Datacommchapter 1

Chapter 1

Part A

Data Communications and Networks Overview

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Syllabus Chapter 1: Data Communications and Networking Overview Chapter 2: Data Communications Fundamental Chapter 3: Characteristics of Data Communication Networks Chapter 4: Reliable Data Communications Chapter 5: Multiple Access Networks Chapter 6: Internetworking Protocols

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1­1   DATA COMMUNICATIONS The term telecommunication means communication at a distance. The word data refers to information presented in whatever form is agreed upon by the parties creating and using the data. Data communications are the exchange of data between two devices via some form of transmission medium such as a wire cable.

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Figure 1.1 Five components of data communication What is data communications? Exchange of data between two devices via a transmission medium Any transfer of data within a computer, between computer and any other devices Exchange of digitally encoded information between two Sides

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Data Representation Ø Text – represented as a bit pattern; codes often used: ASCII; Extended ASCII; Unicode; ISO Ø Numbers – represented by binary equivalent Ø Images – bit patterns representing pixels Ø Audio Ø Video

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Figure 1.2 Data flow (simplex, half-duplex, and full-duplex)

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1­2   NETWORKS A network is a set of devices (often referred to as nodes) connected by communication links. A node can be a computer, printer, or any other device capable of sending and/or receiving data generated by other nodes on the network.

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Categories of Network Based on size, ownership, distance covered, and physical architecture: Ø

Local Area Network (LAN) – smaller geographical area

Ø

Metropolitan Area Network (MAN) – network extended over an entire city

Ø

Wide Area Network (WAN) – large geographical area

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Figure 1.3 Types of connections: point-to-point and multipoint

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Figure 1.4 Categories of topology

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Figure 1.5 A fully connected mesh topology (five devices)

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Figure 1.6 A star topology connecting four stations

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Figure 1.7 A bus topology connecting three stations

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Figure 1.8 A ring topology connecting six stations

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Figure 1.9 A hybrid topology: a star backbone with three bus networks

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Figure 1.10 An isolated LAN connecting 12 computers to a hub in a closet

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Figure 1.11 WANs: a switched WAN and a point-to-point WAN

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Figure 1.12 A heterogeneous network made of four WANs and two LANs

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1­3   THE INTERNET The Internet has revolutionized many aspects of our daily lives. It has affected the way we do business as well as the way we spend our leisure time. The Internet is a communication system that has brought a wealth of information to our fingertips and organized it for our use.

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Figure 1.13 Hierarchical organization of the Internet

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1­4   PROTOCOLS AND STANDARDS In this section, we define two widely used terms: protocols and standards. First, we define protocol, which is synonymous with rule. Then we discuss standards, which are agreed-upon rules.

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Protocols and Standards Why do we need Protocol and Standard? Protocol – set of rules that govern data communication; defines what, how, and when (Key elements – syntax, semantics, timing)

Standard – provides a model for development; allows for interoperability

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Protocols Ø When two computers across a network exchange data, procedures involved can be quite complex. Ø These computers cannot simply send bit streams to each other and expect to be understood. Ø There must be a high degree of cooperation between the two computer systems. Ø For communication to occur, the entities must agree on a protocol, what is communicated, how it is communicated, and when it is communicated

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Key Elements of a Protocol Ø Syntax

f1

Ø

Format of the data blocks

Ø

e.g. What are the fields, how many bits per field, etc.

f2

f3

Ø Semantics Ø

Control information for coordination & operation

Ø

Defines functions of the fields, what does each field do?

Ø

This include error handling information

Ø Timing Ø

Speed matching/synchronizing so that packets can be received properly (especially to know where the protocol frame starts and ends)

Ø

Sequencing so that frames or packets can be received in order (especially for packet-based switching)

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Basic Protocol Architecture Ø Application Layer Ø

Support for different user applications

Ø

e.g. e-mail, file transfer

Ø Transport Layer Ø

Reliable data exchange

Ø

Independent of network being used

Ø

Independent of application

Ø Network Access Layer Ø

Exchange of data between the computer and the network

Ø

Sending computer provides address of destination so that data can be routed

Ø

May invoke levels of service e.g. priority

Ø

1.25 Dependent on type of network used (Ethernet LAN, ATM, WLAN)

Standards Ø A set of agreed-upon rules/protocols which are essential in creating and maintaining an open and competitive market for equipment manufacturers and other service providers, also in guaranteeing international interoperatibility of data and telecommunications technology and processes Ø Standard Organization: Ø

