GENERAL SIR JOHN KOTELAWALA DEFENCE UNIVERsity | Assignment 01 | Advanced Network and Security | | ICT/12/0121 WMIL Wasalage | 9/23/2013 |

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Abstract

This document contains a detailed explanation about the ISO OSI Reference Model used in networking and it also contains details about the networking devices used in setting up LANs & WANs.
Abstract

This document contains a detailed explanation about the ISO OSI Reference Model used in networking and it also contains details about the networking devices used in setting up LANs & WANs.

Acknowledgement

I would like to express my special thanks of gratitude to Mrs. Punsisi Premaratne who gave me this opportunity to do this project in the networking field. Secondly i would like to thank my parents and friends who helped me a lot in finalizing this project within the limited time frame.

Acknowledgement

I would like to express my special thanks of gratitude to Mrs. Punsisi Premaratne who gave me this opportunity to do this project in the networking field. Secondly i would like to thank my parents and friends who helped me a lot in finalizing this project within the limited time frame.

Table of Contents

Abstract 01
Acknowledgement 02
Table of Content 03
List of Figures 04

1.0 Introduction to ISO OSI Reference Model 05
1.1 Physical Layer 07
1.2 Data Link Layer 08
1.3 Network Layer 10
1.4 Transport Layer 11
1.5 Session Layer 12
1.6 Presentation Layer 13
1.7 Application Layer 14

2.0 Device Used in LANs & WANs and their Functionalities 16
2.1 Hubs 16
2.2 Switches 17
2.3 Bridge 18
2.4 Repeaters 19
2.5 Router 20
2.6 Network Interface Card (NIC) 21
2.7 Gateway 22
2.8 Firewall 23
2.9 CSU/DSU 23
2.10 ISDN Adapter 24
2.11 Modems 25

References 26

List of Figures

Figure 1.0: Layers of OSI Reference Model 05
Figure 2.0: Detailed OSI RM model 06
Figure 3.0: Physical Layer 07
Figure 4.0: Transmission Modes 07
Figure 5.0: Networking Topologies 08
Figure 6.0: Physical Layer 08
Figure 7.0: Two Layers of Data Link Layer 09
Figure 8.0: Network Layer Addressing 10
Figure 9.0: Data Segmentation 11
Figure 10.0: Session Layer Interaction 12
Figure 11.0: Presentation Layer interaction 13
Figure 12.0: Application Layer Interaction 15
Figure 13.0: Devices Connected to a Hub 16
Figure 14.0: Physical View of a Switch 17
Figure 15.0: Switch connected Network 17
Figure 16.0: Example of Using a Bridge 18
Figure 17.0: Using of Repeaters 19
Figure 18.0: Usage of Router 20
Figure 19.0: Physical View of NIC 21
Figure 20.0: Network with a Gateway Device 22
Figure 21.0: Basic Idea of a Firewall 23
Figure 22.0: Network with CSU/DSU 24
Figure 23.0: Physical View of an ISDN 24
Figure 24.0: Network with an ISDN Adapter 24
Figure 25.0: System with a Modem 25

1.0 Introduction to ISO OSI Reference Model

When computer networking technology first appeared in the world many years ago, for a network it was compulsory to use devices from the same vendor since equipment from different vendors had different communication rules. To overcome this issue International Standard Organization (ISO) developed a reference model called OSI RM. (Open System Interconnection Reference Model). The International Organization for Standardization (ISO) is an International standards organization responsible for a wide range of standards, including many that are relevant to networking. In 1984 in order to support network interconnection without necessarily requiring complete redesign, the OSI reference model was approved as an international standard for communications architecture. It allowed different network device manufacturers to create different protocols using this model.

Protocol is basically a set of rules and regulations between two computers for the communication. It acts as a language in data communication. There are several different protocols made based on OSI RM model. Although they are different since they have followed the model they can communicate without a problem.

The OSI model defines how information or data makes its way from application programs (such as spreadsheets) through a network medium (such as wire) to another application located on another network. The OSI reference model splits up the problem of moving information between computers over a network medium into seven smaller and more manageable problems and this is known as Layering.

