Unit2.2
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Computer Networks
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basics of computers
Transcript of Unit2.2
Computer Networks
A computer network is defined as the interconnection of 2 or more independent computers or/and peripherals.
Computer Network
Network
Source Destination
Need of Networks
– Communicate and collaborate
– Share information
– Share resources
– Sharing computer files and disk space
– Sharing high-quality printers
– Access to common fax machines
– Access to common modems
– Multiple access to the Internet
1. Local Area Networks (LANs) - a computer network covering a small geographic area, like a home, office, or group of buildings. Typically within 5-mile radius.
2. Metropolitan Area Networks (MANs)- are large computer networks usually spanning a city. (within 30 miles)
3. Wide-Area Networks (WANs) - any network whose communications links cross metropolitan, regional, or national boundaries.
Classification of Networks
The way in which the computers are interconnected together is known as TOPOLOGY.
Network Topology
Types of topologies
• Physical topology• Logical topology
Types of physical topologies
• Bus/Linear • Star • Ring • Tree • Mesh
Linear or bus topology
• Consists of a main cable, known as backbone cable, with a terminator at each end .
• All nodes (file server, workstations, and peripherals) are connected to the cable.
• Ethernet and LocalTalk networks use bus topology.
Advantages of Bus Topology
• Easy to connect a computer or peripheral to a linear bus.
• Requires less cable length.
• Easy to extend.
• If one node of the N/W is faulty, the N/W can still remain working.
• Entire network shuts down if there is a break in the main cable.
• Terminators are required at both ends of the backbone cable.
• Difficult to identify the problem if the entire network shuts down.
• Not meant to be used as a stand-alone solution in a large building.
Disadvantages of Bus Topology
Star topology
• A star topology is designed with each node (file server, workstations, and peripherals) connected directly to a central network hub.
• Data on a star network passes through the hub before continuing to its destination.
• The hub manages and controls all functions of the network.
• It also acts as a repeater for the data flow.
• Easy to install.
• No disruptions to the network other than connecting or removing devices.
• Easy to detect faults and to remove parts.
Advantages of Star Topology
• Requires more cable length than a bus topology.
• If the hub fails, nodes attached are disabled.
• More expensive than bus topology because of the cost of the hub.
Disadvantages of Star Topology
Tree Topology
• A tree topology combines characteristics of bus and star topologies.
• It consists of groups of star-configured workstations connected to a bus backbone cable.
• Tree topologies allow for the expansion of an existing network.
Advantages of a Tree Topology
• Point-to-point wiring for individual segments.
• Supported by several hardware and software venders.
Disadvantages of Tree Topology
• Overall length of each segment is limited by the type of cabling used.
• If the backbone line breaks, the entire segment goes down.
• More difficult to configure than other topologies.
Ring topology
• Is a type of computer network configuration where each network computer and device are connect to each other forming A large circle.
• Data is divided into packets when transmitted.
• Packet is sent around the ring until it reaches its final destination.
Advantages of ring topology
• Requires lesser amount of cable and there are not much of installation problems
• All stations have equal access
Disadvantages of ring topology
• Failure of one computer may impact others• Data transfer is slow
Mesh topology
• It requires that every terminal should be attached to each other.
• All the computers must have adequate number of interfaces for the connections to be made.
• Because of this requirement the installations is somewhat difficult.
• The length of cable required is quite higher as compared to other topologies.
Advantages of mesh topology
• Ease of troubleshooting.
• Data transfer is faster.
Disadvantages of mesh topology
• uses a lot of cabling.
• Complex
• Most expensive topology
Considerations When Choosing a Topology
• Money. A linear bus network may be the least expensive way to install a network; you do not have to purchase concentrators.
• Length of cable needed. The linear bus network uses shorter lengths of cable.
• Future growth. With a star topology, expanding a network is easily done by adding another concentrator.
• Cable type. The most common cable in schools is unshielded twisted pair, which is most often used with star topologies.
Two types of transmission technologies are there:
1. Broadcast NetworkSingle communication channel is shared by all
the machines in the network.
Transmission Technology
2. Point to Point NetworkData is routed directly from source machine to
destination machine directly.
Transmission Technology
Dedicated link
Routing
• When data is to be transmitted between two remote machines using intermediate machines, certain routing techniques have to be applied.
• The intermediate machines may be:1. Gateway2. Hub3. Router4. Repeater5. Bridge
Gateway
• A gateway is a network point that acts as an entrance to another network.
• Acts as a bridge between two networks so that data can be transferred between a number of computers.
• In enterprises, the gateway is the computer that routes the traffic from a workstation to the outside network that is serving the Web pages.
• In homes, the gateway is the ISP that connects the user to the internet.
• For example, when you send an e-mail to a friend or when you log in to a Web site, there is a gateway that allows the connection take place.
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 is
copied to the other ports so that all segments of the LAN can see all packets
Router• A router is a device that forwards data
packets along networks. • A router is connected to at least two
networks.• Routers are located at gateways.• Routers use headers and forwarding tables
to determine the best path for forwarding the packets.