CCITT International Telegraph and Telephone Consultative Committee in Europe [now ITU-T (International Telecommunication Union – Telecommunication standardization sector)]

Ø

ISO – International Standards Organization

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Standards Ø Advantages Ø

Ensures a large market for equipment and software

Ø

Allows products from different vendors to communicate

Ø Disadvantages Ø

Freeze technology

Ø

May be multiple standards for the same thing

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Chapter 1

Part B

Data Communications and Networks Overview

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2­1   LAYERED TASKS We use the concept of layers in our daily life. As an example, let us consider two friends who communicate through postal mail. The process of sending a letter to a friend would be complex if there were no services available from the post office.

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Figure 2.1

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Tasks involved in

2­2   THE OSI MODEL Established in 1947, the International Standards Organization (ISO) is a multinational body dedicated to worldwide agreement on international standards. An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model. It was first introduced in the late

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Motivation of OSI Model Ø Provides a common set of convention, to make communication among heterogeneous machines possible. Ø If functions of each layer are well-defined, standards can be developed independently and simultaneously for each layer – that means faster standardization process. Ø If the boundaries between layers are well-defined, changes in standards in one layer need not affect another layer – easier to introduce new standards Ø the task of communication between applications on different computer is too complex to be handle as a unit. Problem can be decomposed into manageable parts 1.32

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ISO is the organization. OSI is the model.

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Figure 2.2 Seven layers of the OSI model

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Figure 2.3 The interaction between layers in the OSI model the same set of layered functions must exist in 2 systems (transmitting and receiving sides) communication is achieved by having The corresponding (peer) layers in 2 Systems communicate

telecom networks mainly concern the lowest 3 layers

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Figure 2.4 An exchange using the OSI model

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2­3   LAYERS IN THE OSI MODEL In this section we briefly describe the functions of each layer in the OSI model.

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Figure 2.5 Physical layer

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The physical layer is responsible for movements of individual bits from one hop (node) to the next.

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Figure 2.6 Data link layer

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The data link layer is responsible for moving frames from one hop (node) to the next.

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Figure 2.7 Hop-to-hop delivery

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Figure 2.8 Network layer

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The network layer is responsible for the delivery of individual packets from the source host to the destination host.

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Figure 2.9 Source-to-destination delivery

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Figure 2.10 Transport layer

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The transport layer is responsible for the delivery of a message from one process to another.

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Figure 2.11 Reliable process-to-process delivery of a message

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Figure 2.12 Session layer

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The session layer is responsible for dialog control and synchronization.

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Figure 2.13 Presentation layer

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The presentation layer is responsible for translation, compression, and encryption.

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Figure 2.14 Application layer

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The application layer is responsible for providing services to the user.

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Figure 2.15 Summary of layers

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Functions of Each Layer (Sum)

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Layer 7

Layer 1

The basic functions of each layer are summarized below: 1.

Physical- Concerned with transmission of raw bit stream over physical medium; deals with mechanical, electrical, functional and procedural properties of interfaces and physical medium

2.

Link- Responsible for node-to-node validity and integrity of the transmissions; send blocks of data [frames] with synchronization

3.

Network- Provide upper layers with data transmission and switching technologies used to connect systems; establishes the route between sender and receiver

4.

Transport- Provide end-to-end error recovery and flow control

5.

Session- Provide coordination for communication between applications; establishes, manages and terminates connections between cooperating applications

6.

Presentation- Manages the way data is represented to the application processes from difference in data representation

7.

