A network is an interconnection of two or more devices in order to enable transfer of data or information from one place to another.
Advantages
1. Sharing of hardware resources 2. Sharing of software resources 3. Central storage and data security 4. Easier and faster sharing of information Disadvantages
1. Costly hardware and software
2. Need for an administrator to take care of the network Types of Computer Networks
1. Local Area Network (LAN)
A network consisting of two or more computers that are interconnected by means of cable in a single location is called as Local Area Network. In a LAN, the computers can take any one of the three functions as detailed below.
Peer-to-peer : This is an example of a simple network where two or more computers are directly connected to each other and share resources. There is no central control over the network. Peer networks are organized into workgroups. Access to individual resources has to be controlled through a password. There is no restriction on the number of computers on a peer network.
Server Based Network : In this type of networks, there is a main computer called as the SERVER that controls the networks and provides central storage space for information.
The other computers that are connected to the server are called as CLIENTS, or WORKSTATIONS, or DUMB TERMINALS.
Hybrid Network : These types of network are a combination of both peer-to-peer network and server network.
2. Metropolitan Area Network (MAN)
A network of computers that are interconnected within a specific geographical area like a university campus or a city is called as Metropolitan Area Network or Campus Area Network (CAN)
3. Wide Area Network (WAN)
A network of computers that are interconnected over a large area is called as a Wide Area Network. The computers are connected by means of cables, telephone lines, satellites or radio transceivers.
Components of a Computer Network 1. Server
2. Workstations
3. Network Interface Card (NIC) 4. Network media
5. Connector
6. Connecting devices Types of cables Coaxial cable
Coaxial cable has a single strand or multi-strand of copper. The wire is enclosed in plastic foam for insulation. The foam is surrounded by a second conductor, a wire mesh or metallic foil. The wire mesh protects the central conductor from electro magnetic interference. It also provides proper grounding for the central conductor. Coaxial cable transmit data typically at 10 Mbps. Data is transmitted in the form of electric current and are comparatively slower than fiber-optic cables.
Twisted-pair cable
These cables have one or more pairs of copper wires that are twisted. The twisting reduces external interference. There are two types of twisted pair cables. 1. Shielded Twisted Pair (STP) and 2. Unshielded Twisted Pair (UTP). STP cables are shielded
with aluminium or polyester present in between the outer jacket and the wires. UTP cables do not have this shielding.
Fiber-optic cable
These cables are mainly used as backbone in a WAN. Data is transmitted in the form of light signals. The inner core is enclosed in a layer of glass called as a cladding, that reflects light back into the core. A plastic sheath surrounds each fiber. Fiber-optic cables are capable of transmitting data at a very high speed offering data transfer speeds ranging from 100 Mbps to 2 Gbps and are more expensive.
Plenum cable
In coaxial cables, PVC is used as the outer jacket for insulation. When the cables have to be run in the space between the false ceiling and the floor above, a special type of cable called as plenum-grade cable has to be used. When burned, PVC gives off poisonous gases, so a fire resistant cable such as the plenum-grade cable is used. It is more expensive and less flexible than PVC insulated coaxial cables.
Network Connectivity Devices
Any network grows over a period. This growth creates the need for attaching more computers in the network. Expansion of a network is of two types, they are
a) Expansion within a network b) Joining two networks
More computers can be attached in a network by using additional cables and connectors. But once the network architecture is stretched beyond a point, it reduces the data transmission speeds. In such a situation, there arises a need for special devices, which can amplify the signals and send them over longer distances. The following devices can be used to expand a single network without connecting it to other networks.
a) Hubs (Active, Passive and Intelligent) b) Repeaters
c) Switches d) Bridges e) Multiplexers a) Hubs
These are the connecting points in a network where UTP and STP cables are used. Hubs amplify signals and split them so that the signals reach their destination. Hubs can be of three types based on their function. Hubs are available in configuration of 4/8/16/32 ports for connecting 4/8/16/32 computers respectively. To add more computers to the network cascading of hubs can be done. Cascading is nothing but taking the signal from
one hub to another hub. This is allowed only for Active hubs.
