Networking and Digital Communication Fundamentals

1. Introduction

The basic of networking and digital communication and essential elements required to understand how computer network operates in the context of security of information system for out net-centric organisation.

1.1 Topics to be covered

• Types of network
• How network works
• Routers
• Gateways
• Bridges
• OSI Model
• Network Topologies
• Protocols

Network: Two or more devices connected together in such way as to allow them to exchange information.

Protocol: Set of standards that defines all operations within a network, basically it is an agreed upon formate for transmitting data between two devices. It determines a number of things in the network:

• type of error check
• data compression method, if any
• indicate finished sending a message, or received a message
• how devices outside the network can interact with the network

Protocol defines everything from basic networking data structure to upper level application programs.

2. Network Types

A network is a logical grouping of computers that share information and resources.

Common Categorisation:

1. LAN (Local Area Network)
2. WAN (Wide Area Network)

2.1 LAN (Local Area Network)

A LAN is a single network of computers, which are physically located in the same area and rarely spans over more than one location. It usually comprise small number of computers and printers with up to five servers.

Example - Network in a small office or home office.

2.2 WAN (Wide Area Network)

A WAN is a network of LANs. WANs span more than one geographic area and are used to connect remote offices to each other. Basically, a WAN comprises two or more LANs joined together by routers. It can be found in medium - to - large-sized businesses with more then one office location.

Routers are hardware devices that direct traffic from one LAN to another.

Example - A software company might have its headquarters in Delhi, but also have remote office locations, say in Aurangabad and Faridabad. The LAN in the Delhi office would be connected to the LAN in the remote offices forming a WAN.

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LANs and WANs comes in many different varient. The most popular network type is Ethernet. Ethernet network have speeds of 10 Mbps, 100 Mbps or 1 Gbps. The majority of networks today operates at 100 Mbps. An alternative to Ethernet network is token-ring networks. Token-ring networks operates at 4 or 16 Mbps and are usually found in legacy system/networks. Token-ring networks are rarely found in companies today because of their lack of speed.

2.3 Key Terms

1. LAN: It is a system of network computers and other hardware, such as printers, that are in relatively close proximity to one another.
2. Backbone: It is a high-speed network that connects several LANs in a location, to provide services. Such as in a university campus several LANs are connected to provide electronic mail (e-mail) and Internet access service beyond the LAN environment.

3. MAN: MANs are larger than LANs, but smaller than WANs. MANs are usually characterised by very high speed connections using fiber optical cables or other digital media.

   IEEE 802-2001 standard definition:
   A MAN (metropolitan area network) is optimized for a larger geographical area than is a LAN, ranging from several blocks of building to entire cities. MANs can also depend on communications channels of moderate-to-high data rates. A MAN might be owned and operated by a single organization, but it is usually used by many individuals and organizations. MANs might also be owned and operated as public utilities. They will often provide means for internetworking for local network. Metropolitan area networks can span up to 50 km, devices used ate modem and wire/cable.

4. WAN: It is a geographically dispersed communications network, with many owners, linking computers for the purpose of communication with each other, such as a national network for airline reservations or railway reservation.

5. Value-added Network (VAN): It is a communication channel leased from a telephone company to offer customers with modern access to network services through a local or toll-free number.

6. Wireless Network: It is most commonly used to refer to a telecommunication network whose interconnections between nodes is implemented without the use of wires. Wireless telecommunications networks are generally implemented with some type of remote information transmission system that uses electromagnetic waves, such as radio waves.

3. Network Architecture

Network architecture can be described in two ways: peer-to-peer and client/server.

3.1 peer-to-peer

A peer-to-peer network is a grouping of personal computers such that all share information among each other. It usually comprise less than 10 computers. This type of network is for the users that requires very little computer service and security.

Security Issues

• In peer-to-peer network, file storage is scattered among the computers and security is extremely low.
• Susceptible to hackers and other malicious users because there is no solid security policy enforced.

3.2 client/server

A client/server network is a network comprising several workstation and one or more servers. Client (user) log in the server and gain access to their files.

Advantage over peer-to-peer network

• An administrator can control the privileges of each user.
• Files are stored centrally, simplifying data backup.
• Security policies are implemented that protects the users’ information.

4. Network Topologies

Topology refers to the shape of a network or the network’s layout. How different nodes in a network are connected to each other and how they communicate. Each topology is suited to specific tasks and has its own advantages and disadvantages. The choice of topology is dependent on:

1. type and number of equipments being used.
2. planned application and rate of data transfer.
3. required response time.
4. cost.