Application- Defines the rules for applications to gain entrance into the communication system 1.57

2­4   TCP/IP PROTOCOL SUITE The layers in the TCP/IP protocol suite do not exactly match those in the OSI model. The original TCP/IP protocol suite was defined as having four layers: host-to-network, internet, transport, and application. However, when TCP/IP is compared to OSI, we can say that the TCP/IP protocol suite is made of five layers: physical, data link, network, transport, and

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TCP/IP Model Ø Everyone believed that the OSI model would become the ultimate standard for computer communication before 1990, but this did not happen Ø TCP/IP protocol suite became the dominant commercial architecture because it was used and tested extensively in the Internet, while the OSI model was never fully implemented Ø As TCP/IP was developed concurrently with the OSI model, it does not contain specific protocols relating to all the OSI layers Ø The TCP/IP suite is made of five layers Ø The three top-most layers in the OSI model are represented by the applications layer Ø The OSI model specifies functions associated with each layer, whereas TCP/IP layers contain relatively independent protocols that can be mixed and matched 1.59

Figure 2.16 TCP/IP and OSI model

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TCP/IP Model

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TCP/IP Model

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TCP/IP Model n

Application Layer - contains a selection of application protocols [e.g. FTP, SMTP, HTTP, SNMP and TELNET]

n

Transport Layer - represented by 2 transport protocols: i. TCP [Transmission Control Protocol] ii. UDP [User Datagram Protocol] - these are process-to-process protocols responsible for delivery of a message from a process [running program] to another process - TCP provides a reliable connection-oriented service - UDP provides an unrealiable connectionless service by delivering the UDP datagrams on a best-effort basis [when error correction is not needed or for a single short request/response message exchange between two application protocols] 1.63

TCP/IP Model n

Network Layer - The main protocol is IP [Internet Protocol] and other supporting protocols: ARP, ICMP and IGMP - IP is an unreliable and connectionless protocol - a best-effort delivery service, it does its best to get a transmission through to its destination - IP transport data in packets called datagrams [each=IP header + TCP or UDP packet] which travel along different routes to destination - IP is a host-to-host protocol, meaning that it can only deliver a packet from one device to another by routing across multiple networks 1.64

TCP/IP Model n

Data Link Layer - also known as network access layer, and is concerned with the exchange of data between an end system and a network - main functions are: bits

i. Framing IP datagrams into a stream of

ii. Specifying the MAC [Medium Access Control] methods to the networks iii. Specifying MAC [hardware] and CRC in the frame - HDLC [High-level Data Link Control] and ARQ [Automatic Repeat Request] are the two important protocols at this layer 1.65

TCP/IP Model n

Physical Layer - defines the interface between devices and transmission media [type of connectors], type of media, transmission rate, type of encoding for representing data bits in electrical or optical signals, network topology, communication mode [e.g. fullduplex service] and etc

n

There are no specific protocols defined for the lowest two layers

n

Hence, they support all of the standard and proprietary LAN or WAN protocols or technologies

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2­5   ADDRESSING Four levels of addresses are used in an internet employing the TCP/IP protocols: physical, logical, port, and specific.

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Figure 2.17 Addresses in TCP/IP

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Figure 2.18 Relationship of layers and addresses in TCP/IP

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Example 2.1 In Figure 2.19 a node with physical address 10 sends a frame to a node with physical address 87. The two nodes are connected by a link (bus topology LAN). As the figure shows, the computer with physical address 10 is the sender, and the computer with physical address 87 is the receiver.

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Figure 2.19 Physical addresses

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Example 2.2 Most local-area networks use a 48-bit (6-byte) physical address written as 12 hexadecimal digits; every byte (2 hexadecimal digits) is separated by a colon, as shown below:

07:01:02:01:2C:4B A 6-byte (12 hexadecimal digits) physical address.

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Example 2.3 Figure 2.20 shows a part of an internet with two routers connecting three LANs. Each device (computer or router) has a pair of addresses (logical and physical) for each connection. In this case, each computer is connected to only one link and therefore has only one pair of addresses. Each router, however, is connected to three networks (only two are shown in the figure). So each router has three pairs of addresses, one for each 1.73

Figure 2.20 IP addresses

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Example 2.4 Figure 2.21 shows two computers communicating via the Internet. The sending computer is running three processes at this time with port addresses a, b, and c. The receiving computer is running two processes at this time with port addresses j and k. Process a in the sending computer needs to communicate with process j in the receiving computer. Note that although physical addresses change from hop to hop, logical and port addresses remain the same from the 1.75

Figure 2.21 Port addresses

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The physical addresses will change from hop to hop, but the logical addresses usually remain the same.

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Example 2.5 A port address is a 16-bit address represented by one decimal number as shown.

753 A 16-bit port address represented as one single number.

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The physical addresses change from hop to hop, but the logical and port addresses usually remain the same.

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