1. Data and Computer Communications

2. Computer Network An interconnected collection of autonomous computers.
Two computers are said to be interconnected if they are able to exchange information.
A system with one control unit and many slaves is not a network.

3. Computer Network (Cont.)
Distributed Systems
Computer Network
The existence of multiple autonomous computers is transparent to the user.
User must explicitly do everything.
Allocation of jobs to processor and files to disks and all other system functions must be automatic.
Distributed system is a software system built on top of a network.
Overlap between distributed systems and Computer Network
Example:
More files around System can involve the User movement.

4. Computer Network (Cont.)
Uses of Computer Network
Companies
People
Social Issues
Resource Sharing
Access to remote information
News-groups
Geography
Person To Person communication &
Bulletin Boards
High reliability: replication
Interactive Entertainment
Saving money on the flow
Client-server model
Scalability: Ability to increase system performance gradually as the workload grows.

5. 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

6. Simplified Communications Model - Diagram

7. Key Communications Tasks
Transmission system utilization
Interfacing
Signal generation
Synchronization
Exchange management
Error detection and correction
Addressing and routing
Recovery
Message formatting
Security
Network management

8. Transmission Technology Point – To – Point Network
Network Hardware
Transmission Technology
Broadcast Network
Point – To – Point Network
Single communication channel that is shared by all the machines on the network.
Many connections between individual pairs of machines
All the others receive “Packets” in certain contexts, sent by any machine.
A packet may have to visit one or more intermediate machine.
An address field within the packet specifies for whom it is intended.
Routing algorithms play an important role in PTP networks.
Multicasting: transmission to a subnet of the machines.

9. Simplified Data Communications Model

10. 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

11. Simplified Network Model

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. Local Area Networks (Cont.)
LAN
MAN
WAN
INTERNET
LAN CHARACTERISTICS
Size
Transmission Technology
Topology
Restricted in Size
Single Cable
10 to 100 Mbps
Low delay (ms)
Very few Errors
Megabits/Sec. (Unit)
BUS (Ethernet)
Ring (Token ring)

14. MAN Metropolitan Area Network Support data and voice
No switching elements
Standard: DQDB (Distributed Queue Dual Bus)
Two unidirectional buses to which all the computers are connected.
Each bus has a head-end, a device that initiates transmission activity.
Traffic that is destined for a computer to the right of the sender uses the upper bus, traffics to the left uses the lower one.

15. Wide Area Networks Large geographical area
Crossing public rights of way
Rely in part on common carrier circuits
Alternative technologies
Circuit switching
Packet switching
Frame relay
Asynchronous transfer mode (ATM)

16. Wide Area Networks (Cont.)
Host (end system).
Subnet (communication subnet).
WANs typically have irregular topologies.
WAN CONSISTS OF
Transmission Lines:- Circuits, Channels or Tanks
Switching Elements:- Specialized computers used to connect two or more transmission lines.

17. Internet Collection of interconnected networks.
Example: A collection of LAN’s connected by a WAN.
WAN : (router + hosts).
SUBNET : (only routers).

18. Circuit Switching
Dedicated communications path established for the duration of the conversation
E.G. Telephone network

19. Packet Switching Data sent out of sequence
Small chunks (packets) of data at a time
Packets passed from node to node between source and destination
Used for terminal to computer and computer to computer communications

20. Frame Relay
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

21. Asynchronous Transfer Mode
ATM (cell relay)
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
Offers a constant data rate channel

22. Integrated Services Digital Network
ISDN
Designed to replace public telecom system
Wide variety of services
Entirely digital domain
First generation ( narrowband ISDN )
64 kbps channel is the basic unit
Circuit-switching orientation
Contributed to frame relay
Second generation ( broadband ISDN )
100s of mbps
Packet-switching orientation
Contributed to ATM ( cell relay )

23. Protocols Used for communications between entities in a system
Must speak the same language
Entities
User applications
facilities
Terminals
Systems
Computer
Terminal
Remote sensor

24. Protocol Hierarchies Organized as a series of layers or levels.
The purpose of each layer is to offer certain services to the higher layers.
Layer n on one-machine carries on a conversation with layer n on another machine.
Protocol: is an agreement between the communicating parties on how communication is to proceed.
Peers communicate using the protocol.
In reality, no data directly transferred from layer n on one machine to layer n on another machine.

25. Protocol Hierarchies (Cont.)
Each layer passes data and control information to the layer immediately below it.
Between each pair of adjacent layers there is an “interface”.
The design of layers helps in:
Minimizing the amount of information that must be passed between layers
Make it simpler to reduce the implementation of one layer with a completely different one
Protocol stack:
A list of protocol used by a certain system, one protocol per layer.

