1 A Taxonomy of Communication Networks Y. Richard Yang 1/16/2008.

1

A Taxonomy of Communication Networks

Y. Richard Yang

1/16/2008

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Outline

Recap A taxonomy of communication networks Summary

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Recap A protocol defines the format and the order of

messages exchanged between two or more communicating entities, as well as the actions taken on the transmission or receipt of a message or other events.

Some implications of the past: ARPANET is sponsored by ARPA

The initial IMPs (routers) were made by a small company

Many networks

Commercialization

design should survive failures

internetworking: need a network to connect networks

keep the network simple

architecture supporting distributed, autonomous systems

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Backbone ISPISP ISP

Recap: Current Internet Physical Infrastructure

Residential access Cable Fiber DSL Wireless

Campus access, e.g.,

Ethernet Wireless

The Internet is a network of networks

Each individually administrated network is called an Autonomous System (AS)

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Northern CrossRoads (NoX) Aggregation Point (AP)

http://www.nox.org/NoXAPConfig3-01.jpg

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Abilene I2 Backbone

http://weathermap.grnoc.iu.edu/abilene_jpg.html

7Qwest Backbone Map

http://www.qwest.com/largebusiness/enterprisesolutions/networkMaps/preloader.swf

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ATT Global Backbone IP Network

From http://www.business.att.com

9From AT&T web site.

AT&T USA Backbone Map

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Commercial Internet ISP Connectivity Roughly hierarchical

Divided into tiers Tier-1 ISPs are also called

backbone providers, e.g., AT&T, Verizon, Sprint, Level 3, Qwest

An ISP runs (private) Points of Presence (PoP) where its customers and other ISPs connect to it

ISPs also connect at (public) Network Access Point (NAP)

called public peering

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Network Access Point

Interconnect multiple ISP’s

Example: Chicago NAP (http://www.aads.net/main.html)

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Outline

Admin. and recap A taxonomy of communication networks Summary

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A Taxonomy of Communication Networks

So far we have looked at only the topology and physical connectivity of the Internet: a mesh of computers interconnected via various physical media

The fundamental question: how is data (the bits) transferred through a communication network?

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Broadcast vs. Switched Communication Networks

Broadcast networks nodes share a common channel; information

transmitted by a node is received by all other nodes in the network

examples: TV, radio

Switched networks information is transmitted to a small sub-set (usually

only one) of the nodes

communication networks

switchednetworks

broadcastnetworks

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Circuit switching: dedicated circuit per call/session: e.g., telephone, GSM High-Speed Circuit-Switched Data

(HSCSD), Integrated Services Digital Networks (ISDN) Packet switching: data sent thru network in discrete

“chunks” e.g., Internet, General Packet Radio Service (GPRS)

A Taxonomy of Switched Networks

communication networks

switchednetworks

broadcastnetworks

circuit-switchingnetworks

(e.g. telephone, GSM)

packet-switching networks

(e.g. Internet)

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Outline

Admin. and review A taxonomy of communication networks

circuit switching networks packet switching networks circuit switching vs. packet switching datagram switching vs. virtual circuit

switching Summary

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Circuit Switching

Each link has a number of “circuits” sometime we refer to

a “circuit” as a channel or a line

An end-to-end connection reserves one “circuit” at each link

First commercial telephone switchboard was opened in 1878 to serve the 21 telephone customers in New Haven, Connecticut

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Circuit Switching: Resources/Circuits (Frequency, Time and others) Divide link

resource into “circuits” frequency

division multiplexing (FDM)

time division multiplexing (TDM)

others such as code division multiplexing (CDM), color/lambda division

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Circuit Switching: The Process

Three phases1. circuit establishment2. data transfer3. circuit termination

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circuit establishment

DATA

data transmission

circuit termination

Host A Host BNode 1 Node 2

propagation delay from A to Node 1

propagation delay from B To A

processing delay at Node 1

Timing Diagram of Circuit Switching

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Delay Calculation in Circuit Switched Networks

Transmission delay: R = reserved

bandwidth (bps) L = message 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 a source to a destination

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

DATA

d/s

L/R

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An Example

Propagation delay suppose the distance between A and B is 4000 km,

then one-way propagation delay is:

Transmission delay suppose we reserve a one slot HSCSD channel

• a frame can transmit about 115 kbps• A frame is divided into 8 slots• each reserved one slot HSCSD has a bandwidth of about

14 Kbps (=115/8) then the transmission delay of a message of 1 Kbits:

msskmkm 20/000,200

4000

mskbpskbits 7014

1

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An Example (cont.) Suppose the setup message is very small, and the total

setup processing delay is 200 ms Then the delay to transfer a message of 1 Kbits from A to

B (from the beginning until host receives last bit) is:

ms33070202020020

DATA

20 + 200

20

20

70

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Outline


circuit switching networks packet switching networks circuit switching vs. packet switching

Summary

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Packet Switching

Each end-to-end data flow (i.e., a sender-receiver pair) 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?• any analogy in life?

At each node the entire packet is received, processed (e.g., routing), stored briefly, and then forwarded to the next node; thus packet-switched networks are also called store-and-forward networks

Header Data Trailer

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Packet Switching

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Inside a Packet Switching Router

An output queueing switch

incoming links outgoing linksnode

Memory

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Packet Switching: Resources

Resources used as needed On its turn, a packet uses full link

bandwidth

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Outline


circuit switching networks packet switching networks circuit switching vs. packet switching

Summary

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Packet Switching vs. Circuit Switching

The early history of the Internet was a heated debate between Packet Switching and Circuit Switchingthe telephone network was the

dominant network

Need to compare packet switching with circuit switching

Circuit Switching vs. Packet Switching

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circuit switching

packet switching

resource partitioned not partitioned

when using resource

use a single partition bandwidth

use whole link bandwidth

reservation/setup

need reservation (setup delay)

no reservation

resource contention

busy signal (session loss)

congestion (long delay and packet losses)

header no header per packet header

processing fast per packet processing

Key Issue to be Settled

All the other comparisons are easy to understand, a key debating issue was whether resource partition would be inefficient.

