteed serviceAgility

Organization / provider reconfigure service as needed Growth, load balancing, time-zone serving


Cloud OwnershipPublic cloud

Service provider as public utility — sells / rents computing serviceInitial providers leverage large existing infrastructureAmazon, Microsoft, Google, IBM

Menu of services at fixed prices

Private cloudCloud infrastructure for private organizationManaged internally or outsourcedIsolates service developers from implementation issues

Standard development platform

Requirements for economic justificationLarge organization Technology-based servicesFrequent new serviceExample — internet content provider


Programming in the CloudDepends on environment

IaaS — Organization sees virtual hardware environmentPaaS — Organization sees virtual OS environmentSaaS — Organization sees virtual application software environment

IBM BluemixSaaS from IBMFree accounts for students using [email protected] addressBluemix DevOps Services

Develop, track, plan, and deploy software on IBM cloud serviceCollaboration tools — Git, Jazz SCM, GitHubBuild application → deploy to IBM cloud Supports

Arduino, C, C#, C++, CSHTML, Embedded, JavaScript (ejs) Erlang, Go, HTML, abstraction markup language (Haml) Jade, Java, JSON, Lua Objective‐C PHP, Python, Ruby, Swift, Virtual, Basic (vb) VMHTML, XHTML, XML, Xquery, yaml, Launch, file Dockerfile, gitignore, git config, cfignore

"You can go from source code to a running app in minutes."


Some Internet ProtocolsApplication layer transactions

Hypertext Transfer Protocol (HTTP)

Transport layer Transport Control Protocol (TCP)

Reliable transport service

User Datagram Protocol (UDP)Unreliable transport service

Network layerInternet Protocol (IP)

Node addressing

Internet Control Message Protocol (ICMP)Messages about messaging

Routing protocols (RIP, OSPF, IGRP, BGP)Learn network topology for message forwarding

792ICMP

791IP

768UDP

793TCP

2616HTTP

RFCProtocol

RFC — Internet standard


What Internet Protocols Do

Hypertext Transfer Protocol (HTTP)Application layer transactions

Some examples

Responses

Requests

Status of transactionStatus

Contents of requested fileData

Delete file by nameDelete

Replace file by namePost

Retrieve file by nameGet



Domain Name Service (DNS)Translates node name to Internet address (and vice versa)

Example

Some examples

c:\> nslookup www.hadassah.ac.ilName: www.hadassah.ac.ilAddress: 212.179.79.228



Transport Control Protocol (TCP)Reliable transport service

SenderLabel source and destination software by port numberNumber outgoing segmentsWait for ACK (acknowledgment) for outgoing segmentsRetransmit segments if no ACK before timeout Negotiate segment size (for error and congestion control)

ReceiverCheck completeness and order of incoming segments Check incoming segments for errorsSend ACK for good segmentsProvide good incoming segment to destination software

Some examples



Internet Protocol (IP)Best effort network serviceNo guarantee of delivery

IP version 4 addressFour octets 0.0.0.0 to 255.255.255.255 (many reserved addresses)

SenderAttach source and destination network addresses to segmentRoute IP datagram to next hop along route

Receiver Intermediate node — route IP datagram to next hop along routeHost node — provide segment to transport layer

Some examples


Network Infrastructure

ScaleWide Area Network (WAN < earth)Local Area Network (LAN < 2 km)Personal Area Network (PAN < 30 m)

Medium

Traffic statisticsConstant Bit Rate (CBR) — peak data rate = average data rateVariable Bit Rate (VBR) — peak data rate > average data rate

Layers 1 + 2 — bits, bytes, signals, cables, electronics

Copper wire and cableElectrical signals

Requires legal right to transmit radioOpen space

Radio wave signals

Requires legal right to install cablesOptical fiber

Light wave signals


Connectivity = Medium + Topology Point-to-point

Dedicated link from node to nodeFastest and most complex

SwitchDedicated link from node to switchSwitch connects nodes on request

Non-blocking provides n × (n – 1) connectivityBlocking provides n × m connectivity (m < n – 1)

Shared mediumNodes share medium accessContention

Nodes compete for access

PollingCentral controller polls nodes

bus

wireless


Physical Transmission Serial data rate at physical layer

Bits per second = bps = b/sBytes per second = B/s1 B/s = 8 b/s

Capacity (bandwidth)Maximum data rate on mediumFixed by transmitter / medium / receiverLimits

Speed of circuitsSignal to noise ratio (SNR)

01


Physical Transmission Throughput

Takes account ofUtilization = % time transmitter sendingErrors ⇒ re-transmission ⇒ more data on same capacityDelays ⇒ less data received on same capacity

2 3 1 4

utilization = 10 / 16 = 62.5%

0 16

bit errors

bits received

error-free data received per secondthroughput

capacity=


Baud Rate

SymbolPhysical signal that encodes bits

Symbol rate (Baud rate)Symbols transmitted per second

Bit transmission rateBits transmitted per second = (symbols / second) × (bits / symbol)

ExamplePulse amplitude modulation (PAM)Define 2N electrical levels from 0 to 11…1Each symbol (level) transmits N data bits

0001

1011

N = 2 (4 Level) PAM1.00 V

0.50 V

0.75 V

0.25 V

Symbols per second


Baud Rate

33 kbps dial-up modemDefine 210 = 1024 electrical symbols (max for SNR on phone line)Baud rate = 3300 symbols / second

Bits transmitted per secondData rate = (3300 symbols / second) × (10 bits / symbol)

= 33,000 bps

0000000000

00000000010000000010

1111111111

N = 10 (1024 Level) PAM

...

Symbols per second


Data Concentration High capacity link

No single node can utilize link capacityExample

Optical fiber cable with 4 fibers at 25 Gbps = 100 Gbps

Multiplexing Combine multiple nodes onto one linkExample

Optical fiber with 25 Gbps data rateCombine 25 nodes transmitting at 1 Gbps

25 inputsat 1 Gb/s

1 output at25 Gb/s

Multiplexor


Multiplexing MethodsFrequency Division Multiplexing (FDM)

Divide available frequencies (bandwidth) among nodesNodes transmit simultaneously on different frequencies

ExampleFM radio uses 88 MHz to 108 MHz = 20 MHz bandwidthDivide 20 MHz into 100 channels = 200 kHz per FM channel

88 91.3 93.9 95.5 96.6 97.8 101 104.8 MHz

88 מ וס י קה צ"ג ל ' ב צ"ג ל ' ג י ר ושל ים ' ד


Multiplexing MethodsTime Division Multiplexing (TDM)

Divide capacity into time slotsNode transmits in assigned time slot

ExampleE1 digital line transmits at 2048 kbpsDivide 2048 kbps line into 32 time slots = 64 kbps per node

32 x 64 kbps = 2048 kbps = 2.048 Mbps

32 inputsat 64 kbps

1 output at2.048 Mbps

Multiplexor

32 outputsat 64 kbps

1 input at2.048 Mbps

Demultiplexor


E1 Multiplex

1125 s/sample

8000 samples/second= μ

32 inputsat

8000samples/sec

1 output at32 x 8000 x 8 bps = 2.048 Mbps

byte from line 0

byte from line 1

byte from line 2

byte from line 31

0 1 2 ... 31

125 sμ

Every 125 sec multiplexor (MUX)

receives 8‐bit sample from each line

(isochronous)

μ

125 sec/frame3.91 sec/sample

32 samples/frameμ

= μ


GSM CellularMixed Multiplexing

Time Division Multiple Access (TDMA)Used on GSM / UMTS phones — 2G and 3GCombines FDM and TDM

Frequency Division Multiplexing (FDM)GSM bands = 25 MHzDivide 25 MHz into 125 channels = 200 kHz per channelTransmit 270 kbps over 200 kHz channel

Time Division Multiplexing (TDM)Divide 270 kbps into 8 times slots = 33 kbps per user33 kbps = 23 kbps for voice + 10 kbps control


Data Statistics — CBRConstant Bit Rate (CBR)

Isochronous data Equal time interval between bitsBits per second = constant

Average data rateAverage data rate = peak data rate = minimum data rate

ExampleUncompressed digital audioSample analog signal every T seconds

Round-off sample to N-bit number from 0 to 2N – 1

Digital audio stream at N / T bps


Digital Voice on Telco Telephone Sample analog voice signal every 0.125 ms

0.125 ms per voice sample ⇒ 8000 voice samples / second

Round-off sample to 8-bit data

Data ∈ {0, 1, 2, ... , 255}Sample = {158.276, 158.879, 159.724, 159.821, 159.312, 158.791}Data = {158, 159, 160, 160, 159, 159}