4.1 Physical Topology Vs. Logical Topology

Physical topology is the physical layout of devices on a network. The way that the workstations are connected to the network through the actucal cables that transmit data, that is , the physical structure fo the network, is called the physical topology. Every LAN has a topology, that is, the way that the devices on a network are arranged and how they communicate with each other.

The logical topology, in contrast, is the way that the signals act on the network media, or the way that the data pass through the network from one device to the next without regard to the physical interconnection to the device. Logical topology is also called signal topology.

A network’s logical topology is not necessarily the same as its physical topology.

Logical topologies are bound to the network protocols that direct how the data move across a network. The ethernet protocol is a common logical bus topology protocol.

4.2 Mesh Topology

In this topology, the devices are connected with many redundant interconnections between the network nodes. In a true mesh topology, every node has a connection to every other node in the network.

One of the problems with mesh topology is that its cabling expenses are high and it takes extra efforts to track down cable faults.

4.3 Star Topology

In this topology, all devices are connected to a central hub. Nodes communicate across the network by passing data through the hub.

Advantages of star topology are: it becomes easy to add new workstations (nodes), it allows centralised control over the network and it allows centralised monitoring of the hub/network.

Disadvantage: if the hub fails, it cripples all the workstations connected to that hub, bringing the network performance to zero, and hub are expensive network components. in terms of cost.

In the connection with the start topology, the following important terms are used:

1. Hub: A common connection point for devices in a network. Hubs are commonly used to connect segments of a LAN. A hub contains multiple ports. When a packet arrives at one port, it it copied to the other ports so that all segments of the LAN can see all the packets.
   1. Passive Hub: A passive hub serves simply as a conduit for the data, enabling it to go from one device (or segment) to another.
   2. Intelligent Hub: Intelligent hubs includes additional features that enables an administrator to monitor the traffic passing through the hub and to configure each port in the hub. Also called manageable hubs.
   3. Switching Hub: Switching hubs reads the destination address of each packet and the forwards the packet to the correct port.
2. Port: An interface of the computer to which a device can be connected.
3. Packet: A piece of message transmitted over a packet-switching network. One if the key features of a packet is that it contains the destination address in addition to the data.
4. Packet Switching: Protocol in which message are divided into packets before they are sent. Each packet is then transmitted individually and can even follow different routes to its destination. Once all the packets forming a message arrive at the destination, they are recompiled into the original message.
5. Switching Hub: A special type of hub that forwards packets to the appropriate port based on the packet’s address.
6. Segment: A segment, in a LAN or other types of networks, is a section of a network that is bounded by bridges, routers or switches. Hubs and switches are used to connect each segment to the rest of the LAN.

4.4 Bus Topology

In this topology, all devices are connected to a central cable, called the bus or backbone.

In connection with the bus topology, the following important terms are used:

1. Bus: A collection of wires through which data are transmitted from one part of a computer to another.
2. Backbone: Another term for bus, the main wire that connects nodes. The term is often used to describe the main network connection composing the Internet.

In networking, a bus is a central cable that connects all devices on a LAN. It is also called the backbone.

4.5 Ring Topology

In this topology, all devices are connected to one another in the shape of a closed loop, so that each device is connected directly to two other devices, one on either side of it.

Advantages:

1. Cable failures affect limited users.
2. Equal access is provided for all users.
3. Each workstation has full access speed to the ring.
4. Even when workstation numbers increase, performance diminishes only slightly.

Disadvantages:

1. Wiring cost.
2. Difficult connections.
3. expensive adaptors cards need to be used.

4.6 Tree Topology

This is a hybrid topology. Groups of star-configured networks are connected to a linear bus backbone.

5. The OSI Seven Layer Model

The OSI is an International Organisation for Standardisation (ISO) standard for worldwide communications that defines a networking framework for implementing protocols in seven layers.

The OSI reference model is a set of seven layers that defines the different stages that data must go through to travel from one device to another over a network. It is also referred as the OSI reference model or just the OSI model.

5.1 Application Set

5.1.1 Layer 7 - Application

This is the layer that actually interacts with the operating system (OS) or application whenever the user transfers files, reads messages or perform other network related activities. The application layer supports application and end-user processes. Everything at this layer is application specific. This layer provides application services for file transfers, e-mail and other network software services.

5.1.2 Layer 6 - Presentation

It takes the data provided by the application layer and converts it into a standard formate that the other layers can understand. The presentation layer provides independence from differences in data presentation by translating from application to network formate and vice versa.

The presentation layer works to transform data into the form that the application layer can accept. This layer formates and encrypt data to be sent across a network, providing freedom from compatibility problems.