Active hubs : These hubs provide connection points for cables to be attached from each computer in the network. The hub in turn is connected to the main computer called as the Server. Active hubs are mainly used in Star Topology.
Passive hubs : These hubs merely act as junction boxes for extending the network. They cannot amplify or split signals.
Intelligent hubs : These hubs have special instructions stored in the ROM that help in signal regeneration and path selection. An intelligent hub has the ability to choose the path where the signal has to reach instead of sending the signals along all paths. They can also choose the quickest path for sending the signal.
b) Repeaters
The signals that pass in the network media (cables) are prone to weakening called as attenuation. This results in loss of signals. In order to prevent this, a repeater is used to boost the signal allowing it to travel over longer cable distances.
c) Switches
Switches operate at the data link layer and are responsible for receiving and transmitting frames. Switches use MAC (Message Authentication Code) address from the host’s NIC cards to filter the network. Switches work in full-duplex mode (100 Mbps) where the signals can be transmitted and received between the switch and the computer simultaneously. Switches are available in configuration of 8/16/24/32 ports for connecting computers.
d) Bridges
Bridges are used to cross from one circuit, channel or element over to another. It is a device that connects two LAN segments together, which may be of similar or dissimilar types, such as Ethernet and Token Ring. A bridge is inserted into a network to segment it and keep traffic contained within the segments to improve performance. Bridges learn from experience and build and maintain address tables of the nodes on the network. By monitoring which station acknowledged receipt of the address, they learn which nodes belong to the segment.
Bridges are protocol independent; routers are protocol dependent. Bridges are faster than routers because they do not have to read the protocol to glean routing information. Bridges with more than two ports (multiport bridges) perform a switching function. Today's LAN switches are really multiport bridges that can switch at full wire speed.
e) Multiplexers
These devices use a technique called as multiplexing where many different
signals are combined and transmitted and at the receiving end, they are demultiplexed to reach the appropriate destination. The transmission signals of cable TV is an example of multiplexing where multiple channels are send in a single coaxial cable and the demultiplexer present in the TV recognizes the appropriate signal.
Inter-network Connectivity
Any network consisting of two or more networks is called as an inter-network. The Internet is an example of this. An inter-network may have different types of networks
like Ethernet, Token ring, and ATM. The following devices can be used to expand an inter-network by connecting it to other network.
a) Routers
A device that forwards data packets from one local area network (LAN) or wide area network (WAN) to another. Based on routing tables and routing protocols, routers read the network address in each transmitted frame and make a decision on how to send it based on the most expedient route (traffic load, line costs, speed, bad lines, etc.). Routers work at layer 3 in the protocol stack, whereas bridges and switches work at the layer 2.
Routers are used to segment LANs in order to balance traffic within workgroups and to filter traffic for security purposes and policy management. Routers are also used at the edge of the network to connect remote offices.
b) Brouters
c) Gateways
These are sophisticated devices combining both hardware and software. Gateway helps in connecting disparate networks. They take care of protocol conversion so that data received from any network can be interpreted in your network and used. d) CSU/DSU
Channel Service Unit/Digital Service Unit help in using the existing telephone line for transmitting signals. Apart from sending signals CSU/DSU also shield your network from voltage currents that come through the telephone network.
Network Topology
The term topology, or more specifically, network topology, refers to the
arrangements or physical layout of computers, cables, and other components on the network. The three popular topologies are bus, star and ring. Each topology has its strengths and weakness.
These are the three major topologies used in networks. Ethernet uses bus, hub and switch topologies. Token Ring uses ring and switch.
Patch Chord
Patch Chord is just an UTP cable with RJ-45 connectors connected at both the ends. The patch chord may be straight cable or cross over cable depending upon the application where it is used.