The OSI reference model is composed of seven layers as shown in the figure

Figure 1.0: Layers of OSI Reference Model

Figure 1.0: Layers of OSI Reference Model

The OSI Reference Model divided into seven layers, each specifying particular network functions to reduce the complexity. Each layer is responsible for some king of processing and each layer do the communication with their immediate layers only. Each layer communicates with the same layer’s software or hardware on other computers. This communication is governed by an agreed-upon series of rules and conventions called protocols.
The lower 4 layers (transport, network, data link and physical —Layers 4, 3, 2, and 1) are concerned with the flow of data from end to end through the network while the upper three layers of the OSI model (application, presentation and session—Layers 7, 6 and 5) are orientated more toward services to the applications.
Following figure represents a detailed OSI RM model.

Figure 2.0: Detailed OSI RM model
Figure 2.0: Detailed OSI RM model

All the layers of the ISO OSI reference model are explained in the next section of this document.

1.1 Physical Layer
Physical Layer is the first and lowest layer of the OSI Model. It defines the physical and electrical characteristics of the network. Physical layer can be identified as the interface between network medium and devices. The physical layer is responsible for establishing, maintaining and ending physical connections (point to point) between computers. The physical layer specifies the electrical, mechanical, procedural, and functional requirements for activating, maintaining, and deactivating a physical link between the data terminal equipment (DTE) and the data communication equipment (DCE).

Figure 3.0: Physical Layer
Figure 3.0: Physical Layer

This layer defines optical, electric and mechanical characteristics such as voltage levels, timing of voltage changes, physical data rates, transmission distances and physical connections

This layer defines the duration of a bit, which is how long it lasts. Data bits to be transmitted, must be encoded in to signals. This layer gets the frames sent by the data link layer and converts them into signals compatible with the transmission media.

• If metallic cables (copper or coaxial) is used then physical layer convert data into electrical signals.
• If fiber optics cables is used then physical layer convert data into light signals.
• If use wireless signals then physical layer convert data into electromagnetic signals.

The physical layer also defines the direction of transmission between two devices: simplex, half-duplex, or full-duplex. Only one device can send in simplex mode and the other can only receive. It’s a one way communication. In half-duplex mode, two devices can send and receive, but one at the same time. In full-duplex, two devices can send and receive at the same time.

Figure 4.0: Transmission Modes
Figure 4.0: Transmission Modes

Furthermore the physical layer concern with the defining of the topology of the network. Network topology represents how the devices within the network are interconnected with each other. The most commonly used topologies are ring topology, star topology, bus topology star topology or hybrid topology.

Figure 5.0: Networking Topologies
Figure 5.0: Networking Topologies

1.2 Data Link Layer

The second layer of the OSI model is the data link layer which is located in between physical layer and the network layer. In this layer, the data packets sent by the network layer are converted in to frames. A frame consists of collection of bits with start bit and the stop delimiters. Frame is a uniform way of sending the data along with the address information and error checking.

Figure 6.0: Physical Layer
Figure 6.0: Physical Layer

If these frames are to be distributed to different systems on the network, data link layer adds header to define the sender and/or receiver of the frame and if the frame is send for a system outside the sender’s network, the receiver address is the address of the device that connects the network to the next one. Cyclic Redundancy Check (CRC), which is a common technique used for detecting data transmission error is used for this purpose. If the CRC fails at the receivers end, it will request the sender to retransmit the packet.

Figure 7.0: Two Layers of Data Link Layer
Figure 7.0: Two Layers of Data Link Layer

The data link layer is divided into two sub layers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sub layer controls how a computer on the network gains access to the data and permission to transmit it. The MAC sub layer keeps MAC addresses for communicating with other devices on the network .The LLC layer controls frame synchronization, flow control and error checking.

According to Microsoft support documents (2013) summary of main functions provides by this layer are

* Link establishment and termination * Frame traffic control * Frame sequencing * Frame acknowledgment * Frame delimiting * Frame error checking * Media access management

Common networking components that function at layer 2 include:

• Network interface cards
• Ethernet and Token Ring switches
• Bridges

Protocols used in this layer are IEEE 802.2, 802.3, 802.5.

