1. CS408/533 Computer Networks
Text: William Stallings Data and Computer Communications, 6th edition
Chapter 1 - Introduction

2. A Communications Model
Source
generates data to be transmitted
Transmitter
Converts data into transmittable EM signals
Transmission System
Carries data
Receiver
Converts received signal into data
Destination
Takes incoming data

3. Simplified Communications Model - Diagram

4. Key Communications Tasks (also an overview of the course)
Transmission System Utilization - efficient use of resources (e.g. Multiplexing, Congestion Control)
Interfacing and Signal Generation - hook up and generate transmittable and interpretable signals
Synchronization - to know beginning and end of data
Error detection and correction
Flow control - do not overwhelm the destination
Addressing and routing - destination identification and path selection
Security - only the intended recipient must read
Network Management – Complex System, need to be managed (planning, configuration, status monitoring)

5. 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

6. Simplified Network Models

7. Wide Area Networks Large geographical area Resemble “highways”
interconnected switching nodes
Data are switched from one node to another towards the destination
These nodes are not interested in the data
Alternative technologies
Circuit switching
Packet switching
Frame relay
Asynchronous Transfer Mode (ATM)

8. Circuit Switching
Dedicated communications path established for the duration of the connection
Channels are pre-allocated
Minimal (if not none) delay at each node
e.g. telephone network

9. Packet Switching No dedicated channels Data sent out in a sequence
Small chunks (packets) of data at a time
Packets passed from node to node between source and destination
Packets are
received
stored briefly (necessary for packet processing)
forwarded to the next node

10. Frame Relay
Packet switching systems have large overheads to compensate for errors
Modern transmission systems are more reliable
The ideas behind frame relay
Errors can be caught at the end points
Let’s take the advantage of high data rates and low error rates
Most overhead for error control is stripped out
Data Rate
2 Mbps as compared to 64Kbps of X.25 (packet switching)

11. Asynchronous Transfer Mode
ATM
Evolution from frame relay
Little overhead (even less than frame relay) for error control
higher layers of end users are responsible for error check
Fixed packet (called cell) length
48 bytes of data + 5 bytes of header
Anything from 10Mbps to Gbps range

12. ISDN Integrated Services Digital Network
Voice, Data and other traffic has a single interface
network devices will understand the required service and process accordingly
Designed to replace Public Telephone Network
could not do it
Narrowband ISDN (or just ISDN)
Broadband ISDN (B-ISDN)

13. Local Area Networks Smaller scope
Building or small campus
Usually owned by same organization as attached devices
requires set up and maintenance
Data rates much higher than WANs
Usually broadcast systems
Now some switched systems and ATM are being introduced

14. Protocols Cooperative action is necessary
computer communications is not only to exchange bytes
huge system with several utilities and functions. For examples
error detection
encryption
For proper communication, entities in different systems must speak the same language
Mutually acceptable conventions
what is communicated?
when is communicated?
how is communicated?
Those conventions and associated rules are referred as “PROTOCOLS”

15. Protocol Architecture
Task of communication broken up into modules
For example file transfer could use three modules
File transfer application
Communication service module
Network access module

16. Simplified File Transfer Architecture
File Transfer Application Layer: Application specific commands, passwords and the file(s) transmission – high level data
Communications Service Module: reliable transfer of those data – error detection, ordered delivery of data packets, etc.
Network Module: actual transfer of data and dealing with the network – if the network changes, only this module is affected, not the whole system

17. A General Three Layer Model
Generalize the previous example for a generic application
we can have different applications ( , file transfer, …)
Network Access Layer
Transport Layer
Application Layer

18. Network Access Layer
Exchange of data between the computer and the network
Sending computer provides address of destination
so that network can route
Software is dependent on type of network used (LAN, packet switched etc.)
Independent of upper layers

19. Transport Layer Reliable data exchange
To make sure that all the data packets arrived in the same order in which they are sent out
Independent of network being used
Independent of application

20. Application Layer Support for different user applications
e.g. , file transfer

21. Addressing Requirements
Two levels of addressing required
Each computer needs unique network address
Each application on a (multi-tasking) computer needs a unique address within the computer
The service access point or SAP

22. Protocol Architectures and NetworksX Z Y

23. Protocol Data Units (PDU)
User data is passed from layer to layer
Control information is added/removed to/from user data at each layer
header
each layer has a different header
Data + header = PDU (Protocol Data Unit)
each layer has a different PDU

24. Transport PDU Transport layer may fragment user data
Each fragment has a transport header added
Destination SAP
Sequence number
Error detection code
This is transport protocol data unit

25. Network PDU Adds network header
network address for destination computer
optional facilities from network (e.g. priority level)

26. Operation of a Protocol Architecture