The presentation layer is sometimes called the syntax layer.

5.1.3 Layer 5 - Session

This layer establishes, maintains and ends communication with the receiving device. It establishes, manages and terminates conversations, exchanges and dialogues between the applications at each end. It deals with session and connection coordination.

5.2 Transport Set

5.2.1 Layer 4 - Transport

This layer maintains flow control of data and provides for error checking and recovery of data between the devices.

Transport layer looks to see if data are coming from more then one application and integrates each application’s data into a single stream for the physical network. This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery. It ensures complete data transfer.

5.2.2 Layer 3 - Network

The format, in which the transmitted data will be sent to the recipient device, is determined in this layer. Logical protocols, routing and addressing are handled here.

Network layer provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, inter-networking, error handling, congestion control and packet sequencing.

5.2.3 Layer 2 - Data

In this layer, the appropriate physical protocol is assigned to the data. Also, the type of network and packet sequencing are defined. At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronisation.

In Institute of Electrical and Electronics Engineer (IEEE) 802 networks, the data link control (DLC) layer is divided into two sublayers:

1. The media access control (MAC) layer
2. logical link control (LLC) layer

MAC is a hardware address that uniquely identifies each node of a network. The MAC layer interface directly with the network medium. Consequently, each different type of a network medium requires a different MAC layer. The MAC sublayer controls how a computer on the network gains access to the data and permission to transmit it.

The LLC layer controls frame synchronisation, flow control and error checking.

5.2.4 Layer 1 - Physical

This is the level of the actual hardware. It defines the physical characteristics of the network such as connections, voltage levels and timing. This layer conveys the bit stream electric impluse, light and radio signal through the network at the electric and mechanical levels. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects.

6. Network Components

The network components can be both software and hardware elements. Some of the key hardware components of computer networks are:

1. Cables
   1. Coaxial Cable
      1. Thin Ethernet
      2. Thick Ethernet
      3. Twisted Pair
2. network adapter cards
3. hubs and switching hubs
4. routers
5. gateways and bridges
6. firewalls.

6.1 Network Cables

There are two most popular types of network cabling:

1. twisted pair (also known as 10BaseT)
2. thin coax (also known as 10Base2)

10BaseT was used with 10Mbps. 10BaseT cabling looks like an ordinary telephone wire, except that is has eight wires inside instead of four.

Thin coax looks like the copper coaxial cabling that is often used to connect a videocassette recorder to a television set.

10baseT or Cat5 cables are used for most home and small business networks. They are thin, easy to manipulate are easy to purchase as they come in custome lengths. The 10baseT/Cat5 cables have connectors that resemble a phone cord connector, only larger. These are called RJ-45 connectors.

6.2 Network Adapter Cards

Network adapter cards (NIC: Network Interface Card) plays an important role in computer networks on which modern IS reside. A network computer is connected to the network cabling with an NIC.

6.3 Hub and Switching Hub or the Switch

A hub is the central connecting device in a network. It is a box that is used to gather groups of PCs together at a central location with 10BaseT cabling.

For small networks (such as peer-to-peer network), it is possible to get by with a hub, some 10BaseT cables and a handful of network adapters.

Larger network often use a thin coax ‘backbone’ that connects a row of 10BaseT hubs together. Each hub, in turn, may connect a handful of computers together using 10BaseT cabling, which allows you to build networks of tens, hundreds or thousands of computers.

The switching hub, sometimes called a ‘switch’, is a more advanced unit over the basic hub. In a basic hub, all the computers connect to it and the speed of the Internet is defined by the slowest computer network card connected.

Switching hub treats each network card independently. The switching hub allows all the faster connections to remain at a higher speed and still interacts with the slow system.

6.4 Routers

Network is a heterogeneous environments. In such environments, there is a need of connection device that would interconnect two different technologies.

The router serves as a routing switchboard. It is a special computer that is dedicated to the task of interconnecting networks. It moves the information from its source to its destination regardless of the middleware.

Routers connect two or more networks and forward data packets between them. When data arrive from one of the segments, the router decides, according to its routing table, to which segment to forward the data.

6.5 Gateways and Bridges

A gateway is a legacy term, that is, it is a term that was used in the earlier days of computer networking. It once used to refer to a routing device. Today, in the TCP/IP world, the term gateway now refers to special-purpose devices that performs protocol conversions. Gateways implement application layer conversions of information received from various protocols.

A bridge is an electric device that connects and passes packets between two network segments. In general, a bridge will forward or discard an incoming frame based on the MAC address of that frame.

