1. Dr. Mohammed Aldasht and Dr. Mahdi Nasereddin
PALESTINE POLYTECHNIC UNIVERSITY College of Administrative sciences and Informatics Department of Information Technology Data communications and networks
William Stallings Data and Computer Communications 7th Edition
Slides modified and updated by:
Dr. Mohammed Aldasht and Dr. Mahdi Nasereddin
Summer 2006

2. William Stallings Data and Computer Communications 7th Edition
Chapter 1 Data Communications and Networks Overview

3. A Communications Model
Source: generates data to be transmitted, examples are telephones and personal computers.
Transmitter: converts data into transmittable signals;
e.g. the modem takes a digital bit stream from an attached device such as a PC and transforms that bit stream into an analog signal that can be handled by the telephone network.
Transmission System: carries data, this can be a single line or a complex network connecting source and destination.
Receiver: converts received signal into data;
e.g. a modem will accept an analog signal coming from a network or transmission line and convert it into a digital bit stream.
Destination: takes incoming data from the receiver.

4. Communications Tasks I
Transmission system utilization: refers to the need to make efficient use of transmission facilities that are typically shared among a number of communicating devices.
Multiplexing.
Congestion control.
Interface: to communicate, a device must interface with the transmission systems. Most systems use electromagnetic signals.
Signal generation: the form and intensity of the signal must be capable of being propagated through the transmission systems and interpretable as data at the receiver.

5. Communications Tasks II
Synchronization: the receiver must be able to determine when a signal begins to arrive and when it ends.
Exchange management: establish the connection between the two parties, end the connection, decide for both to transmit simultaneously or must take turns.
Error detection and correction: for example, in transferring a file from one computer to another, it is not acceptable for the contents of the file to be accidentally altered.
Flow control: the source should not send data faster than the destination can process it.
Addressing: a source system must indicate the identity of the intended destination.

6. Communications Tasks III
Routing: a specific route through the transmission system (network) must be chosen.
Recovery: needed in situations in which an information exchange is interrupted due to a fault somewhere in the system.
Message formatting: has to do with an agreement between two parties as to the form of the data to be exchanged.
Security: the sender may wish to be assured that only the intended receiver actually receives the data. And the receiver may wish to be assured that the received data have not been altered in transit.
Network management: capabilities are needed to configure the system, monitor its status, reacts to failures and overloads, and plan for future growth.

7. Simplified Communications Model - Diagram

8. Simplified Data Communications Model

9. Networking Point to point communication not usually practical
Devices are too far apart. It would be inordinately expensive.
Large set of devices. It is impractical to provide a dedicated wire between each pair of devices.
Solution to this problem is to attach each device to a communications network
Wide Area Network (WAN):
Local Area Network (LAN): covers a small area, building or a floor.

10. Wide Area Networks
Cover a large geographical area (country or continent)
Rely at least in part on circuits provided by a common carrier.
WAN technologies
Circuit switching: a dedicated communications path is established between the sender and receiver through the network. Example: telephone network.
Packet switching: data are sent out in a sequence of small chunks, called packets.
Packets passed from node to node between source and destination.
Used for terminal to computer and computer to computer communications.

11. WAN technologies (Cont.)
Frame Relay (unreliable)
Packet switching systems have large overheads to compensate for errors
Modern systems are more reliable
Errors can be caught in end system
Most overhead for error control is stripped out
Asynchronous Transfer Mode (ATM) (unreliable)
Evolution of frame relay
Little overhead for error control
Fixed packet (called cell) length
Anything from 10Mbps to Gbps
Constant data rate using packet switching technique
Although they are capable of doing error checking, this is not enough to make Frame relay reliable.

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

13. LAN Configurations Switched Wireless
Switched Ethernet LAN: which may consist of a single switch with a number of attached devices, or a number of interconnected switches.
ATM LAN: ATM network in a local area.
Wireless
Widely used in business environments.
Advantages:
Mobility
Ease of installation

14. Metropolitan Area Networks
MAN: occupies a middle ground between LANs and WANs.
Point-to-point and switched network techniques used in WANs may be inadequate for the growing needs of the organizations.
Recently, wireless networks and metropolitan extensions to Ethernet have been implemented in MANs.
A MAN in intended to provide the required capacity at lower costs and greater efficiency than obtaining an equivalent service from the local telephone company.

15. Networking Configuration

16. William Stallings Data and Computer Communications 7th Edition
Chapter 2
Protocols and Architecture

17. 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 (port on TCP/IP stacks)

18. Protocol Data Units (PDU)
The combination of data from the next higher layer and control information is known as a PDU.
At each layer, protocols are used to communicate
Control information is added to user data at each layer
e.g.
Transport layer may fragment user data
Each fragment has a transport header added
Destination SAP
Sequence number
Error detection code
This gives a transport protocol data unit

19. Protocol Data Units Network PDU: adds network header
Network address for destination computer
Facilities requests (Control Information)

20. PDU Headers Transport PDU header Includes the following:
Destination SAP: when the destination transport layer receives the transport PDU, it must know to whom the data are to be delivered.
Sequence number: if the PDUs arrive out of order, the destination transport entity may reorder them.
Error-detection code: the sending transport entity may include a code that is a function of the contents of the remainder of the PDU. The receiving transport protocol performs the same calculation and compares the result with the incoming code
Network PDU header Includes the following:
Destination computer address: the network must know to which computer on the network the data are to be delivered.
Facilities requests: the network access protocol might want the network to make use of certain facilities, such as priority.

