EECS122 – Lecture 2 Department of Electrical Engineering and Computer Sciences University of California Berkeley.

EECS122 – Lecture 2

Department of Electrical Engineering and Computer Sciences

University of CaliforniaBerkeley

EECS 122 - UCB 2

TOC: Applications/Design

Web BrowsingTelephone CallMultiplexingProtocolsIETF

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Applications: Web Browsing

ExampleLocating Resource: DNSConnectionEnd-to-endPacketsBitsPoints to remember

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Click Link or URL get content from localor remote computerURL:

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

Web: Example

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Web: Locating Resourcewww.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?

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

H ost cnn.com

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Web: End-to-end

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

End-to-end pacing andflow control

Routing

www.google.com

host

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Web: 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

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Web: 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

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Web: Points to remember

Separation of tasks send 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]

Scalability routers don’t know about connections names and addresses are hierarchical

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Applications: Telephone

Telephone NetworkDialing a NumberSetting up a CircuitPhone ConversationReleasing the Circuit

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Telephone: Telephone Network

Switches

Network

Logic Diagram

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Telephone Network: Switches

5ESS (Lucent)

DMS100 (Nortel)

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Telephone Network: Network

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Telephone Network: Logic Diagram

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Telephone: Dialing a Number

A Off-HookS1 ListensA dialsS1 Registers

A

BS1

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Telephone: Setting Up a Circuit

A

Bring

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.

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Telephone: Phone Conversation

A

B

Voice signals use the reserved circuits

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Telephone: Release Circuits

A

B

A or B goes Off-HookCircuits get released

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Design: Multiplexing

OverviewOperationsTDM/FDMStatistical MultiplexingAnalysis of TDM/FDMAnalysis of Statistical Multiplexing

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Multiplexing: Overview

Networks are shared resourcesSharing via multiplexingFundamental Question:

how to achieve controlled sharing

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Multiplexing: Operations

Methods for sharing a communication channelTradeoff between utilization and predictabilityCommon Approaches: TDM (time-division multiplexing) Statistical Multiplexing

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MPX: Time Division Multiplexing

Multiplexern linksrate r bpseach

1 link, rate nr bps

Frame:

Time “slots” are reservedbps = bits per second

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MPX: Statistical Multiplexing

Multiplexern linksany rate 1 link, any rate

TraceExcerpt:

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

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MPX: Analysis of TDM/FDM

TDM, FDM (frequency division multiplexing), and WDM (wavelength) may under-utilize channel with idle sendersApplicable only to fixed numbers of flowsRequires precise timer (or oscillator and guard bands for FDM)Resources are guaranteed

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MPX: Analysis of SM

Traffic 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 size prevents domination of single sender

Resources are not guaranteed

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Design: Protocols

DefinitionExamplesStandards

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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 go semantics: what they mean and what to

do with them

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Protocols: Examples

Internet Protocol (IP) if you can generate and understand

IP, you can be on the Internet media, OS, data rate independent

TCP and HTTP if you can do these, you are on the

web

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Protocols: 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

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Design: 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