• They use protocols to communicate with each other and configure the best route between any two hosts.
Switch
• In networks, a device that filters and forwards packets between LAN segments.
• LANs that use switches to join segments are called switched LANs.
Repeater
• Repeaters are used in transmission systems to regenerate signals distorted by transmission loss.
• Analog repeaters frequently can only amplify the signal while digital repeaters can reconstruct a signal to near its original quality.
Bridge
• A device that connects two local-area networks (LANs), or two segments of the same LAN that use the same protocol is known as bridge.
More Concepts…
• Network protocols.
• Layering.
• Network/protocol architecture.
Network Protocols
• A communication protocol is a set of rules that specify the format and meaning of messages exchanged between computers across a network.
• A set of related protocols that are designed for compatibility are called protocol suite.
Human and Computer Protocols
Hi
Hi
Got thetime?
2:00<data>
time
Human Protocol Computer Protocol
Web client
Web server
openconnection
OK
send me data
Protocol Architecture
• Task of communication broken up into modules
• For example file transfer could use three modules– File transfer application– Communication service module– Network access module
Layers
• Layers are the different components that need to be designed/implemented when designing/implementing networks.
• Each layer responsible for a set of functions.
• Top layer relies on services provided by bottom layer.
• Layer makes it service available to higher layer through an interface.
Layering
• Building complex systems is hard!– Approach: “Divide and conquer”.– Split job into smaller jobs, or layers.
• Analogy to other fields.– Building a house: digging, foundation,
framing, etc.– Car assembly line…
• Basic idea: each step dependent on the previous step but does not need to be aware of how the previous step was done.
Analogy: Air Travel
• The problem: air travel.• Decomposed into series of steps:
Arrival at airport
Check-in
Boarding
Takeoff
Departure from airport
Baggage claim
Deplane
Landing
Traveling
More on the air travel analogy…
Arrival
Check-in
Boarding
Takeoff
Dep
art
ing
air
port
Departure
Baggage claim
Deplane
Landing
Arr
ivin
g a
irport
Traveling
intermediate air traffic sites
Airplane routing Airplane routing
A Three Layer Model
• Application Layer
• Transport Layer
• Network Access Layer
Network Access Layer• Exchange of data between the computer
and the network
• Sending computer provides address of destination
• May invoke levels of service
• Dependent on type of network used (LAN, packet switched etc.)
Transport Layer• Reliable data exchange
• Independent of network being used
• Independent of application
Application Layer• Support for different user applications
• e.g. e-mail, file transfer
Layered Protocol Design
• Layering model is a solution to the problem of complexity in network protocols
• The model divides the network protocols into layers, each of which solves part of the network communication problem– Each layer has its own protocol!
• Each layer implements a service to the layer above– Relying on services provided by the layers
below.
Network/Protocol Architecture
• Set of layers, what their functions are, the services each of them provide, and the interfaces between them.
• A.k.a, protocol architecture or protocol stack.
• Examples: – ISO-OSI 7 layer architecture.– TCP-IP architecture (Internet).
Protocol Data Units (PDU)• At each layer, protocols are used to
communicate.
• At the source, control information is added to user data at each layer, a.k.a., encapsulation.
• At the receiver, control information is stripped off at each layer going up the stack, a.k.a., decapsulation.
Example 1: ISO OSI Architecture
• ISO: International Standards Organization
• OSI: Open Systems Interconnection.
Physical
Data link
Network
Transport
Session
Presentation
Application
ISO Model• Layer 7: Application
– Application-specific protocols (e.g. ftp, http, smtp)• Layer 6: Session
− establish sessions between machines.• Layer 5: Presentation
− performs proper co ordination between machines• Layer 4: Transport
– Delivery of data between computers (end-to-end).• Layer 3: Network
– Data routing across a network.• Layer 2: Data Link
– Reliable transmission over physical medium.• Layer 1: Physical
- Transmission of bits between two nodes.
Example 2: TCP/IP Architecture
• Model employed by the Internet.
Physical
Data link
Network
Transport
Session
Presentation
ApplicationApplication
Transport
Internet
NetworkAccess
Physical
TCP/IP ISO OSI
TCP/IP Protocol Architecture
Routing Technologies
• Computer Networks may use the following routing technologies:
1. Circuit Switching
2. Packet Switching
3. Message Switching
Circuit Switching
• A physical circuit is established between two machines.
• Once the connection is established, the data transfer takes place and then the connection is released.
• The data transfer rate is high but error prone.
Packet Switching
• Data is divided into small, fixed size packets.• Each packet contains the address of
destination.• All the intermediate machines inspect the
address and route it to the right machine.• If any packet is missing, the destination
machine can request the source machine to retransmit it.
Message Switching
• Message as a whole is transmitted.• Each message contains the address of the
destination.• Message switching N/W is “Store and
Forward” N/W.• Once the message arrives at the intermediate
machine, it is stored in it completely and transmitted when the line is free.
