Introduction 1-1

02 - Switches and Access

2

Chapter 1Introduction

Computer Networking: A Top Down Approach Featuring the Internet, 5rd edition. Jim Kurose, Keith RossAddison-Wesley, July 2010.

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All material copyright 1996-2010J.F Kurose and K.W. Ross, All Rights Reserved

3

The Network Core

mesh of interconnected routers

the fundamental question: how is data transferred through net? circuit switching:

dedicated circuit per call: telephone net

packet-switching: data sent thru net in discrete “chunks”

4

Network Core: Circuit Switching

End-end resources reserved for “call”

link bandwidth, switch capacity

dedicated resources circuit-like

(guaranteed) performance

call setup required

5

Network Core: Circuit Switching

network resources (e.g., bandwidth) divided into “pieces”

pieces allocated to calls

resource piece idle if not used by owning call (no lending)

dividing link bandwidth into “pieces” Frequency Division

Multiplexing (FDM) Time Division

Multiplexing (TDM)

6

Circuit Switching: FDM and TDM

FDM

frequency

time

TDM

frequency

time

4 users

Example:

7

Packet Switching: Statistical Multiplexing

Sequence of A & B packets does not have fixed pattern statistical multiplexing.

(In TDM each host would get same slot in revolving TDM frame.)

A

B

C10 MbsEthernet

1.5 Mbs

D E

statistical multiplexing

queue of packetswaiting for output

link

8

Network Core: Packet Switching

each end-end data stream divided into packets

Packets from different users share network resources

each packet uses full link bandwidth

resources used as needed

resource contention: aggregate resource

demand can exceed amount available

congestion: packets queue, wait for link use

store and forward: packet must be completely received before being forwarded

packet loss: drop a packet from the queue, when too many packets

Bandwidth division into “pieces”

Dedicated allocationResource reservation

9

Packet switching versus circuit switching

Great for bursty data resource sharing simpler, no call setup

Does not restrict the number of simultaneous users

Takes advantage of “silent periods” of users Excessive congestion: packet delay and loss

protocols needed for reliable data transfer, congestion control

Is packet switching a “slam dunk winner?”

10

Packet-switched networks: forwarding

Goal: move packets through routers from source to destination we’ll study several path selection (i.e. routing)algorithms

(chapter 4)

datagram network (Internet): destination address in packet determines next hop routes may change during session analogies: USPostal mail, asking directions when driving

virtual circuit network (X.25, frame relay, ATM): each packet carries tag (virtual circuit ID), tag determines

next hop fixed path determined at call setup time, remains fixed thru

call routers maintain per-call state

11

Network Taxonomy

Telecommunicationnetworks

Circuit-switchednetworks

FDM TDM

Packet-switchednetworks

Networkswith VCs

DatagramNetworks

Telephones Internet

Introduction 1-12

Access networks and physical media

Q: How to connect end systems to edge router?

residential access nets institutional access

networks (school, company)

mobile access networks

Introduction 1-13

Residential access: point to point access

Dialup via modem up to 56Kbps direct access

to router (often less) Can’t surf and phone at

same time: can’t be “always on”

ADSL: Asymmetric Digital Subscriber Line up to 1 Mbps upstream up to 8 Mbps downstream FDM:

Introduction 1-14

Residential access: cable modems

HFC: Hybrid Fiber Coax asymmetric: up to 1Mbps upstream, 10

Mbps downstream network of cable and fiber attaches homes

to ISP router shared access to router among homes

deployment: available via cable companies.

Introduction 1-15

Cable Network Architecture: Overview

home

cable headend

cable distributionnetwork (simplified)

Typically 500 to 5,000 homes

Introduction 1-16

Cable Network Architecture: Overview

home

cable headend

cable distributionnetwork (simplified)

Introduction 1-17

Cable Network Architecture: Overview

home

cable headend

cable distributionnetwork

server(s)

Introduction 1-18

Cable Network Architecture: Overview

home

cable headend

cable distributionnetwork

Channels

VIDEO

VIDEO

VIDEO

VIDEO

VIDEO

VIDEO

DATA

DATA

CONTROL

1 2 3 4 5 6 7 8 9

FDM:

Introduction 1-19

Company access: local area networks

company/univ Local Area Network (LAN) connects end system to edge router

Ethernet: shared or dedicated

link connects end system and router

10 Mbs, 100Mbps, Gigabit Ethernet

deployment: institutions, home LANs happening now

LANs: chapter 5

Introduction 1-20

Wireless access networks

shared wireless access network connects end system to router via base station aka “access

point” wireless LANs:

802.11b (WiFi): 11 Mbps

basestation

mobilehosts

router

Introduction 1-21

Internet structure: network of networks

roughly hierarchical at center: “tier-1” ISPs (e.g. Sprint, AT&T, UUNet,

Level3), national/international coverage treat each other as equals

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-1 providers interconnect at public Network Access Points (NAPs)

Introduction 1-22

Tier-1 ISP: e.g., SprintSprint US backbone network

Introduction 1-23

Internet structure: network of networks

“Tier-2” ISPs: smaller (often regional) ISPs Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet tier-2 ISP is customer oftier-1 provider

Tier-2 ISPs also peer privately with each other, interconnect at NAP

Introduction 1-24

Internet structure: network of networks

“Tier-3” ISPs and local ISPs last hop (“access”) network (closest to end systems)

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

Local and tier- 3 ISPs are customers ofhigher tier ISPsconnecting them to rest of Internet

Introduction 1-25

Internet structure: network of networks

a packet passes through many networks!

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP