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CSE401NComputer Networks

Lecture-2Network Structure[KR-1.2+1.3+1.4]

S. M. Hasibul HaqueDept. of CSE

BUET

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A closer look at network structure: network edge:

applications and hosts network core:

routers network of networks

access networks, physical media: communication links

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The network edge: end systems (hosts):

run application programs e.g., WWW, email at “edge of network”

client/server model client host requests,

receives service from server e.g., WWW client (browser)/

server; email client/server

peer-peer model: host interaction symmetric e.g.: Gnutella, KaZaA

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Network edge: connection-oriented service

Goal: data transfer between end sys.

handshaking: setup (prepare for) data transfer ahead of time Hello, hello back human

protocol set up “state” in two

communicating hosts

TCP - Transmission Control Protocol Internet’s connection-

oriented service Why not connected?

TCP service [RFC 793] reliable, in-order byte-

stream data transfer loss: acknowledgements

and retransmissions

flow control: sender won’t overwhelm

receiver

congestion control: senders “slow down

sending rate” when network congested

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Network edge: connectionless service

Goal: data transfer between end systems same as before!

UDP - User Datagram Protocol [RFC 768]: Internet’s connectionless service unreliable data

transfer no flow control no congestion

control

App’s using TCP: HTTP (WWW), FTP

(file transfer), Telnet (remote login), SMTP (email)

App’s using UDP: streaming media,

teleconferencing, Internet telephony

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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”

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Network Core: Circuit Switching

End-end resources reserved for “call”

link bandwidth, switch capacity

dedicated resources: no sharing

circuit-like (guaranteed) performance

call setup required

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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 sharing)

dividing link bandwidth into “pieces” frequency division time division

dividing link bandwidth into “pieces” frequency division time division

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Network Core: Circuit Switching

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Circuit Switching: TDMA and TDMA

FDMA

frequency

time

TDMA

frequency

time

4 users

Example:

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Circuit Switching: Resources (Frequency and Time)

Divide link bandwidth—the resource--into “pieces” frequency division

multiplexing (FDM) time division

multiplexing (TDM)

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Circuit Switching: The Process

Three phases1. circuit establishment2. data transfer3. circuit termination

If circuit not available: “busy signal”

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circuit establishment

DATA

data transmission

circuit termination

Host 1 Host 2Node 1 Node 2

propagation delay from Host 1 to Node 1

propagation delay from Host 2 To Host 1

processing delay at Node 1

Timing Diagram of Circuit Switching

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Delay Calculation in Circuit-Switched Networks

Transmission delay: R = bandwidth (bps) L = packet length

(bits) time to send a

packet into link = L/R

Propagation delay: d = length of physical link s = propagation speed in

medium (~2x105 km/sec) propagation delay = d/s

Propagation delay: delay for the first bit to go from source to destination

Transmission delay: time to pump data onto link at reserved rate

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An Example

Propagation delay suppose the distance between host 1 and host 2 is 4000 km,

then one-way propagation delay is:

Transmission delay suppose we reserve one slot of a T1 line, which

• has a bandwidth of 1.536 Mbps• is divided into 24 slots, and thus• each reserved slot has a bandwidth of 64 Kbps

then the transmission delay of a file with 6.4 Kbits is

Suppose the setup message is very small, and the total setup processing delay is 200 ms

msskmkm 20/000,200

4000

mskbpskbits 10064

4.6

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An Example (cont.)

Then the delay to transfer a 6.4 Kbits file from host 1 to host 2 (from the beginning until host receives last bit of the file) is:

ms360100202020020

DATA

20 + 200

20

20

100

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Network Core: Packet Switching

each end-end data stream divided into packets

user A, B packets 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: packets move one hop at a time transmit over link wait turn at next

link

Bandwidth division into “pieces”Dedicated allocationResource reservation

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Packet Switching

Each end-to-end data flow divided into packets Packets have the following structure:

• Header and Trailer carry control information (e.g., destination address, check sum)

• (where is the control information for circuit switching?)

At each node the entire packet is received, stored briefly, and then forwarded to the next node (Store-and-Forward Networks)

Each packet is passed through the network from node to node along some path (Routing)

Header Data Trailer

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Packet Switching

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Inside a Packet Switching Router

A node in a packet switching network

incoming links outgoing linksnode

Memory

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Packet Switching: Resources

Each packet waits for its turn at the output link On its turn, a packet uses full link bandwidth Resources used as needed Aggregate resource demand can exceed amount

available Congestion: packets queue, wait for link use

Bandwidth division into “pieces”Resource reservationDedicated allocation

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A Taxonomy of Packet-Switched Networks According to Routing

Goal: move packets among routers from source to destination we’ll study several routing algorithms later in the course

Two types of packet switching datagram network

• each packet of a flow is switched independently virtual circuit network:

• all packets from one flow are sent along a pre-established path (= virtual circuit)

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Datagram Packet Switching

Example: IP networks Each packet is independently switched

each packet header contains complete destination address

receiving a packet, a router looks at the packet’s destination address and searches its current routing table to determines the next hop

routes may change during session routers do not keep any state about a flow

An example of datagram-style routing in daily life?