DS-0 stream(8000 samples / second) × (8 bits / sample) = 64 kbps64 kbps digitized voice (no compression)

158159

160 160159 159

157

158

159

160

161

t


Multiplexing StatisticsDeterministic multiplexing (CBR)

N Nodes = N time slotsNode reserves fixed time slot

Guaranteed transmission capacityNode transmits in assigned time slot

Example E1 multiplex for wired telephone — 32 x 64 kbps = 2048 kbpsE2 multiplex — 4 x 2048 kbps = 8192 kbps

N Nodesassigned

fixedtime slot

DeterministicMultiplexor

N time slots at B bps

N x B bps


Data Statistics — VBR Variable Bit Rate (VBR)

Bursty dataPeak data rate B > average data rate λAssume packets are independent (Poisson statistics)

ExampleData sent by time-of-day client

Request time-of-day (1000 bits) once every hour (3600 seconds)Average data rate = 1000 bits / 3600 seconds = 0.28 bps

Peak data rate = 55 Mbps on 802.11g WiFiPeak data rate 55 Mbps > average data rate = 0.28 bps

( )

( ) ( )

, ,

, ,!

kT

P k T kT

TP k T e

kλ

λ

λ

λλ −

=

=

probability of bits arriving

in seconds when average rate =


Multiplexing StatisticsStatistical multiplexing (VBR)

M nodes > N time slotsBursty data

Average data rate λ < peak data rate B

Average traffic rate = M x λ < capacity rate = N x BActual traffic < capacity ⇒ OK

Actual traffic > capacity ⇒ data delayed or lost

Example Internet routers

M Nodesrequest

time slots

StatisticalMultiplexor

M > N time slots at B bps

N x B bps


Overflow in VBROverflow

Actual traffic > capacity Short time (a few time slots) ⇒ data delayed

Long time (many time slots) ⇒ buffer overflow ⇒ data lost

Overflow probabilityAverage traffic rate = M x λ

Average data arriving in time T = M x λ x T

Capacity rate = N x BData capacity in time T = N x B x T

Overflow in time TActual data arriving in time T > N x B x T

N x B x T + 1 or N x B x T +2 or N x B x T +3 or ...Independent outcomes

( ) ( ) ( ) ( )

1

1 2 ...!

overflow∞

− λ

= +

λ= + + = ∑

k

k

P P or or ek

M T

NBT

M TNBT NBT


Overflow ExampleAverage traffic on network

λ = 10 packets / second per nodeM = 10 nodes

Average packets in 0.1 second = M x λ x T= 10 nodes x (10 packets / second per node) x 0.1 second

= 10 packets

Maximum traffic on network (capacity)B = 25 packets / second per node

N = 4 nodes

Maximum packets in 0.1 second = N x B x T

= 4 nodes x (30 packets / second per node) x 0.1 second

= 12 packets

Overflow condition for T = 0.1 secondOverflow if actual traffic > N x B x T

( ) ( ) ( )

!10

13

10overflow 0.21 21%

∞−

=

= = =∑k

k

P ek


SwitchingSwitch

Multiplexor + DemultiplexorData at input_porti → output portji,j = 0, 1, 2, ... , N - 1

Example

N inputs x B bps= N x B bps

N outputs x B bps= N x B bps

Capacity = C bps

switch

1

2

3

4 1

2

3

4


Circuit SwitchingDeterministic multiplexing

Capacity C = N × BDedicated (reserved) link

input_porti → output portjNo competitionGuaranteed capacity B — if used or not

ExampleBezeq phone call64 kbps from telephone to telephone (even if no one speaks)



Capacity = C bps

switch


Packet SwitchingStatistical multiplexing

Capacity C = M × B < N × BDynamical time slot assignment (on request)

input_porti → output portjCompetition

More ports than capacity

Demand > capacity ⇒ delay

ExampleInternet routerPacket queue — first come first served



Capacity = C bps

switch


Connection TypesConnection

State machine associated with data exchange

Connection-orientedFirst set-up data channelMultiple data transactions associated with connection stateMonitor channel state during data exchangeClose channel after data exchangeExample — phone call