6.6 Firewalls

A firewall is a system designed to prevent unauthorised access to or from a private network. Firewall can be implemented in both hardware or software, or a combination of both. They are frequently used to prevent unauthorised Internet users from accessing private networks connected to the Internet, especially intranets. All messages entering or leaving the intranet pass through the firewall, which examines each message and blocks those that do not meet the specified security criteria.

Types of firewall techniques:

1. Packet filter: This involves looking at each packet entering or leaving the network and accepts or rejects it on the basis of user defined rules.
2. Application gateway: This applies security mechanisms to specific applications, such as file transfer protocol (FTP) and Telnet severs.
3. Circuit-level gateway: In this, security mechanism are applied when a TCP or UDP connection is established. Once the connection has been made, packets can flow between the hosts without further checking.
4. Proxy server: This is used to intercept all messages entering and leaving the network. The proxy server effectively hides the true network addresses.

7. Network Protocols

A network protocol is basically an agreed-upon format for transmitting data between two devices. It is a standard set of rules that determines how systems will communicate across network. Two different systems can communicate and understand each other only if they use the same protocol in spite of their differences.

7.1 TCP/IP

TCP is one of the main protocols in TCP/IP networks. Whereas the IP deals only with the packets, TCP enables two hosts to establish a connection and exchange streams of data. TCP guarantees delivery of data and also guarantees that packets will be delivered in the same order in which they were sent. IP specifies the format of packets, also called datagrams, and the addressing scheme. Most networks combines IP with a higher level protocol called TCP, which establishes a virtual connection between a destination and source.

7.2 Link Protocol

Link protocol plays a role at layer 2 of the OSI model (the data link layer). In communication, there is transmission of a unit of data from one node to another. It is responsible for ensuring that the bits received are the same as the bits sent. There are two categories of data transmission:

1. synchronous
2. asynchronous

Asynchronous means not synchronised, that is, not occourring at perdetermined or regular intervals. The term ‘asynchronous’ is usually used to describe communication in which data can be transmitted intermittently rather than in a steady steam.

The difficulty with asynchronous communication is that the receiver must have a way to distinguish between valid data and noise. In computer communications, this is usually accomplished through a special start bit and a stop bit at the beginning and end of each piece of data. For this reason, asynchronous communication is sometimes called start-stop communication.

Synchronous communication occurs at regular intervals.

7.3 OSI Protocol

OSI is an information exchange standard developed by the ISO in the early 1980s. The structure of the OSI in terms of its underlying protocols is as follows:

1. physical layer
2. datalink layer
3. network layer
4. transport layer
5. session layer
6. presentation layer
7. application layer

7.4 Routing Protocol

Routing protocols allow different computer networks to communicate. Some commonly used routing protocols are:

1. router information protocol (RIP)
2. open shortest path first (OSPF)
3. interior gateway routing protocol (IGRP)
4. enhanced IGRP (EIGRP)
5. border gateway protocol (BGP)
6. intermediate system to intermediate system (IS - IS)
7. constrained shortest path first (CSPF)

IGRP and EIGRP are proprietary Cisco protocols, and are supported on Cisco routers or other vendors’ routers to which Cisco has licensed the technology.

7.5 Tunneling Protocol

Tunneling is to do with VPN (Virtual Private Network). A tunneling protocol is a network protocol that encapsulates open protocol or session inside another. Protocol A is encapsulated within protocol B, such that A treats B as though it were a data link layer. Tunneling may be used to transport a network protocol through a network that would not otherwise support.

Examples of Tunneling Protocols:

1. Datagram - based
   1. layer 2 tunneling protocol (L2TP)
   2. multi-protocol label switching (MPLS)
   3. generic routing encapsulation (GRE)
   4. general packet radio service (GPRS) tunneling protocol (GTP)
   5. point-to-point tunneling protocol (PPTP)
   6. point-to-point protocol over Ethernet (PPPoE)
2. Stream-based
   1. transport layer security (TLS)
   2. secure shell (SSH)

8. Working of Network and the Internet

No one really ‘owns’ the Internet! It is a global collection of networks, both big and small. Under the Internet scheme, networks all over the world connect together in many different ways to form the single entity that is known as the ‘Internet’.

Typically, people connect to the Internet in two ways: while at work and while using the LAN connection through the ISP contract. The ISP may then connect to a larger network and become part of it. While at home or a remote location, most people typically connect to the Internet using the combination of modem and dial-up service. Either ways, when anyone connects to the Internet, his/her computer becomes a part of a network whose span is global.

POP (Point of Presence)
Most large corporations have their own dedicated backbones connecting various regions. In each region, the company has a point of presence (POP). The POP is a place for local users to access the company’s network, often through a local phone number or dedicated line.