IEEE 802.2 - Divides the data link layer into two sub layers
802.3 - Describes the MAC layer for bus networks that use CSMA/CD.
802.5 - Describes the MAC layer for token-ring networks.

1.3 Network Layer

Network layer provides means for communicating open systems to establish, maintain and terminate network connection. Basically this layer is responsible for all the addressing issues. The IP protocol lives in this layer and all the routing devices in the network works in this layer. This layer helps the data packets to travel to their destination.

Figure 8.0: Network Layer Addressing
Figure 8.0: Network Layer Addressing

Routers are special devices which computers used to build the network. When the router sends the packet generated by the network layer, it uses the data stores in the routing table. In the routing tables all the accessible addresses in the network are stored and the router selects the best route or path to send minimizing the data traffic.

There are three ways in which the packets are routed to their destinations.

1. There could be a static route through the entire network that will never be changed. 2. There could be a static line only used during a particular session between the sender and receiver. 3. The packets could be dynamically sent through the network using changing paths in order to prevent bottlenecks.

Network layer is responsible for logical addressing. It is used to identify the computer on the network as well as the network that the system is located on. It is used by the network layer protocols to deliver the packets to the correct destination. The logical addressing used in the network layer is also commonly known as IP addressing. Logical and physical address mapping is also done in the network layer. It means the translation of logical address, or names into the physical address.

Frame fragmentation is also done in this layer. If it identify that a downstream router’s maximum transmission unit size (MTU) is less than the frame size, a router can fragment a frame for transmission and assemble the fragmented frames again at the destination.

IP, ICMP, ARP, PING, Traceroute are the protocols that work in the network layer of OSI model.

IP (Internet Protocol) - format of packets and the addressing schemes are specified by this protocol.
ICMP (Internet Control Message Protocol) - an extension of IP which supports packets containing error control and informational messages.
ARP (Address Resolution Protocol) - used to convert an IP address to a physical address.
Trace route - utility that tracks a packet from your computer to an internet host showing how many hops and how long it took.

1.4 Transport Layer

TCP lives in this Transport layer. The transport layer regulates information flow to ensure end to end connectivity between host applications reliably and accurately. On the senders end transport layer is responsible for breaking the data into smaller packets and at the receivers end all the packets are reconstructed to get the original data.

Figure 9.0: Data Segmentation
Figure 9.0: Data Segmentation

Not only data segmentation this layer also provides flow control and error handling to ensure complete data transfer. It also checks whether the segments are in correct order and not in duplicated form. If there is any error in the data transmission this layer request for a data retransmission.

When a message is broken into segments and sent over the network, transport layer header information include the control information such as start message and message end flags o identify the message boundaries at the other end.

Some of the main functionalities done by the transport layer are TCP segment sequencing, service point addressing, segmentation and reassembling, connection control and error control and flow control.
TCP segment sequencing is a connection where TCP protocol connects the sender and the receiver of the data transmission using a socket which is determined by IP address and port number. UDP on the other hand is a connectionless communication which does not guarantee the packet delivery between sender and receiver. In simply the transport layer can transmit data connectionless or connection oriented method. In connectionless methods each data packet is considered independent and sends into the target address. But in connection oriented method initially a connection is established and then the packets are transferred. After the transmission of packets is complete the connection is terminated.

Another service provided by the transport layer is that it provides facilities to specify the service address for the service or application on the source and the destination computer. It will help to identify the application which the request is coming from and to which application the request is sent to. TCP and UDP are some protocols used in this layer.

TCP (Transmission Control Protocol) - enables two hosts to establish a connection and exchange streams of data.
UDP (User Datagram Protocol) - offering a direct way to send and receive datagrams over an IP network with very few error recovery services unlike TCP

1.5 Session Layer

The session layer is responsible for establishing, managing and terminating connections between applications at each end of the communication link. The session layer allows the applications to start a communication session over the network. When two computers need to communicate over a network first they establish a session in between them. When a session is established between two devices, a set of rules are proposed and both parties must agree to these set of rules to successfully establish the session. The communication session is terminated when the session is complete and communication ends gracefully. This layer also provides dialog control between devices and the data synchronization facilities.