Enter number → answer call → extended conversation → disconnect

ConnectionlessTransmit data with no prior channel set-upNo channel state defined by nodesEach message independentExample — email message

Send email → hope message arrives → hope message is found / read


Datagram Service Network of routers and links

Packet switchingConnectionless

Each datagramHas source and destination address in header

Data Link header or Network header

Routed individually through networkDatagrams may follow separate routesExample

B → 1 → 4 → 6 → FB → 1 → 5 → 6 → F

AB

C

E

F

D

1

2 3

4

5

6

datasrc = B dest = F


Switched Virtual Circuit (SVC) Network of switches and links

Circuit switching or packet switchingConnection-oriented

Switched Virtual Circuit (SVC) Set-up / close messages carry source and destination addresses

Example

Packet routing by VC ID in header (layer 2 or layer 3)Every packet follows same VC route Example

AB

C

E

F

D

1

2 3

4

5

6

Set-up VC – 1: B → 1 → 4 → 6 → F

dataVC – 1


A to D — circuit mode (deterministic SVC)B to E — packet mode (statistical SVC)B to F — packet mode (statistical SVC)C to F — packet mode (datagram service)

Switching Example

AB

C

E

F

D

1

2 3

4

5

6


Transmission Delay

Transmission delay TT

TT = Time to inject bits into line = (bits in packet) / (bits per second)

Processing delay Tproc

Packet process time in intermediate nodeSVC with fixed route ⇒ shorter delay than datagram routing

Propagation delay Tprop

Tprop = (length of cable) / (signal speed)

Queuing delay TQ

Time packet waits in buffer for previous packets (congestion)TQ = (service time per packet) × (packets waiting in buffer)

Example: 1000 Mb / 100 Mbps = 10 sec

Example: 4 km / (2 × 108 km/s) = 2 × 10-8 sec << 10 sec

TT TpropTQ NodeTprocNode


Example of Queuing Delay

Queuing delay TQ

TQ = (service time per packet) × (packets waiting in buffer) Packets waiting in buffer = 1 / (1 – utilization)

Queuing delay exampleService time per packet = 10 ms / packet

Service rate = 1 / (10 ms / packet) = 100 packets / secondAverage traffic = S = 85 packets / second

Utilization = (85 packets / second) / (100 packets / second) = 0.85Buffer level = 1 / (1 – 0.85) = 6.67

TQ = (10 ms / packet) × 6.67 packets = 67 msC = switch capacity = service rate = 100 packets / second

Demand > 100 buffer ⇒ overflow ⇒ excess delay

( ) ( ) 85

1 1 101

85 0.05! !

demand demand k k

S

k C k C k

SP C P k e ek k

∞ ∞ ∞− −

= + = + =

> = = = = =∑ ∑ ∑

TT TpropTQ NodeTprocNode


Error ControlBit error

Data 1 received as 0 or data 0 received as 1

Packet LossCongestion or buffer overflow → packet discarded

Error detectionError correction code / redundancy code / checksumChecksum transmitted with data in header / trailerReceiver compares independent hash with transmitted code

Error controlRequired

Discard corrupt packet

Optional Retransmit discarded / missing packets

bit errors in received dataBit Error Rate (BER)

bits in received data=

packets lostPacket loss rate

packets transmitted=


Network ScalePrivate network

Small Office / Home Office (SOHO) Small number of computers in a few roomsSimple Ethernet / WiFi LAN

EnterpriseMany nodes in large building / campusComplex Intranet

Access networkProvide user connection to Internet coreInfrastructure provider manages layers 1 and 2Internet Service Provider (ISP) manages layers 3 and 4

Internet coreNetwork of routers and links at layer 3Infrastructure provider manages links at layers 1 and 2Links are typically built over complex network systems


Private Networks Simple Ethernet / WiFi LAN

Ethernet switching hub4 to 16 nodesFull connectivity (non-blocking)10 / 100/ 1000 Mbps

WiFi hubMore nodes lowers performanceNodes compete to transmit to hub11 / 54 / 100+ Mbps

Complex IntranetMultiple LAN hubsHubs connected

Directly (bridging)Indirectly (routing)


