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SEG 3255
Communicationand Networking
Winter 2008 course notes
A. Williams
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What is a Network?
A set of processing nodes connected by communication links. Many topologies possible:
Many types of communication media:
twisted (copper) pair
coaxial (copper) cable
radio infrared
fiber optic cable
satellites
busring mesh star
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Types of Communication Links
Point-to-point
Shared / broadcast
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Circuit Switching
All resources (e.g. communication links) needed by a call arededicated to that call for its duration.
Example: a voice telephone call
Call from A to F blocks calling from B to E.
Resource reservation: resources are always available when needed
by a call, providing a guaranteed quality of service.
F
B C D
EA
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Packet Switching
Data entering network is divided into small chunks calledpackets.
Packets traversing the network share network resourceswith other packets.
Demand for resources may exceed resources available:
Contention: two packets arrive simultaneously at D
destined for E or F Queuing (waiting) for resources.
Statistical sharing of resources.
F
B C D
EA
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Why resource sharing?
To save/make money! Example: 1 Mbit/sec link; each user requires 100 Kbits/sec
when transmitting; each user has data to send only 10% ofthe time.
Circuit switching: give each caller 100 Kbits/sec capacity.10 callers can be supported.
Packet switching: with 35 calls in progress, the probabilitythat 10 or more callers are simultaneously active is less than0.0004. Many more callers can be supported with only a
small probability of contention. If users are bursty, then packet switching is advantageous.
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Elements of a Network
Communication links
Buffers: to hold packets when contention for communications link.
Network: set of nodes (hosts, routers, gateways) within a singleadministrative domain (e.g. university department, company).
A
CBD
internal
view of C
Three networks forming an internetwork
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Elements of a network (2)
Internetwork: a collection of interconnected networks
Active network elements: hardware running protocols:
Host: hardware running applications which use network (e.g. A).
Router: hardware (often without application level functions)routing packets from input line to output line (e.g. C).
Gateway: a router connected directly to two or more networks(e.g. B and D).
A
CBD
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Protocols
Rules by which active network elementscommunicate with each other is a protocol
Protocols define the formats and timing ofmessages exchanged, and actions taken on receiptof messages for peer entities
Protocols in everyday life:
Rules by which two or more people communicateto provide a service, or to get something done
Example: traffic lights guiding traffic flow Example: military precedence for entering a
vehicle
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Layered Architecture
Architecture of a complex system canbe simplified by layering.
Layer N relies on services of layerN 1 to provide a service to layerN + 1
Service required from lower layer isindependent of how that service isimplemented
Interfaces define how services arerequested
Benefits:
Information/complexity hiding
Layer N change doesnt affectother layers
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interface
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Layering and Protocols
The network consists of geographically distributed hardware andsoftware components
A distributed, layered view:
Principal challenge: how to provide services when:
resources and information needed are distributed
communication via unreliable medium
A B C
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Layering and Protocols
Peer entities (processes) in layer N provide service bycommunicating (via messages) with each other using thecommunication service provided by layer N 1
logical versus physical communication:
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A B C
Logical communication:
Layer 4 to Layer 4
Actual
communication
path
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Generic Layer Issues (1)
Error control: making a channel more reliable, and handlinglost or out of sequence messages.
Flow control: avoid flooding a slower peer entity.
Resource allocation: mediating contention for physical (e.g.buffers) or logical (e.g. data structures) resources
Fragmentation: dividing chunks of data into smaller pieces,and subsequent reassembly
Multiplexing: combining several higher layer sessions
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Generic Layer Issues (2)
Connection setup: initiating logical communicationwith peer entity
Addressing / naming: managing identifiers
Compression: reducing data rate
Encryption: provide data security
Timer management: bookkeeping and error
recovery
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OSI Reference Model
OSI (Open Systems Interconnect) referencemodel adopted in 1984
Model consists of a 7 layer stack:
Application
Presentation
Session
TransportNetwork
Data Link
Physical
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Functions of the OSI Layers
Physical The bits that are transmitted over the
communication media.
Deals with network hardware, bit encoding.
Examples: copper, fibre, radio, satellite.
Data Link
Activates, maintains, and deactivates the
physical link between two adjacent nodes. Deals with framing, windowing, flow control,
error detection and recovery.
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Functions of OSI Layers (2)
Network Determines how best to route packets of data
from source to destination via intermediatenetwork nodes.
Deals with addressing, routing, fragmentation,and congestion.
Transport
Ensures that data is transmitted reliablybetween source and destination.
Deals with end to end integrity and quality ofservice.
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Functions of OSI Layers (3)
Session (often omitted) Controls the dialogue between two host applications.
Provides check points and error recovery.
Reports exceptions to upper layers.
Presentation (often done by application) Resolves data representation differences.
Performs data compression and encryption.
Application
Perform functions to implement network applications. Examples: e-mail, teleconferencing.
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Data Transmission in the OSI model
Send side layer N takes a protocol data unit (PDU) from layer N + 1, addsits own fields to form a new PDU, and passes it to layer N 1
Receive side layer N takes the PDU from layer N 1, strips the layer Nfields, and passes it to layer N + 1
T: transport header: e.g. sequence numbers, error correction bits, timestamp info
N: network header: e.g. source and destination addresses
L: link header: e.g. error detection bits, acknowledgment field
A
network
Application
PresentationSession
Transport
Network
Data Link
Physical
AS
AST
ASTN
ASTN L2L1
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The Internet Reference Model
De facto model that was defined after ARPANETwas up and running.