Figure 10.0: Session Layer Interaction
Figure 10.0: Session Layer Interaction

Dialog control management is a service provided by the session layer. It allows the two devices to have a dialog between each other. It allows the communication process to be take place either in half duplex (one direction at a time) or in full duplex (both directions at the same time). The session layer determines who has the ability to transfer at the current time.

The other valuable service is the data synchronization, which means the ability to insert checkpoints during the data transfers which will help to resume the data transmission if an error occurs in the communication network. This is very much useful when transferring larger files in size.

SQL and RPC are the protocols used in this layer.

SQL (Structured Query Language) - A standardized query language for requesting information from a database.
RPC (Remote Procedure Call) - allows a program on one computer execute a program on a server.

1.6 Presentation Layer

The presentation layer is located just below the application layer in the OSI model in networking. The presentation level is the translator between the application and network layers. Unlike lower layers, its concern with the syntax and semantics of the information transmitted where syntax is defined as how data is represented while semantics id defined as what data means. Presentation layer translates the data into a standard format which is used in the network, this layer is also known as syntax layer.

Figure 11.0: Presentation Layer interaction
Figure 11.0: Presentation Layer interaction

When presentation layer receives data from application layer it checks whether it’s in the common format if not it translates it into the common format. At the other end the presentation layer convert the common format into the application understandable format and send it to the application layer. While sending security is also added in this layer using encryption and decryption.

Following are some of the main formatting functions done at this layer.

Data Compression: This is used to reduce the number of bits that need to be sending over the network.

Data Encryption: Encryption of data is done to increase the security of data during transmission. Encryption means that the data that needed to be send over the network is converted into another for and send over the network. It prevents others from intercepting data and being able to get the real meaning of the bits.

Data Decryption: Reversing of the data encrypted in to original format is known as decryption.

Data Translation: The process of converting data into bits is called translation. Data should be converted because different computers may have different formats.
Ex: ASCII to EBCDIC

Protocols such as ASCII, EBCDIC, MIDI, MPEG, and JPEG are used in this layer.

ASCII (American Standard Code for Information Interchange) - A code for representing English characters as numbers.
EBCDIC (Extended Binary-Coded Decimal Interchange Code) - Code used by IBM to represent characters as numbers.
MIDI (Musical Instrument Device Interface) - Used by the electronic music industry for controlling devices that emit music.
MPEG (Moving Pictures Experts Group) - The family of digital video compression standards and file formats developed by the ISO group.
JPEG (Joint Photographic Experts Group) - A format for color images

1.7 Application Layer
Application layer is the closest layer to the users of the network. it provides the interface between the program that is sending or receiving data and the protocol stack. Application layer is the one seen by the user and applications are loading to this layer. In other words application layer provides network services to the user applications outside the OSI model such as spreadsheet programs, word processing programs, browsers and bank terminal programs. Application layer provides data to and obtains data from presentation layer.

Figure 12.0: Application Layer Interaction

Figure 12.0: Application Layer Interaction

Application layer is responsible for identifying the communication partners, quality of service, authentication of users, privacy and any constraints of data syntax. Microsoft support documents states that common functions of application layer are * Resource sharing and device redirection * Remote file access * Remote printer access * Inter-process communication * Network management * Directory services * Electronic messaging (such as mail) * Network virtual terminals

Several protocols are used in this layer to provide various functions. Some of the commonly used protocols are HTTP, FTP, SMTP, DNS, TFTP, NFS and TELNET.

HTTP (Hypertext Transfer Protocol)-This protocol is used by web servers and browsers to communicate with each other.
FTP (File Transfer Protocol) - Used to transfer files over the internet using TCP/IP
SMTP (Simple Mail Transfer Protocol) - Used for the purpose of sending email messages between servers.
DNS (Domain Name Service) - This is a service which converts domain names into IP addresses.
TFTP (Trivial File Transfer Protocol) - A simplified version of the FTP protocol which some security features of FTP is absent.
NFS (Network File System)-Protocol developed by Sun Microsystems which allows a user on a client computer to access files over a network in a similar how local storage is accessed.

2.0 Device Used in LANs & WANs and their Functionalities

To create a good computer network it is essential to use different networking devices such as router, hub, switch, gateway and etc. in the following some of the important devices used in creating networks are listed and briefly explained their functions.