26. Key Elements of a Protocol
Syntax
Data formats
Signal levels
Semantics
Control information
Error handling
Timing
Speed matching
Sequencing

27. Design Issues for the Layers
Addressing.
Data transfer.
Simplex communication.
Half-duplex communication.
Full-duplex communication.
Number and priorities of the logical connection channels. Many networks provide at least two logical channels per connection, one for normal data and one for urgent data.
Error control.
Error detecting code.
Error correcting code.

28. Design Issues (Cont.) How to receive data in order (sequence no.).
How to keep a fast sender from swamping a slow receiver with data (flow control).
Size of the message: disassembling >transmitting >reassembling messages.
Routing: multiple paths between source and destination.

29. 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

30. Simplified File Transfer Architecture

31. A Three Layer Model Network access layer Transport layer
Application layer

32. Network Access Layer
Exchange of data between the computer and the network
Sending computer provides address of destination
May invoke levels of service
Dependent on type of network used (LAN, packet switched etc.)

33. Transport Layer Reliable data exchange
Independent of network being used
Independent of application

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

35. Interfaces and Services
Active elements in each layer are called ENTITIES.
Entity.
Software [example: process.].
Hardware [example: intelligent I/O chip.].
The entities in layer n implement a service used by layer n+1.
Layer n called service provider.
Layer n + 1 called service user.
Services are available at sap’s (service access points).
Each SAP has an address that uniquely identifies it.

36. Interfaces and Services (Cont.)
IDU: interface data unit.
ICI: interface control info.
SDU: service data unit.
At a typical interface, the layer n + 1 entity passes an IDU to the layer n entity through the SAP.
In order to transfer the SDU, the layer n entity may have to fragment it into several pieces, each of which is given a header and send to as a separate PDU (protocol data unit) such as a packet.

37. 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

38. Protocol Architectures and Networks

39. Protocols in Simplified Architecture

40. Protocol Data Units (PDU)
At each layer, protocols are used to communicate
Control information is added to user data at each layer
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

41. Network PDU Adds network header
Network address for destination computer
Facilities requests

42. SERVICES Connection Oriented Connectionless
Modeled after the telephone system
Modeled after posted system
Establish a connection
Use the Connection
Release the connection
Acts like a tube: receive data by the same order was sent
Messages could be received in different order than it was sent with
Reliable connection oriented service
Unreliable connectionless service
(not acknowledged)

43. Request reply service
Sender transmits a single datagram containing a request, the reply contains the answer.
Used to implement communication in the client-server model.

44. Operation of a Protocol Architecture

45. Service Primitives
A service is formally specified by a set of primitives (operations) available to a user or other entity to access the service.
Primitive tells the service to
Perform some action OR
Report an action by a peer entity.
Example: Connection oriented service with 8 service primitives.
CONNECT.request – Request a connection to be established.
CONNECT.indication – Signal the called party.

46. Example (Cont.)
CONNECT.response – Used by the caller to accept/reject calls.
CONNECT.confirm – Tell the caller whether the call was accepted.
DATA.request – Request the data be sent.
DATA.indication – Signal the arrival of data.
DISCONNECT.request – Request that a connection be released.
DISCONNECT.indication – Signal the peer about the request.
Service Could be.
Confirmed (Example: CONNECT).
Unconfirmed (Example: DISCONNECT).

47. Relationship of Services to Protocols
Service: is a set of primitives (operations) that a layer provides to the layer above it.
Protocol.
A set of rules governing the format and meaning of the frames, packets, or messages that are exchanged by the peer entities within a layer.
Entities use protocols in order to implement their service definitions.
Entities are free to change their protocols, provided they do not change the service visible to their users.
REFERENCE MODELS
OSI References Model
TCP/IP Reference Model

48. TCP/IP Protocol Architecture
Developed by the US defense advanced research project agency (DARPA) for its packet switched network (ARPANET).
Used by the global internet.
No official model but a working one.
Application layer.
Host to host or transport layer.
Internet layer.
Network access layer.
Physical layer.

49. Physical Layer
Physical interface between data transmission device (e.G. Computer) and transmission medium or network
Characteristics of transmission medium
Signal levels
Data rates
Etc.

50. Network Access Layer Exchange of data between end system and network
Destination address provision
Invoking services like priority

51. Internet Layer (IP) Systems may be attached to different networks
Routing functions across multiple networks
Implemented in end systems and routers

52. Transport Layer (TCP) Application Layer Reliable delivery of data
Ordering of delivery
Application Layer
Support for user applications
e.g. http, SMPT

53. TCP/IP Protocol Architecture Model

54. OSI Model 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

55. OSI References Model International Standards Organization.
OSI (Open Systems Interconnection).
Reference model: deals with connecting open systems that are; Open for communication with other systems.

56. Principles
A layer should be created where a different level of abstraction is needed.
Each layer should perform a well-defined function.
The function of each layer should be chosen with an eye toward defining internationally standardized protocols.
The layer boundaries should be chosen to minimize the information flow across the interfaces.
The number of layers should be large enough that distinct functions need not be thrown together on the same layer out of necessity.

