Applications/Design

Web BrowsingTelephone CallMultiplexingProtocolsIETFSummary

TOC – Applications

Web Browsing

ExampleLocating Resource: DNSConnectionEnd-to-endPacketsBitsPoints to remember

TOC – Applications – Web Browsing

Example

Click Link or URLget content from local

or remote computerURL:

http://www.google.com/stringSpecifies- Protocol: http- Computer: www.google.com- StringComputer (server) selects contents based on string

TOC – Applications – Web - Example

Locating Resource

www.google.com is the name of a computerNetwork uses IP addressesTo find the IP address, the application uses a hierarchical directory service called theDomain Name System

local

com

host

www.google.com?IP = a.b.c.d

IP = a.b.c.d

www.google.com?

TOC – Applications – Web - Locating

ConnectionThe protocol (http) sets up a connection between the host and google.com to transfer the pageThe connection transfers the page as a byte stream, without errors: pacing + error control

Host google.com

connect

OK

get page

page; close

TOC – Applications – Web - Connection

End-to-endThe byte stream flows from end to end across many links and switches: routing (+ addressing)That stream is regulated and controlled by both ends: retransmissionof erroneous or missing bytes; flow control

End-to-end pacing andflow control

Routing

www.google.com

host

TOC – Applications – Web – E2E

PacketsThe network transports bytes grouped into packetsThe packets are “self-contained” and routers handle them one by oneThe end hosts worry about errors and flow control:

Destination checks packet for errors (using error detection code CKS) and sends ACKs with sequence number #Source retransmits packets that were not ACKed and adjusts rate of transmissions

C

A | B | # , CKS | bytes

B C

www.google.comIP address: A

HostIP address: B

Destination

Next Hop

TOC – Applications – Web - Packets

Bits

Equipment in each node sends the packets as a string of bitsThat equipment is not aware of the meaning of the bits

01011...011...110

Transmitter Physical Medium Receiver

01011...011...110

OpticalCopperWireless

TOC – Applications – Web - Bits

Points to remember

Separation of taskssend bits on a link: transmitter/receiver [clock, modulation,…]

send packet on each hop [framing, error detection,…]

send packet end to end [addressing, routing]

pace transmissions [detect congestion]

retransmit erroneous or missing packets [acks, timeout]

find destination address from name [DNS]

Scalabilityrouters don’t know about connectionsnames and addresses are hierarchical

TOC – Applications – Web - Remember

TelephoneTelephone NetworkDialing a NumberSetting up a CircuitPhone ConversationReleasing the Circuit

TOC – Applications – Telephone

Telephone Network

Switches

Network

Logic Diagram

TOC – Applications – Telephone - Network

Switches

5ESS (Lucent)

DMS100 (Nortel)

TOC – Applications – Telephone – Network - Switches

Network

TOC – Applications – Telephone – Network - Network

Logic Diagram

TOC – Applications – Telephone – Network - Logic

Dialing a Number

S1 B

A

A Off-HookS1 ListensA dialsS1 Registers

TOC – Applications – Telephone – Dialing

Setting Up a Circuit

ring B

A

Circuit = capacity to carry one phone call (shown by thin lines)Circuit is allocated to the call between A and BCircuits are not shared; they are dedicated.

TOC – Applications – Telephone – Setup

Phone Conversation

B

A

Voice signals use the reserved circuits

TOC – Applications – Telephone – Conversation

Release Circuits

A

B

A or B goes Off-HookCircuits get released

TOC – Applications – Telephone – Release

Multiplexing

OverviewOperationsTDM/FDMStatistical MultiplexingAnalysis of TDM/FDMAnalysis of Statistical Multiplexing

TOC – Applications – Multiplexing

Overview

Networks are shared resourcesSharing via multiplexingFundamental Question:

how to achieve controlled sharing

TOC – Applications – Multiplexing – Overview

Operations

Methods for sharing a communication channelTradeoff between utilization and predictabilityCommon Approaches:

TDM (time-division multiplexing)Statistical Multiplexing

TOC – Applications – Multiplexing – Operations

Time Division Multiplexing

n linksrate r bpseach

Multiplexer1 link, rate nr bps

Frame:

Time “slots” are reservedbps = bits per second

TOC – Applications – Multiplexing – TDM

Statistical Multiplexing

n linksany rate

Multiplexer1 link, any rate

TraceExcerpt:

Variable-sized “packets” of data are interleavedbased on the statistics of the senders

TOC – Applications – Multiplexing – SM

Analysis of TDM/FDMThe rate of the outgoing link must be at least equal to the sum of the rates of the incoming linksConsequently, that outgoing link may be under-utilized if the incoming links are not constantly busyFDM (frequency division multiplexing) divides the outgoing link into separate frequency bands instead of time slotsWDM (wavelength) separates outgoing signals by sending them on different wavelengthsThese schemes are applicable only to fixed numbers of flowsRequires precise timer (or oscillator and guard bands for FDM)Resources are guaranteed

TOC – Applications – Multiplexing – Analysis TDM

Analysis of SM

The rate of the outgoing link must exceed the sum of the average rates of the incoming packet streamsTraffic is sent on demand, so channel is fully utilized if there is traffic to sendAny number of flowsNeed to control sharing:

packets are limited in sizeprevents domination of single sender

Resources are not guaranteed

TOC – Applications – Multiplexing – Analysis SM

Protocols

DefinitionExamplesStandards

TOC – Applications – Protocols

Definition

Agreement dictating the form and function of data exchanged between two (or more) parties to effect a communicationTwo parts: syntax and semantics

syntax: where bits gosemantics: what they mean and what to do with them

TOC – Applications – Protocols - Definition

Examples

Internet Protocol (IP)if you can generate and understand IP, you can be on the Internetmedia, OS, data rate independent

TCP and HTTPif you can do these, you are on the web

TOC – Applications – Protocols - Examples

Standards

New functions require new protocolsThus there are many (e.g. IP, TCP, UDP, HTTP, RIP, OSPF, IS-IS, SMTP, SNMP, Telnet, FTP, DNS, NNTP, NTP, BGP, PIM, DVMRP, ARP, NFS, ICMP, IGMP)Specifications do not change frequentlyOrganizations: IETF, IEEE, ITU

TOC – Applications – Protocols - Standards

The IETF

Specifies Internet-related protocolsProduces “RFCs” (www.rfc-editor.org)Quotation from IETF T-shirt:

We reject kings, presidents and voting.We believe in rough consensus and running code.

--- David Clark

TOC – Applications – Protocols - IETF

Summary

Network Structure:

Finding Destination: Directory ServiceRegulating Flows of Packets: E2E TransportFinding a path to destination: RoutingSwitching: Circuits (telephone) or Packets (Internet)Sharing a link = Multiplexing: TDM (telephone) or SM (Internet)Framing packets inside bit stream: LinkTransmitting bits

• Protocols, Standards, IEEE, IETF, …

TOC – Applications – Summary