9. Telecommunication Links and Other Important Related Topics

9.1 Circuit Switching and Packet Switching

Circuit Switching is a type of communication in which a dedicated channel (or circuit) is established for the duration of a transmission. The most ubiquitous circuit-switching network is the telephone system, which links together wire segments to create a single unbroken line for each telephone call. This system is ideal for communications that requires data to be transmitted in real time. These networks are sometimes called connection oriented networks.

Packet switching is a method in which messages are divided into packets and sends each packet individually. The Internet is based on a packet-switching protocol - the TCP/IP. Packet switching networks are more efficient if some amount of delay is acceptable. These networks are connectionless networks, a packet switching network can be made connection oriented by using a higher level protocol. Example: TCP, makes IP networks connection oriented.

9.2 Host

A host is a computer system that is accessed by a user working at a remote location. Typically, the term is used when there are two computer systems connected by modems and telephone lines.

9.3 Dedicated Server

Dedicated means being reserved for a specific use. A dedicated server is a single computer in a network reserved for serving the needs of the network.

9.4 Workstation

A ‘workstation’ is a type of computer used for engineering applications, desktop publishing, software development and other type of applications that require a moderate amount of computing power and relatively high-quality graphic capabilities. In the context of networking, workstation refers to any computer connected to a LAN. It could be workstation or a PC.

9.5 Channel and Node

A channel is a transmission path. A transmission channel is the path between two nodes of a network that a data communication follows. The term can refer to the physical cabling that connects the nodes on a network, the signal that is communicated over the pathway or a subchannel in a carrier frequency.

A node is a processing location in a network. A node can be a computer or some other device, such as printer. Every node has a unique network address, called as MAC address.

9.6 Contention

In the context of computer networks, contention means competition for resources. The term is used especially in network to describe the situation where two or more nodes attempt to transmit a message across the same wire at the same time. A contention is also a type of network protocol that allows nodes to contend for network access. That is, two or more nodes may try to send message across the network simunltaneously. The contention protocol defines what happens when this occurs.

9.7 CSMS/CD

CSMA/CD is a set of rules determining how network devices respond when two devices attempts to use a data channel simultaneously (called a ‘collision’).

9.8 Twisted-Pair Cable

This is a type of cable that consists of two independently insulated wires twisted around one another.

9.9 Coaxial Cable

A coaxial cable is a type of wire that consists of a centre wire surrounded by insulation and the a grounded shield of braided wire.

9.10 Fiber Optics

Fibre optics is a technology that uses glass (or plastic) threads (fibers) to transmit data. A fiber optic cable consists of a bundle of glass threads, each or which is capable of transmitting messages modulated onto light waves.

9.11 Token, Token Passing and Token-Ring Network

In the networking context, a token is a special series of bits that travels around a token ring network. As the token circulates, computers attached to the network can capture it. The token acts like a ticket, enabling its owner to send a message across the network. There is only one token for each network, so there is no possibility that two computers will attempt to transmit message at the same time.

Token passing uses a token, or series of bits, to grant a device permission to transmit over the network. Whichever device has the token, can put data into the network. When its transmission is complete, the device passes the token along to the next device in the topology.

A token ring network is a type of computer network in which all the computers are arranged in a circle. A token, which is a special bit pattern, travels around the circle. To send a message, a computer catches the token, attaches a message to it and then lets it continue to travel around the network.

9.12 IEEE 802.5 Token Ring

A short message (called a token) is circulated around the ring, passed from station to station. A station that wants to transmit waits for the token to arrive.

When the token arrives, the station change it from a token to a connector message and appends its message. This new message is then placed on the outgoing side of the ring. Each station passes on received tokens if they have nothing to transmit.

The workstations monitor connector messages to see if the message is addressed to them. If connector message are addressed to them, they copy the message, modify it to signify its receipt and then send it on around the ring. Connector messages that are not addressed to them are passed directly on to the next station in the ring. When the connector message travels full circle and arrives at the original sending station, it checks the message to see if it has been received. It then discards the message and replace it with a token.

9.13 CAM and Polling Mechanisms

CAM is short for channel access method, a protocol for how data are transmitted in the bottom two layers of the OSI mode (the physical and the data link layer). Polling, contention and token passing are three examples of CAMs.

Polling is CAM. In a master-slave scenario, the master queries each slave device in turns as to whether it has any data to transmit. If the slave answers yes, then the device is permitted to transmit its data. If the slave answers no, then the master moves on and polls the next slave device. This process is repeated continuously.