2.1 Hubs

Hub is a network device which is used to connect various devices within the network and connect small networks in order to share data and resources among each other. Hubs operate at the Physical Layer (Layer 1) of the Open Systems Interconnect (OSI) Reference Model.
Hubs have number of ports such as 6, 8, 12, 16 and 24 etc. A hub with 6 ports is known as a work group and a hub with 12 or more ports is known as a destiny hub. Hub is not an intelligent device. When a packet arrives it is sent to all the other ports. Another fact is that hub does not know to set the minimum traffic like routers when transmitting the data.

Figure 13.0: Devices Connected to a Hub
Figure 13.0: Devices Connected to a Hub

Hubs can be categorized into Active hubs and Passive hubs. Active hubs take energy from a power supply to regenerate network signals and send them again. It functions like a repeater. Active hubs are also identified as multi-port repeaters. This amplification process helps to ensure the reliability of the transmitting of the data. On the other hand passive hubs do not regenerate bits; they only allow two or more hosts to connect to the same cable segment.

Another classification of hubs is intelligent and dumb hubs. Intelligent hubs have console ports that mean they can be programmed to manage network traffic. Dumb hubs merely take an incoming networking signal and repeat it to every port without the ability to do any management.

Advantages

* Inexpensive * Easy to configure

Disadvantages

* Hub broadcast all the information to every node connected. * Bandwidth is shared by all of the PC's connected to the hub * Hub create loops

2.2 Switches Switch is a network device similar to hub which helps to connect networks together but it is more intelligent than a hub. Switch works in the layer 2 of OSI reference model. There are different switches available with different number of ports such as 8, 26, 24, 32 and etc. A switch consist of 32 of ports can be defined as high density switch. Switch gets the signals from the devices which has been connected to it then regenerate them and send to the destination the new copy of the generated signal.

Figure 15.0: Switch connected Network
Figure 15.0: Switch connected Network

Figure 14.0: Physical View of a Switch
Figure 14.0: Physical View of a Switch

The difference between the hub and switch is that switches can make decisions based on MAC addresses and hubs don't make decisions at all. Because of the results that switches make, they make a LAN much more efficient. When a data is transfers through a switch it can identify the intended device ad use that data into that device only unlike hubs.

Another reason for referring switch as an intelligent device is as it is capable of the transfer data with the minimum traffic. The switch can transfer data without occurring collision. The transmission of the data in a switch is done in half duplex way. Data is not be transmitted both sides at the same time. This half duplex can be developed to get the better performance by the process called full duplex. In the full duplex type, it will be used more switches to transmit data among the devices.

Advantages

* Passes any transmission only to the correct port unlike hubs. * Reduces collisions on the network and minimize the traffic. * Reduces the number of Broadcast domains. * CAM table for Port to MAC mapping
Disadvantages

* Handling Multicast packets needs quite a bit of configuration & proper designing. * Not as good as a router in limiting Broadcasting. * Relatively with hubs and bridges more expensive.

2.3 Bridge

Bridge is a layer 2 device used to connect two LAN network segments. The sections of a main network are known as subnets. Bridge is an intelligent device than hubs and switches. Bridges are capable of identifying the Media Access Control (MAC) address of each device connected to the network. Every networking device has a unique MAC address on the NIC, the bridge keeps track of which MAC addresses are on each side of the bridge and makes its decisions based on this MAC address list.

Figure 16.0: Example of Using a Bridge Device
Figure 16.0: Example of Using a Bridge Device

The larger networks have been divided into small sections when transferring data and they are connected through the bridges. When data transfers bridge set up a network segment between the two required devices and control and maintains the flow of transmission. When data receive to the bridge it will be checked whether the MAC address of the destination is available on the data packet and if the MAC address is available data will be transferred to the particular destination and if the address is not available it will be blocked the transmission of data and prevent.

There are 3 types of bridges namely Transparent bridges, Source-Root bridges and Translational bridges. A transparent bridge does nothing except block or forward data based on the MAC address. Source Route Bridge used in Token Ring networks. The source route bridge derives its name from the fact that the entire path that the packet is to take through the network is embedded within the packet. Translational bridge used to convert one networking data format to another; for example, from Token Ring to Ethernet and vice versa.