Tool used to analyze the issue: queueing theory

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Analysis of Circuit-Switching Blocking (Busy) Time

Assume N circuits Session arrival: per second Session service rate: per second What is the percentage of time that a

new session is blocked?

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For a demo of M/M/1, see: http://www.dcs.ed.ac.uk/home/jeh/Simjava/queueing/mm1_q/mm1_q.html

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Analysis of Delay at a Link

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

The focus is on transmission delay and propagation delay

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The Key Case: To Partition or not to Partition?

Case 1 (not reserve): all arrivals into a single link with rate R, what is the queueing delay + transmission delay?

Assume:R = link bandwidth (bps)L = average packet length (bits)a = average packet arrival rate

(pkt/sec)R/L = packet service rate (pkt/sec)

Case 2 (reserve): the arrivals are divided into n links with rate R/n each, what is the queueing delay + transmission delay?

Setup: n streams; each stream has an arrival rate of a/n

Comparison: each stream reserves 1/n bandwidth or not

Analysis of Queueing + Transmission Time

Consider a single queue Packet arrival rate: per second Packet service rate: per second What is the queueing + transmission

time of each packet?

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Delay: M/M/1 Model

1

:delay queueing averageR

Lw



1

1

R

Ltransqueueing

R

La

LR

anutilizatio

/ :

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Queueing Delay as A Function of Utilization

~ 0: average queueing delay small

-> 1: delay becomes large > 1: more “work” arriving

than can be serviced, average delay infinite !

Assume:R = link bandwidth (bps)L = packet length (bits)a = average packet arrival rate


R

La

LR

anutilizatio

/ :

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Statistical Multiplexing Gain

Case 1 (not reserve): all arrivals into a single link with rate R



Case 2 (reserve): the arrivals are divided into n links with rate R/n each

Setup: n streams; each stream has an arrival rate of a/n

Comparison: each stream reserves 1/n bandwidth or not

1

1

R

Ltransqueueing

1

1

R

nLtransqueueing

any analogy in life?

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One of the major issues facing the Internet: How to provide circuit-like behavior (in terms of quality of service)? virtual circuit switching is originally mainly for

routing efficiency

bandwidth guarantees needed for audio/video apps

still an unsolved problem

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Outline


circuit switching networks packet switching networks circuit switching vs. packet switching routing in packet switching networks

Summary

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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 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 determine the possible next hops, and pick one

Discussions: an example of datagram-style communication in

daily life? advantages of datagram switching? potential problems of packet switching?

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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 A Host BNode

1Node 2

propagationdelay fromHost A to Node 1

transmission time of Packet 1at Host A

Timing Diagram of Datagram Switching

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Virtual-Circuit Packet Switching

Example: Asynchornous Transfer Mode (ATM) networks; Multiple Label Packet Switching (MPLS) in IP 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 state do routers maintain for datagram switching?

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 over virtual circuit switching?

What are the benefits of virtual circuit switching over datagram switching?

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Summary of the Taxonomy of Communication Networks

circuit-switchednetwork

communication network

switchednetwork

broadcastcommunication

packet-switched network

datagram network

virtual circuit network

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Backup Slides

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Packet SwitchingPacket-switching:

store and forward behavior

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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

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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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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Relationship Between Switching Techniques and End Host Services

End hosts determine end host services: connection-oriented or connectionless what an application wants

Network service providers determine network services: circuit-switching, packet switching, datagram switching, or virtual circuit switching how the ISP builds their networks

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Packet Switching: Resources

Resources used as needed On its turn, a packet uses full link bandwidth Aggregated resource demand can exceed amount

available congestion: packets queue, wait for link use buffer overflow: packet losses

Bandwidth division into “pieces”

Resource reservation

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Advantages of packet switching over circuit switching most important advantage of packet-switching over circuit

switching is statistical multiplexing, and therefore efficient bandwidth usage

no call setup (for datagram switching only) no per-flow state information (for datagram switching only) simple to implement

Disadvantages of packet switching potential congestion: packet delay and high loss

• protocols needed for reliable data transfer, congestion control

packet header overhead per packet processing overhead

Questions: why does the current Internet use datagram?

The early history of the Internet was a heated debate between Packet Switching and Circuit Switching

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Statistical Multiplexing

Case 1 (not reserve): all arrivals into a single link with rate R, what is the queueing delay + transmission delay?



1R

Lw

Case 2 (reserve): the arrivals are divided into n links with rate R/n each, what is the queueing delay + transmission delay?

Setup: N flows; each flow has an arrival rate of a/n

Comparison: each flow reserves 1/n bandwidth or not

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Queueing Delay: Kendall Notation Arrival/Service/NumberServers

E.g., M/M/1, M/D/1

Arrival process: the inter-arrival time D: fixed/deterministic inter-arrival time M: Markov, i.e., in each unit of time, where small,

the probability of exactly one arrival is , where is called arrival rate

Service process: the service time D: fixed/deterministic service time M: Markov, i.e., in each unit of time, where small,

the probability of finish serving is , where is called service rate

For a demo of M/M/1, see: http://www.dcs.ed.ac.uk/home/jeh/Simjava/queueing/mm1_q/mm1_q.html

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Queueing Delay: M/M/1 Model

1

:delay queueing averageR

Lw



12

1 :delay queueing average

R

LwA more realistic model of

modern routers: M/D/1

1 A Taxonomy of Communication Networks Y. Richard Yang 1/16/2008.

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