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Data Communication and Networks. Lecture 1 Introduction and Overview September 7, 2006. A Communications Model. Source generates data to be transmitted Transmitter Converts data into transmittable signals Transmission System Carries data Receiver Converts received signal into data

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data communication and networks

Data Communication and Networks

Lecture 1

Introduction and Overview

September 7, 2006

a communications model
A Communications Model
  • Source
    • generates data to be transmitted
  • Transmitter
    • Converts data into transmittable signals
  • Transmission System
    • Carries data
  • Receiver
    • Converts received signal into data
  • Destination
    • Takes incoming data
key communications tasks
Key Communications Tasks
  • Transmission System Utilization
  • Interfacing
  • Signal Generation
  • Synchronization
  • Error detection and correction
  • Addressing and routing
  • Recovery
  • Message formatting
  • Security
  • Network Management
networking
Networking
  • Point to point communication not usually practical
    • Devices are too far apart
    • Large set of devices would need impractical number of connections
  • Solution is a communications network
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”

The Network Core
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

Network Core: Circuit Switching
circuit switching fdm and tdm

Example:

4 users

FDM

frequency

time

TDM

frequency

time

Circuit Switching: FDM and TDM
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

Network Core: Packet Switching

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
    • Node receives complete packet before forwarding
packet switching statistical multiplexing
Sequence of A & B packets does not have fixed pattern  statistical multiplexing.

In TDM each host gets same slot in revolving TDM frame.

D

E

Packet Switching: Statistical Multiplexing

10 Mb/s

Ethernet

C

A

statistical multiplexing

1.5 Mb/s

B

queue of packets

waiting for output

link

packet switching versus circuit switching
Great for bursty data

resource sharing

simpler, 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?”

Packet switching versus circuit switching
local area networks
Local Area Networks
  • Smaller scope
    • Building or small campus
  • Usually owned by same organization as attached devices
  • Data rates much higher
  • Usually broadcast systems
protocols
Protocols
  • Used for communications between entities in a system
  • Must speak the same language
  • Entities
    • User applications
    • e-mail facilities
    • terminals
  • Systems
    • Computer
    • Terminal
    • Remote sensor
key elements of a protocol
Key Elements of a Protocol
  • Syntax
    • Data formats
    • Signal levels
  • Semantics
    • Control information
    • Error handling
  • Timing
    • Speed matching
    • Sequencing
what s a protocol
human protocols:

“what’s the time?”

“I have a question”

introductions

… specific msgs sent

… specific actions taken when msgs received, or other events

network protocols:

machines rather than humans

all communication activity in Internet governed by protocols

What’s a protocol?

protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt

what s a protocol17
a human protocol and a computer network protocol:

TCP connection

reply.

Get http://gaia.cs.umass.edu/index.htm

Got the

time?

2:00

<file>

time

What’s a protocol?

Hi

TCP connection

req.

Hi

in summary a protocol is
In Summary, a protocol is ....
  • An agreement about communication between two or more entities
  • It specifies

– Format of messages

– Meaning of messages

– Rules for exchange

– Procedures for handling problems

protocol specification
Protocol Specification
  • As designers, we can specify a protocol using
    • Event-Time Diagrams
    • Transition Diagrams
  • We can implement a protocol with a Finite State Machine (FSM)
  • Internet Protocols are formalized by RFCs which are administered by IETF
  • You can find any RFChere
event time diagrams
Event -Time Diagrams
  • Define causal ordering
  • Define indication/request/response actions
transition diagram
Transition Diagram
  • Illustrates
    • States
    • Input (the Event that causes transition)
    • Transitions (to new states)
protocol layers
Networks are complex!

many “pieces”:

hosts

routers

links of various media

applications

protocols

hardware, software

Question:

Is there any hope of organizing structure of network?

Or at least our discussion of networks?

Protocol “Layers”
why layering
Why layering?

Dealing with complex systems:

  • explicit structure allows identification, relationship of complex system’s pieces
    • layered reference model for discussion
  • modularization eases maintenance, updating of system
    • change of implementation of layer’s service transparent to rest of system
    • e.g., change in gate procedure doesn’t affect rest of system
  • Can layering sometimes be undesirable?
internet protocol stack
application: supporting network applications

FTP, SMTP, HTTP

transport: process-process data transfer

TCP, UDP

network: routing of datagrams from source to destination

IP, routing protocols

link: data transfer between neighboring network elements

PPP, Ethernet

physical: bits “on the wire”

application

transport

network

link

physical

Internet protocol stack
encapsulation

network

link

physical

link

physical

M

M

M

Ht

M

Hn

Hn

Hn

Hn

Ht

Ht

Ht

Ht

M

M

M

M

Hn

Ht

Ht

Hl

Hl

Hl

Hn

Hn

Hn

Ht

Ht

Ht

M

M

M

source

Encapsulation

message

application

transport

network

link

physical

segment

datagram

frame

switch

destination

application

transport

network

link

physical

router

slide26
OSI
  • Open Systems Interconnection
  • Developed by the International Organization for Standardization (ISO)
  • Seven layers
  • A theoretical system delivered too late!
  • TCP/IP is the de facto standard
osi the model
OSI - The Model
  • A layer model
  • Each layer performs a subset of the required communication functions
  • Each layer relies on the next lower layer to perform more primitive functions
  • Each layer provides services to the next higher layer
  • Changes in one layer should not require changes in other layers
standards
Standards
  • Required to allow for interoperability between equipment
  • Advantages
    • Ensures a large market for equipment and software
    • Allows products from different vendors to communicate
  • Disadvantages
    • Freeze technology
    • May be multiple standards for the same thing
how do loss and delay occur
packets queue in router buffers

packet arrival rate to link exceeds output link capacity

packets queue, wait for turn

packet being transmitted (delay)

packets queueing (delay)

free (available) buffers: arriving packets

dropped (loss) if no free buffers

How do loss and delay occur?

A

B

four sources of packet delay
1. nodal processing:

check bit errors

determine output link

transmission

A

propagation

B

nodal

processing

queueing

Four sources of packet delay
  • 2. queueing
    • time waiting at output link for transmission
    • depends on congestion level of router
delay in packet switched networks
3. Transmission delay:

R=link bandwidth (bps)

L=packet length (bits)

time to send bits into link = L/R

4. Propagation delay:

d = length of physical link

s = propagation speed in medium (~2x108 m/sec)

propagation delay = d/s

transmission

A

propagation

B

nodal

processing

queueing

Delay in packet-switched networks

Note: s and R are very different quantities!

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