RJ 45 Jack
RJ45 Colors and Wiring Standards
5 T-568B Straight-Through Ethernet Cable
Both the T-568A and the T-568B standard Straight-Through cables are used most often as patch cords for your Ethernet connections. If you require a cable to connect two Ethernet devices directly together without a hub or when you connect two hubs together, you will need to use a Crossover cable instead.
6 RJ-45 Crossover Ethernet Cable
How to Build an Ethernet Cable Instructions:
1. Pull the cable off the reel to the desired length and cut using wire cutters or scissors. If you are pulling cables through holes, it's easier to attach the RJ-45 plugs after the cable is pulled. The total length of wire segments between a PC and a switch or between two PC's cannot exceed 100 Meters (328 feet) for 100BASE-TX and 300 Meters for 10BASE-T.
2. Start on one end and strip the cable jacket off (about 1") using a wire stripper or a knife. Be extra careful not to nick the wires, otherwise you will need to start over. 3. Spread, untwist the pairs, and arrange the wires in the order of the desired cable end. Flatten the end between your thumb and forefinger. Trim the ends of the wires so they are even with one another, leaving only 1/2" in wire length. If it is longer than 1/2" it will be out-of-spec and susceptible to crosstalk. Flatten and insure there are no spaces between wires.
4. Hold the RJ-45 plug with the clip facing down or away from you. Push the wires firmly into the plug. Inspect each wire is flat even at the front of the plug. Check the order of the wires. Double check again. Check that the jacket is fitted right against the stop of the plug. Carefully hold the wire and firmly crimp the RJ-45 with the crimp tool.
5. Check the color orientation, check that the crimped connection is not about to come apart, and check to see if the wires are flat against the front of the plug. If even one of these are incorrect, you will have to start over. Test the Ethernet cable.
Ethernet Cable Tips:
• A straight-thru cable has identical ends. • A crossover cable has different ends.
• A straight-thru is used as a patch cord in Ethernet connections.
• A crossover is used to connect two Ethernet devices without a hub or for connecting two hubs.
• A crossover has one end with the Orange set of wires switched with the Green set.
• Odd numbered pins are always striped, even numbered pins are always solid colored.
• Looking at the RJ-45 with the clip facing away from you, Brown is always on the right, and pin 1 is on the left.
• No more than 1/2" of the Ethernet cable should be untwisted otherwise it will be susceptible to crosstalk.
• Do not deform, do not bend, do not stretch, do not staple, do not run parallel with power cables, and do not run Ethernet cables near noise inducing components. TCP/IP
TCP/IP stands for Transmission Control Protocol/Internet Protocol. It was
developed for the US Department of Defense to allow communication between different types of computer and networks. Now it is a widely used networking protocol. TCP and IP are the two best known protocols in the suite. The Transmission Control Protocol (TCP) provides reliable sequence delivery of packets between clients. The Internet Protocol (IP) provides packet delivery between hosts. IP Addressing
In a TCP/IP environment communication between hosts and servers on the internetwork is transparent. This is because each node using the TCP/IP protocol suite has at least one unique 32 bit Internet Protocol (IP) address.
A router “reads” the destination address in an IP packet and makes a routing decision based on this address. All hosts on an inter-network require unique addresses. IP address can be represented as a group of four decimal number (in the range 0 to 255), separated by decimal points. This is known as dotted decimal notation.
They can also be represented in binary or hex. Each IP address identifies a hosts within the inter-network. Part of an IP address identifies a particular network, part identifies a subnet and part identifies a specific host within that sub-network.
An IP address can be represented in dotted decimal notation or binary or hexadecimal notation. For example :
Dotted decimal 130.57.30.56
Binary 10000010.00111001.00011110.00111000 Hexadecimal 82.39.IE.38
Logically an IP address contains two parts. They are 1. Network ID
2. Host ID
In order to provide a method for assigning addresses in simple and easy manner, the IP address is classified for different types of networks like large, medium and small networks.