21. Standardized Protocol Architectures
Required for devices from different vendors to communicate
Vendors have more marketable products
Customers can insist on standards based equipment
Two standards:
OSI Reference model: never lived up to early promises
TCP/IP protocol suite: most widely used
Also: IBM Systems Network Architecture (SNA)

22. OSI Open Systems Interconnection
Developed by the International Organization for Standardization (ISO)
Seven layers: application, presentation, session, transport, network, data link, and physical layer.
A theoretical system delivered too late!
TCP/IP is the de facto standard

23. OSI - The Model
The communications functions are partitioned into a hierarchical set of layers.
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

24. OSI Layers

25. The OSI Environment

26. Elements of Standardization
Protocol specification
Operates between the same layer on two systems
May involve different operating system
Protocol specification must be precise
Format of data units
Semantics of all fields
Allowable sequence of PDUs
Service definition
Functional description of what is provided, but not how the services are to be provided.
Addressing
Each layer provides services to entities at the next higher level. These entities are referenced by means of a service access point (SAP).

27. Service Primitives and Parameters
Services between adjacent layers expressed in terms of primitives (actions) and parameters
Primitives specify function to be performed
Parameters pass data and control information.
The actual form of a primitive is implementation dependent. An example is a procedure call.

28. Primitive Types REQUEST
A primitive issued by a service user to invoke some service and to pass the parameters needed to specify fully the requested service
INDICATION
A primitive issued by a service provider either to:
indicate that a procedure has been invoked by the peer service user on the connection and to provide the associated parameters, or to:
notify the service user of a provider-initiated action
RESPONSE
A primitive issued by a service user to acknowledge or complete some procedure previously invoked by an indication to that user
CONFIRM
A primitive issued by a service provider to acknowledge or complete some procedure previously invoked by a request by the service user

29. Data transfer between two peer entities
Steps occur for data transfer from an (N) entity to a peer (N) entity in another system:
The source (N) entity invokes its (N-1) entity with a request primitive.
Associated with the primitive are the parameters needed, such as the data to be transmitted and the destination address.
The source (N-1) entity prepares an (N-1) PDU to be sent to its peer (N-1) entity.
The destination (N-1) entity delivers the data to the appropriate destination (N) entity via an indication primitive, which includes the data and source address as parameters.
If an acknowledgment is called for, the destination (N) entity issues a response primitive to its (N-1) entity.

30. Timing Sequence for Service Primitives

31. OSI Layers (1)
Physical: Covers the physical interface between devices, and rules by which bits are passed from one to another. Has the following characteristics:
Mechanical: physical properties like: pluggable connectors and cables (called circuits)
Electrical: bits representation (e.g. voltage levels) and transmission rate.
Functional: specifies the function of each circuit, (control, data)
Procedural: specifies the sequence of events by which bit streams are exchanged across the physical medium.

32. OSI Layers (2)
Data Link
The data link layer attempts to make the physical link reliable.
Provides means of activating, maintaining and deactivating a reliable link.
Error detection and control
Higher layers may assume error free transmission, but if the communication is between two systems that are not connected directly (different data links).
The higher layers are not relived of an error control responsibility.

33. TCP/IP Protocol Architecture
Developed by the US Defense Advanced Research Project Agency (DARPA) for its packet switched network (ARPANET)
Consists of a large collection of protocols that have been issued as Internet standards by the Internet Architecture Board (IAB)
No official model but a working one. Five layers:
Application layer
Host to host or transport layer
Internet layer
Network access layer
Physical layer

34. TPC/IP Layers Physical Layer: Network Access Layer:
Physical interface between data transmission device (e.g. computer) and transmission medium or network
Specify the characteristics of transmission medium, nature of the signals and data rate.
Network Access Layer:
Exchange of data between end system and network
Destination address provision
Invoking services like priority

35. TCP/IP Layers (Cont.) Internet Layer (IP) Transport Layer (TCP)
Systems may be attached to different networks
Concerned with routing data across multiple networks
Implemented in end systems and routers
Transport Layer (TCP)
Reliable delivery of data (error handling)
Ordering of delivery (sequencing)
Application Layer
Support for various user applications
e.g. http, SMTP, POP, …

36. OSI v TCP/IP

37. TCP Usual transport layer is Transmission Control Protocol Connection
Reliable connection
Connection
Temporary logical connection (using port values) between entities in different systems
TCP PDU
Called TCP segment
Includes source and destination port (called SAP)
Identify respective users (applications)
Connection refers to pair of ports
TCP tracks segments between entities on each connection

38. UDP Alternative to TCP is User Datagram Protocol
Not guaranteed delivery (unreliable)
No preservation of sequence
No protection against duplication
Minimum overhead (connectionless)
Adds port addressing to IP (full address)

39. TCP/IP Concepts

40. Addressing level Level in architecture at which entity is named
Unique address for each end system (computer) and router
Network level address
IP or internet address (TCP/IP)
Network service access point or NSAP (OSI)
Process within the system
Port number (TCP/IP)
Service access point or SAP (OSI)

41. Trace of Simple Operation
Process associated with port 1 in host A sends message to port 2 in host B
Process at A hands down message to TCP to send to port 2
TCP hands down to IP to send to host B
IP hands down to network layer (e.g. Ethernet) to send to router J
Generates a set of encapsulated PDUs

42. PDUs in TCP/IP

43. TCP Header Information
Destination port: when TCP entity at the receiver receives the segment, it must know to which application the data must be delivered.
Sequence number: if the segments arrive out of order, the TCP entity at the receiver can order them.
Checksum:
Sent segment includes a code that is a function of the contents of the remainder of the segment.
Receiving TCP performs the same calculation and compare the result with the incoming code to detect errors.

44. Some Protocols in TCP/IP Suite