Advantages * Inexpensive * Easy to use * Help to lower the data load over the data link layer

Disadvantages

* Cannot build a communication network between the networks of different architectures. * Unable to read specific IP address(routing is not possible) * Unable to handle more complex and variable data load * Extremely large networks cannot rely on bridges

2.4 Repeaters

There are several types of media used in networking and each one of them has their own pros and cons. For example a major problem face in cable media is the maximum length of the cable which the signal can be carried because of the noise signal strength will gradually reduce. For instance maximum length for an Unshielded Twisted Pair (UTP) cable to carry a good signal is 100m. To spread the network to broader than this a device called repeater is used. Repeater is used to regenerate or replicate a signal. Repeaters are classified as Layer one devices in the OSI model, because they performance only on the bit level and look at no additional information.

Figure 17.0: Using of Repeaters
Figure 17.0: Using of Repeaters

Purpose of repeater is to retransmit those weaker electrical, wireless or optical signals in the network. Repeaters remove the unwanted noise also from the incoming signals. A set of repeaters from point to point in the network can be used to spread a network in a wider area.

Repeaters can be divided into two groups’ namely analog repeaters and digital repeaters. Analog repeaters frequently can only amplify the incoming signal while digital repeaters can reconstruct a signal to near its original quality.

Advantages * Simple to connect * Simple to connect * Ability to strengthen signal

Disadvantages

* May not boost signal strength as much as desired. * Compatibility issues.

2.5 Router

Router is a Layer 3 gateway device operates at the network layer of the OSI model. It is a small physical device which is used to connect two or more networks together, such as LAN to LAN, LAN to WAN and LAN to internet. Router is a very intelligent device. Routers forward the data packets along the network. In the data packet forwarding process routers use header and forwarding tables to identify the best route to send the data packet.

Figure 18.0: Usage of Router
Figure 18.0: Usage of Router

The main characteristic of the router is that, it responsible of controlling the flow of the information or the data between source and the destination and send only the desired data. When a packet of data arrives at the router, it checks the header of the data packet for the destination address. If there are no issues in the destination address using the data in the forwarding table it choose the best route to transmit the data packet. And if there is no problems with choosing the path it well send the data to the next hop where hop is a term used to indicate the next point of transmission. It is possible that the hop can be either the destination of the data packet or another router in the transmission path in the network.

Advantages

* Router can be used in both LAN & WAN networks. * Different media & architectures can be connected using routers. * Determine best route for data to reach destination. * Ensure a steady, reliance availability of network connectivity

Disadvantages

* Routers are more expensive than Hub, Bridge & Switch. * One cannot configure a particular port. * Router only work with routable protocol. * Increase latency due to greater degree of packet filtering. 2.6 Network Interface Card (NIC)

NIC is the interface between a computer and a network. Some NIC cards work with wired connections while others are wireless. In new computers NIC is attached on your mother board while some times it is connected through an expansion slot. The network interface connects you to the network via a small receptacle called a port. For wired networks, you attach the network cable into this port. Instead, for wireless networks, the port includes a transmitter/receiver that sends/receives radio signals.

Figure 19.0: Physical View of NIC
Figure 19.0: Physical View of NIC

A major point when comes to NIC are the MAC address of the card which is used to identify the nodes in the network uniquely. The MAC address helps route information within your local area network and is used by interconnecting devices such as switches and bridges

2.7 Gateway

Gateways operate at the network layer of the OSI reference model and it is a device used to connect networks using different types of protocols. It can be also defined as a device that serves as an entrance to another network. A network gateway can be implemented completely using software or completely using hardware, or as a combination of both. The default t gateway is where the IP sends packets that are destined for remote networks. If no default gateway is specified, communication is limited to the local network

Figure 20.0: Network with a Gateway Device
Figure 20.0: Network with a Gateway Device

Advantages

* Can translate information between different networks data formats * Can also be programmed to grant or deny certain users privileges * allow for user authentication * Adds flexibility to your network

Disadvantages

* Gateways require greater processing limits due to protocol conversion * Slower than the other network devices * Installation and configuration of Gateway is more difficult, therefore requires special technicians. * It is more expensive than other devices
2.8 Firewall

In this rapidly developing technology environment firewall has become an essential device for the security of a network. . Firewalls can be either software based or a hardware based. Firewall is responsible for security controlling, controlling the data transmission and access control. Firewall has been programed with a set of rules or regulation according to the organization. In the transmission of data through the firewall, it will check whether any of the rules implemented in the firewall are violated by the packet before allowing it to enter. Example for a hardware oriented fire wall is sonic wall.