ApplicationPresentation
Session
Transport
Network
Data Link
Physical
Application
Transport
Internet
OSI TCP/IP
Data Link
Physical
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Disadvantages of layering
Layering has many conceptual advantages, but fanaticaladherence to layering is problematic.
Layer N may duplicate lower layer functionality:
Example: error recovery on both a hop by hop and end toend basis.
Different layers may need the same information.
Example: time stamps
Layer N may need to know non-adjacent layer information.
Example: choosing packets to drop if congested
OSI session layer has not proven to be particularly useful.
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Standards Bodies
Formal accredited standards bodies produce national andinternational standards.
National standards bodies
Industry Canada
National Research Council (Canada) (NRC-CNRC)
Canadian Standards Association (CSA)
American National Standards Institute (ANSI)
US National Institute of Standards and Technology(NIST)
International standards bodies International Organization for Standardization (ISO)
International Telecommunications Union (ITU)
Non-governmental organizations
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Standards Bodies
ISO (www.iso.ch)
Non-treaty agency of the United Nations.
Collaborates standards development for informationtechnology.
ITU (www.itu.int) ITU-T: telecom sector of ITU
UN treaty agency that sets telecommunications standards.
ANSI (www.ansi.org)
The US national standards body.
Coordinates and accredits standards development across theUS.
IEEE (www.ieee.org)
US based international professional organization. Develops standards and submits to ANSI for approval.
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Standards Bodies
Telcordia (www.telcordia.com)
Coordinates and develops standards for US telephone service
ETSI (www.etsi.org)
European Telecomunication Standards Institute
Similar to Telcordia, but for Europe
IAB / IETF / IRTF Internet Architecture Board (www.iab.org)
Internet Engineering Task Force (www.ietf.org)
Internet Research Task Force (www.irtf.org)
Object Management Group (OMG) (www.omg.org)
Consists of many companies Develops/co-ordinates CORBA/IDL, UML standards
WWW consortium (www.w3.org)
Develops/co-ordinates standards such as HTTP, HTML, XML,
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Protocol Description Techniques In order to ensure that protocol implementations are
interoperable, there should be an unambiguous definition fora protocol.
The protocol operation is typically described using one ormore of the following techniques:
State diagram State transition table
Standardized communications formal descriptiontechnique (FDT)
Specification and Description Language (SDL)
Message sequence charts (MSCs) Unified Modelling Language (UML)
OSI Service primitives
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Message Sequence Charts (MSCs) An exchange of messages over time is illustrated in order to
describe an aspect of the protocol operation (a scenario).
Since there are typically an infinite set of allowable messagesequences, a set of MSCs cannot completely describe aprotocol.
MSC format has been standardized by ITU-T (standardZ.120, revised 2000)
Phone Switch
lift receiver
dial tone
hang up
dial tone off
time
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OSI Service Primitives
Four classes defined in the OSI model:
Request: An entity wants the service to do some work
Indication: An entity is informed about an event
Response: An entity wants to respond to an event
Confirm: An entity receives confirmation of a previousrequest
Layer
N + 1
Layer
N
Layer
N
Layer
N + 1
1 request
peer protocol
3 response2 indication4 confirm
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OSI Service Primitives (2)
A set of service primitives that could describe a simple connection-based
protocol:
CONNECT.request Request a connection to be established
CONNECT.indication Signal the called party of an incoming request
CONNECT.response Used by the called party to accept or reject a call
CONNECT.confirm Tells the caller whether the call was accepted.
DATA.request Request that data be sent
DATA.indication Signal the arrival of data
DISCONNECT.request Request that the connection be released
DISCONNECT.indication Signal that the connection has been released
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The Internet
Grew out of the US defense department AdvancedProjects Research Agency network (ARPANET)
As other networks were connected to theARPANET, notably the National Science
Foundation network (NSFNET), the resultinginternetwork has become known as the Internet
Foundation is the TCP/IP (1983) protocols forcommunication
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The Internet
What does it mean to be on the Internet? machine runs TCP/IP protocol stack
machine has IP address
machine can send IP packets to other internet
hosts (connected to IP router) Four classic (1980s) Internet applications:
electronic mail
Usenet news
remote login
file transfer
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TCP/IP layers (2)
Internet Layer: Official packet format and protocol: Internet
Protocol (IP).
Layer function is to deliver IP packets to their
destination.
IP Version 4 in use, version 6 is being phased in(slowly)
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TCP/IP layers (2)
T
ransport Layer: Designed to allow source to destination
conversation.
Transmission Control Protocol (TCP) provides a
reliable, connection-oriented service User Datagram Protocol (UDP) is an unreliable,
connectionless protocol, used where promptdelivery is the chief concern
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TCP/IP layers (3)
Application Layer: file transfer (FTP)
e-mail (SMTP)
virtual terminal (T
ELNET
) news (NNTP)
World Wide Web (HTTP)