Non‐Private Networks

Service infrastructure Routing + accounting nodes in office buildings

Link infrastructure Cables + radio channels on public / private property

Legal and licensing issues

Controlled by companies in cable businessesTelephone companies (Telco)Cable TV companies Electric companies Railroads companies

Choices for small business Intranet at 3 locationsPay service provider monthly Or

Purchase LAN hubs and routersLease cables from Telco

Access + core


Telephone Network

Local loopWired connection to most buildingsCan carry 1 Mbps (up to 4 km) to 25 Mbps (up to 300 m)Voice network

Analog voice channel from 300 to 3300 HzDigitized voice at 64 kbps

Local presence (central office) in every neighborhoodLocal loop attached to non-blocking switches

Tree network of switchesCentral offices connect to regional offices on fiber optic backbone

Global broadband switched virtual circuit (SVC) networkCircuit mode switches (ESS7) for 64 kbps voiceCircuit / Packet mode layer 2 switches (ATM) up to 2.5 GbpsPrivate routers throughout network for Internet traffic

It's everywhere


Telephone Network

local loop

fiber optic cables

fiber optic cablesup to 40 Gbps

ESS ATM

Central Office

Router

local loop

ESS ATM

Central Office

Router

local loop

ESS ATM

Central Office

Router

switched virtual circuit (SVC)network

up to 2.5 Gbps


Cellular NetworkWireless to base station — uses Telco network for WAN service

Base System (BS)

Telco VoiceNetwork

CellController

ClusterController

Mobile SwitchingCenter (MSC)

Public Land Mobile Network

Mobile Station(MS)

HLRVLR

CellCluster

GPRS

Internet

SGSN

GGSN

Voice

Data


SOHO Access Networks Dial-up modem (modulator / demodulator)

Converts digital bits from computer to analog signals for phone lineUser modem connects to ISP modem by phone call56 kbps downstream / 33 kbps upstream

Digital Subscriber Line (DSL)FDM on local loopVoice channel connected to telephone voice networkData channel — 15 Mbps downstream / 750 kbps upstream

ATM link between DSL modem and Telco central officeDatagrams routed to ISP on Telco router network

Cable modemFDM on TV cableTV channels connected to TVData channel — 30 Mbps downstream / 2 Mbps upstream (shared)

Ethernet link between cable modem and cable head officeDatagrams routed to ISP on Telco router network


Enterprise Access Networks Leased line

Telco line to DCE on customer premises2.048 Mbps to 40 GbpsCarrier Ethernet — Ethernet extensions for metropolitan networks

Asynchronous Transfer Mode (ATM)Telco system for broadband switched virtual circuits (SVC)Optimized for multimedia transmissionLayer 2 ATM switch on customer premisesTelco line up to 2.5 Gbps

Frame Relay (FR)Telco system for broadband permanent virtual circuits (PVC)Layer 2 FR switch on customer premisesTelco line up to 45 Mbps

WiMaxWireless metropolitan networkApplies cellular technology for 40 Mbps data


Internet Core Internet backbone

Collection of core routers and fast links

Core routerFast router with very high I/O capacityUp-to-date routing protocolsHandle multiple layer 1 and layer 2 protocols

Fast linksVarious layer 2 protocolsSome simpleSome complex

Simple Layer 2 ProtocolFiber Optic Cable

Complex Mixture of Protocolsand Physical Media

Internet Core


Documentation Standards

Formal documentation of systems, algorithms, protocolsAdopted by international committeesRecord technical background and implementation requirements

Standards organizations

American National Standards InstituteUS government standards organization

ANSI

Association of Computing Machinery ACM

Internet Engineering Task ForceThe Internet Society inherited Internet from US government in 1989Internet standards called RFC (request for comment)Available at http://www.ietf.org/rfc.html

IETF

Institute of Electrical and Electronics EngineersIEEE

International Telecommunications Union - Telecommunications SectorUnited Nations standards organization (formerly CCITT)

ITU-T

International Standards OrganizationOrganization of governmental standards organizations

ISO

Documents

Overview of Computer Networkingcs.hac.ac.il/staff/martin/Networks/slide01-1.pdf · Computer Networks — Hadassah College — Fall 2015 Overview Dr. Martin Land 5 Topology Node Host