Figure 21.0: Basic Idea of a Firewall
Figure 21.0: Basic Idea of a Firewall

Advantages

* Protect the computer against internet viruses and malware. * Can be configured to provide a great deal of security * Can protect multiple systems simultaneously

Disadvantages

* Packet filtering by a software firewall can degrade your system's performance * Can lend users a false sense of security * Hardware firewalls are expensive * Difficult to install and upgrade

2.9 CSU/DSU

A CSU/DSU is a device that combines the functionality of a channel service unit (CSU) and a data service unit (DSU). These devices are used to connect a LAN to a WAN, and they take care of all the translation essential to convert a data stream between these two methods of communication. The channel service unit (CSU) receives and transmits signals to and from the WAN line and provides a barrier to electrical interference from either side of the apparatus. The CSU can also echo loopback signals from the phone company for testing. . The DSU provides a modem-like interface between the computer as Data Terminal Equipment (DTE) and the CSU.

Figure 22.0: Network with CSU/DSU
Figure 22.0: Network with CSU/DSU

Advantages

* CSU/DSU’s allow computers from different remote locations to communicate with each other via data communications lines. * Easier, faster network troubleshooting

Disadvantages

* High cost involvement.

2.10 ISDN Adapter

ISDN (Integrated Services Digital Network) is a data communication method used over the regular telephone lines. Advantages of ISDN connections over regular dial up connections is that they provide a higher bandwidth and ISDN modem can use for both voice and data at the same time. For the use of ISDN connection installation of some adapters known as ISDN terminal adapters are required. ISDN Terminal Adapter works similar as the digital modem and it converts the signals from digital to analog. ISDN Terminal adapter is plugged into the serial port of the system. Some ISDN adapters have the capability of switching between digital and analog modes. There are 2 types of ISDN modems, such as Symmetric Digital Subscriber Line (SDSL) and Asymmetric Digital Subscriber Line (ADSL).

Figure 24.0: Network with an ISDN Adapter
Figure 24.0: Network with an ISDN Adapter
Figure 23.0: Physical View of an ISDN Adapter
Figure 23.0: Physical View of an ISDN Adapter

2.11 Modems

A modem is communication device which used on dialup networks converts the digital data into analog (modulation) and analog into digital (demodulation). Faster types of the modems are used by the internet such as DSL modem, cable modem and optical modems. Modems can be connected externally (ex: ADSL Modems), connected to the computers serial port by an RS-232 cable or internal in one of the computers expansion slots. Modems are connected to the phone line by standard telephone RJ-11 connectors.

Figure 25.0: System with a Modem
Figure 25.0: System with a Modem

References

* Microsoft , The OSI Model's Seven Layers Defined and Functions Explained,2013
Available from: http://support.microsoft.com/kb/103884 * School of Information Sciences, 7 Layers of OSI, 2013, University of Pittsburgh
Available from: http://www.sis.pitt.edu/~icucart/networking_basics/7layersofOSI.htm

* University of Princeton, Network File System (protocol),2013, University of Princeton Available from: http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Network_File_System_(protocol).html

* Cisco, Introduction to LAN protocols, 2012,IOS Technology Handbook Available from: http://docwiki.cisco.com/wiki/Introduction_to_LAN_Protocols

* Orbit-Computer Solutions, WAN Devices, Orbit-Computer Solutions.com,2013
Available from: http://www.orbit-computer-solutions.com/WAN-Devices.php

* Behrouz A., Data Communication and Networking, 4th edition, McGraw Hill Higher Education * Stallings W., Data and Computer Communications. 7th edition, 2003, Prentice~Hall of India Pvt Limited.