IP Address Classes
Depending on your networking requirements, you are assigned (by your service provider) specific classes of IP address. They are five classes of IP addresses: A, B, C, D and E.
Class A: Address range from 1 to 126. These addresses use the first byte to specify the network and the last three bytes to specify the host. Class A addresses are assigned to system in a small number of networks and a large number of hosts.
Class B: Address range from 128 to 191. These addresses use the first two bytes to specify the network and next two bytes to specify the host. Class B addresses are assigned to systems in universities and commercial organizations.
Class C: Address range from 192 to 223. These addresses use the first three bytes to specify the network and the last byte to specify the host. Class C addresses are assigned to systems in a network with small number of hosts.
Class D: Address range from 224 to 239. These addresses are used for broadcasting messages over an inter-network. These addresses are also used for multicasting.
Class E: Address range from 240 to 255. These addresses are reserved for research purposes and future use.
Subnet Mask
Subnet is a physical segment in a TCP/IP environment that uses IP addresses derived from a single network ID.
Subnetting: One part is used to identify the segment as a unique network other part used to identify the hosts. This is known as subnetting.
Example Class room Students in the class Here Class room - Subnet
Students - Network ID Students name - Host ID
Subnet mask is a 32b bit address and is used to (1) block out a portion of the IP address to distinguish the network ID from the host ID (2) specify whether the destination host IP address is located on a local network or remote network.
Each host on a TCP/IP network requires a subnet mask number
Default subnet masks – used when a network is not divided into subnets Custom subnet masks – used when a network is divided into subnets
Default gateway
IP routing is the process of sending data from a host on one network to a remote host on another network through a router or routers. In IP terminology routers are referred to as gateways. The router uses a path to deliver packets, which is defined in its routing table. Routing table contains the IP address of router interfaces that connect one router with other.
Important Points to Remember: Bridge
A combination of hardware and software that recognizes the messages on a network and passes on those addressed to nodes in other networks.
Browser
Software on the user’s computer that allows the user to access the Internet via the service provider, using a graphical interface.
Data communications systems
Computer systems that transmit data over communications lines such as telephone lines or cables. Download files Retrieve from another computer and store them.
E-mail The ability to send messages directly from one computer to another.
File Transfer Protocol
(FTP) A standard way to transfer copies of files on the Internet. Host computer The mainframe computer in a WAN.
Hub A device that repeats signals and connects a group of computers to a network.
Internet A rapidly growing web of networks from around the world -- simply, a network of networks.
Internet Service Provider (ISP)
Provides the server computer and the software required for you to connect to the Internet.
Listserv Widely used automatic mailing manager.
Local Area Network (LAN) A network of computers that share hardware, software, and data in small geographic area.
Modem A device that converts a digital signal to an analog signal and vice versa.
Newsgroups An informal network of computers that allows the posting and reading of messages in newsgroups that focuses on specific topics.
Plug-ins Software that enhances the value of a browser by increasing its features.
Router A special computer that directs communications traffic when several networks are connected together.
Uniform Resource
Locator (URL)
A unique address on the WWW.
Web pages Documents that contain text, graphics, sound, and/or video and have built-in connections called hyperlinks. Web site A related collection of Web pages.
Wide Area Network (WAN)
A network of geographically distant computers and terminals.
World Wide Web (WWW) The largest and most popular part of the Internet; it is the graphical part of the Internet.
Exercise 3:
Complete each of the statements below by filling in the blanks with terms found in the Computer Network Concepts module.
1. Computer systems that transmit data over communications lines such as telephone lines or cables are called _____________________________. 2. A ___________________________ is a device that converts a digital signal
to an analog signal and vice versa. It is short for modulate/demodulate.
3. ____________________is a technology that is capable of moving data at 128,000 bps over any modem.
4. A network of geographically distant computers and terminals is a