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Host A

Host BHost E

Host D

Host C

Node 1 Node 2

Node 3

Node 4

Node 5

Node 6 Node 7

Datagram Packet Switching

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Packet 1

Packet 2

Packet 3

Packet 1

Packet 2

Packet 3

Packet 1

Packet 2

Packet 3

processing and queueing delay of Packet 1 at Node 2

Host 1 Host 2Node

1Node 2

propagationdelay fromHost 1 to Node 1

transmission time of Packet 1at Host 1

Timing Diagram of Datagram Switching

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Virtual-Circuit Packet Switching

Example: Asynchornous Transfer Mode (ATM) networks Hybrid of circuit switching and datagram switching

each packet carries a short tag (virtual-circuit (VC) #), tag determines next hop

fixed path determined at Virtual Circuit setup time, remains fixed thru flow

routers maintain per-flow state

What advantages do virtual circuit have over datagram? Guarantees in-sequence delivery of packets However: Packets from different virtual circuits may be

interleaved

Incoming Interface

Incoming VC#

Outgoing Interface

Outgoing VC#

1 12 2 22

1 16 3 1

2 12 3 22

…

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Host A

Host BHost E

Host D

Host C

Node 1 Node 2

Node 3

Node 4

Node 5

Node 6 Node 7

Virtual-Circuit Switching

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Virtual-Circuit Packet Switching

Three phases 1. VC establishment2. Data transfer3. VC disconnect

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Packet 1

Packet 2

Packet 3

Packet 1

Packet 2

Packet 3

Packet 1

Packet 2

Packet 3

Host 1 Host 2Node

1Node

2

propagation delay between Host 1 and Node 1VC

establishment

VCtermination

datatransfer

Timing Diagram of Virtual-Circuit Switching

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Discussion: Datagram Switching vs. Virtual Circuit Switching

What are the benefits of datagram switching?

What are the benefits of virtual circuit switching?

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Delay at a Router in Packet Switching A packet experiences delay at each hop Four types of delay at each hop

nodal processing delay: check errors & routing queueing: time waiting for its turn at output link transmission delay: time to pump packet onto a link at link speed propagation delay: router to router propagation

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Delay in Datagram Networks

Packet 1

Packet 2

Packet 3

Packet 1

Packet 2

Packet 3

Packet 1

Packet 2

Packet 3

nodal processing and queueing delay of Packet 1 at Node 2

propagationdelay betweenHost 1 and Node 2

transmission time of Packet 1at Host 1

Host 1 Host 2Node 1 Node 2

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Network Core: Packet Switching

Packet-switching versus circuit switching: human restaurant analogy

other human analogies?

A

B

C10 MbsEthernet

1.5 Mbs

45 Mbs

D E

statistical multiplexing

queue of packetswaiting for output

link

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Packet switching versus circuit switching

1 Mbit link each user:

100Kbps when “active”

active 10% of time

circuit-switching: 10 users

packet switching: with 35 users,

probability > 10 active less than .0004

Packet switching allows more users to use network!

N users

1 Mbps link

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Packet switching versus circuit switching

Great for bursty data resource sharing no call setup

Excessive congestion: packet delay and loss protocols needed for reliable data transfer,

congestion control Q: How to provide circuit-like behavior?

bandwidth guarantees needed for audio/video apps

still an unsolved problem (chapter 6)

Is packet switching a “slam dunk winner?”

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Access networks and physical media

Q: How to connection end systems to edge router?

residential access nets institutional access

networks (school, company)

mobile access networks

Keep in mind: bandwidth (bits per

second) of access network?

shared or dedicated?

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Residential access: point to point access

Dialup via modem up to 56Kbps direct access

to router (conceptually) ISDN: integrated services

digital network: 128Kbps all-digital connect to router

ADSL: asymmetric digital subscriber line up to 1 Mbps home-to-router up to 8 Mbps router-to-home ADSL deployment:

happening

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Residential access: cable modems

HFC: hybrid fiber coax asymmetric: up to 10Mbps upstream, 1

Mbps downstream network of cable and fiber attaches homes

to ISP router shared access to router among home issues: congestion, dimensioning

deployment: available via cable companies, e.g., MediaOne

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Residential access: cable modems

Diagram: http://www.cabledatacomnews.com/cmic/diagram.html

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Institutional access: local area networks

company/univ local area network (LAN) connects end system to edge router

Ethernet: shared or dedicated

cable connects end system and router

10 Mbs, 100Mbps, Gigabit Ethernet

deployment: institutions, home LANs happening now

LANs: chapter 5

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Wireless access networks

shared wireless access network connects end system to router

wireless LANs: radio spectrum replaces

wire e.g., Lucent Wavelan 11

Mbps

wider-area wireless access CDPD: wireless access

to ISP router via cellular network

basestation

mobilehosts

router

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Home networks

Typical home network components: ADSL or cable modem router/firewall Ethernet wireless access point

wirelessaccess point

wirelesslaptops

router/firewall

cablemodem

to/fromcable

headend

Ethernet(switched)

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Physical Media

physical link: transmitted data bit propagates across link

guided media: signals propagate in

solid media: copper, fiber

unguided media: signals propagate

freely, e.g., radio

Twisted Pair (TP) two insulated copper

wires Category 3: traditional

phone wires, 10 Mbps Ethernet

Category 5 TP: 100Mbps Ethernet

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Physical Media: coax, fiber

Coaxial cable: wire (signal carrier)

within a wire (shield) baseband: single

channel on cable broadband: multiple

channel on cable

bidirectional common use in

10Mbs Ethernet

Fiber optic cable: glass fiber carrying

light pulses high-speed operation:

100Mbps Ethernet high-speed point-to-

point transmission (e.g., 5 Gps)

low error rate

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Physical media: radio

signal carried in electromagnetic spectrum

no physical “wire” bidirectional propagation

environment effects: reflection obstruction by objects interference

Radio link types: microwave

e.g. up to 45 Mbps channels

LAN (e.g., WaveLAN) 2Mbps, 11Mbps

wide-area (e.g., cellular) e.g. CDPD, 10’s Kbps

satellite up to 50Mbps channel (or multiple

smaller channels) 270 Msec end-end delay geosynchronous versus LEOS

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Thank YOU