57. OSI Layers Application Presentation Session Transport Network
Data link
Physical

58. The Physical Layer
Deals with transmitting raw bits over a communication channel.
How many volts for 1 or 0.
How many microseconds a bit lasts.
Mechanics, electrical and procedural interfaces.

59. Data link Layer Break the input data up into data frames.
Process the acknowledgement frames sent back by the receiver.
Insert the frame delimiter.
Solve the problems caused by damaged, lost and duplicate frames.
Flow control.
Full duplex transmission (piggybacking)
Medium access sub layer deals with how to control access to the shared channel in broadcast networks.

60. Network Layer Routing packets from source to destination.
Routes can be static or dynamic
Bottleneck, congestion
Connect heterogeneous networks (different addressing method, larger packet service).
In broadcast networks, routing problem is simple, so the network layer is thin.

61. Transport Layer
Accept data from the session layer, split it up into smaller units if needed, pass these to the network layer and ensure that the all pieces arrive correctly at the other end
Under normal conditions, the transport layer creates a distinct network connection for each transport connection required by the session layer
If the transport connection requires a high throughput, the transport layer might create multiple network connections, dividing the data among the network connections to improve throughput

62. Transport Layer (Cont.)
Transport layer determines what type of service to provide the session layer with and ultimately, the users of the entire network
The transport layer is a true end-to-end layer, from source to destination
Multiple connections will be entering and leaving each host. There is a need to tell which message belongs to which connection (transport header)
Establishing and deleting connections across the network
Flow control between hosts (as oppose between routers) so fast host cannot overrun a slow one

63. Session Layer
Allows users on different machines to establish sessions between them
A session might be used to allow a user to log into a remote timesharing system or to transfer a file between two machines
Example: token management. Only the side holding the token may perform the critical operation.
Synchronization: insert a checkpoint.
Example: sending file for 20 hours. After a crash the portion after the checkpoint will be resend again.

64. Presentation Layer
Concerned with the syntax and semantics of the information transmitted.
A typical example of a presentation service is encoding data in a standard agreed upon way. [Character strings, integers, floating-point numbers…].

65. Application Layer
The application layer contains a variety of protocols that are commonly needed.
Example: incompatible terminal type.
One way to solve this problem is to define an abstract network virtual terminal that editor can be written to deal with. To handle each terminal type, a piece of s/w must be written to map the functions of the network virtual terminal onto the real terminal.
Other application is file transfer(ftp).

66. TCP/IP and OSI Protocol Architectures

67. Example Of Networks Novell NETWARE. Client-server model. IPX/SPX.
Network layer runs IPX (internet packet exchange).
IPX uses 10 byte address (IP uses 4 bytes) flat addressing.
Transport protocol.
NCP (network core protocol).
Transport service & other services.
SPX (sequenced packet exchange):
Just transport service.

68. Example Of Networks (Cont.)
The application can choose between NCP & SPX
Transport control field counts how many networks the packet has traversed.
About once a minute, each server broadcasts a packet giving its address and telling what services it offers.
SAP (Service Advertising Protocol) is used for broadcasting
Routers run some kind of special agent processes to construct databases of which servers are running.
When a client is booted, it sends a request for a server. The agent on the local router machine sees this request, and matches up the request with the best server.

69. Example Of Networks (Cont.)
The APRANET.
Packet switched network, consisting of subnet and host computers.
IMPS (interface message processors) connected by transmission lines.
Each IMP would be connected to at least two other imps.
Each node consists of IMP and a host.
Host sends messages of up to 8063 bits to its IMP.
IMP breaks the message into packets of at most 1008 bits and forwards them independently toward the destination.
56-kbps lines leased from telephone companies interconnect the IMPS.
By 1990, the ARPANET had been overtaken by newer networks.

70. Example Of Networks (Cont.)
NSFNET
By 1984 NSF Fig 1.26(the U.S. national science Foundation) began designing a high-speed successor to the ARPANET that would be open to all university research groups.
By 1995 the NSFNET backbone was no longer needed to interconnect the NSF regional networks because numerous companies were running commercial IP Networks.

71. Example Of Networks (Cont.)
The Internet.
In 1992, the internet society was set up, to promote the use of the internet.
Four main applications. .
News.
Remote login: telnet, rlogin.
File transfer: FTP.

72. Example Of Networks (Cont.)
Gigabit TESTBEDS.
The backbones operate at megabit speeds.
Gigabit networks provide better bandwidth but not always much better delay.
Example: sending a 1-kbit packet from NYC to san Francisco at (1 mbps) take.
1 msec to pump the bits out and 20 msec for the delay, for a total of 21 msec. A 1-Gbps network can reduce this to msec.
For some applications, bandwidth is what counts, and these are the applications for which gigabit networks will make a big difference.
Examples:- telemedicine & virtual meeting.

73. Example Data Communication Services
SMDS
X.25
FRAME RELAY
BROADBAND ISDN AND ATM