Introduction Computer Networks www.techstudent.co.cc
Computer Networks • “Computer network” is an interconnectedcollection of autonomous computers. • A system of interconnected computers • Two computers are said to be interconnected if they are able to exchange information
Uses of Computer Networks • Business Applications (for Companies) • Home Applications (for people) • Mobile Users • Social Issues
Business Applications of Networks • A network with two clients and one server.
Business Applications of Networks • The client-server model involves requests and replies. Advantages: Resource sharing Money saving
Home Network Applications • Access to remote information • Person-to-person communication • Interactive entertainment • Electronic commerce
Home Network Applications • In peer-to-peer system there are no fixed clients and servers.
Home Network Applications • Some forms of e-commerce.
Mobile Network Users • Combinations of wireless networks and mobile computing.
Transmission Technology • Broadcast networks • Point-to-point networks
Broadcast Networks Single communication channel shared by all the m/c s. Short msgs(packets)sent by any m/c are received by all other m/cs An address field within the packet specifies for whom it is intended for. After receiving, a m/c checks the address field-if it is for this m/c it processes the packet else ignored
Broadcasting • Broadcast systems allow the possibility of addressing a packet to all destinations by using a spl. code in the address field • When a packet with this code is transmitted, it is received and processed by every m/c on the network. • This mode of opertn is called BROADCASTING
Multicasting • Some broadcast systems support txn to a sub- set of m/c s known as MULTICASTING. • For this 1 bit of the address filed is reserved. • When a packet is sent to a certain group it is delivered to all m/cs subscribing to that group
Point-to-Point • Consist of many connections between individual pairs of m/cs. • From src to detsn a packet on this type of n/w may have to visit one or more intermediate m/cs. • Multiple routes of diffrnt lengths r possible-so routing algrthm play a vital role in p2p n/w. Smaller geographically localized n/ws tend to use brodcasting. Larger n/ws are point-to-point.
Network Hardware • Local Area Networks • Metropolitan Area Networks • Wide Area Networks • Wireless Networks • Home Networks • Internetworks
Classification of interconnected processors by scale(by physical size)
Local Area Networks(LAN) • Privately owned n/ws within a single buildng or campus of upto a few kms size. • Widely used to connect PCs and workstations in companies & factories to share resources and exchange infrmn. • LANs r distinguished from other n/ws by 3 characteristics: • Size,Transmission Technology &Topology
Size: Worst-case txn time is bounded and known in advance.So design is easy • Txn Technology • Often use a txn technology consisting of a single cable to which all the m/cs are attached. • Traditional LANs run at a speed of 10 to 100 Mbps(Megabits/Second-1,000,000 bits/s).
Topologies (a) Bus (b) Ring
Bus • In this at any instant 1 m/c is the master and is allowed to transmit. • All other m/cs are refrain from sending • An arbitration mechanism is needed to solve conflicts if 2 or more m/cs want to transmit simultaneously(may b centralized or distributed) • IEEE 802.3(Ethernet) is a bus based broadcast n/w with decentralized control operating at 10 to 100 Mbps. • m/cs on ethernet can transmit at any time ;if 2 or more packets collide, each computer waits a random time and tries again later
Ring • In this each bit propagates around on its own,not wait for the rest of the packet to which it belongs. • Each bit circumnavigates the entire ring in the time it takes to transmit a few bits,often before the complete packet has been transmitted. • IEEE 802.5(the IBM token ring) is a popular ring-based LAN operating at 4 and 16 Mbps.
Metropolitan Area Networks • A metropolitan area network based on cable TV. • MAN covers a city (Eg:cable TV n/w).
Wide Area Networks(WAN) • Spans a large geographical area(a country /continent). • It contains collection of m/cs intended for running user pgms-known as HOSTS. • Hosts are connected by a communication SUBNET. • Job of the subnet is to carry msgs from host to host
In most WAN the subnet consists of 2 distinct components: • Transmission lines and switching elements. • Transmission lines (circuits,channesl or trunks) moves bits betwn m/c s • Switching elements are specialized computers used to connect 2 or more txn lines. • When data arrive on an incoming line the switching element must choose an outgoing line to forward them. • These switching computers r known as router.
In abov fig.Each host is connected to a LAN on which a router is present. • The collection of communication lines and routers form the subnet. • When a packet is sent from one router to another via one or more intermediate routers,the packet is received at each intermediate router ,stored there until the required o/p line is free and then forwarded. • A subnet organized according to this principle is called store-and-forward or packet-switched subnet.
Packet switching principle • When a process on some host has a msg to send to a process on some other host ,the sending host first cuts the msgs into packets, each one having its number in sequence. • These packets are then injected into the n/w one at a time in quick succession. • The packets are transported individually over the n/w and deposited at the receiving host, where they r reassembled into the original msg and delivered to the receiving process.
Wireless Networks Categories of wireless networks: • System interconnection • Wireless LANs • Wireless WANs
System interconnection • Interconnecting the components of a computer using short-range radio. • Bluetooth is an Eg for this. • It allows different components like mobiles, digital cameras etc to connect to a computer by merely being brought within range.
Wireless LAN • s/ms in which every computer has a radio modem and antenna with which it can communicate with other s/ms. • There is a standard for wireless LANS called IEEE 802.11,which most s/ms implement and which is becoming more widespread.
Wireless WANs • The radio n/w used for cellular phones is an eg for low-b/w wireless s/ms. • Wireless LANs can operate at rates up to about 50 Mbps over distance of 10 mtrs.
Wireless Networks • (a) Bluetooth configuration • (b) Wireless LAN
Wireless Networks • (a) Individual mobile computers • (b) A flying LAN
Home Network Categories • Computers (desktop PC, PDA, shared peripherals • Entertainment (TV, DVD, VCR, camera, stereo, MP3) • Telecomm (telephone, cell phone, intercom, fax) • Appliances (microwave, fridge, clock, furnace) • Telemetry (utility meter, burglar alarm, babycam).
Internetworks • A collection of interconnected networks is called internetwork or just internet • Two different and frequently incompatible networks are connected using machines called gateways
Network Software The philosophy of connecting together two entities. “Layering” is the key word.
Network Software PROTOCOL HIERARCHIES Layers - The concept that network software is organized functionally into levels. A level on one host talks to the same level on another host (its peer). Protocol - The protocol is the convention or standard that a layer uses to talk to the other layer. An agreement or standard on the conversation. Physical Medium - Underneath the layers is the wire or fiber or whatever. Interface - Defines the services that one layer offers another (either up or down.) Important that each layer perform specific actions.
Protocol Hierarchies • The philosopher-translator-secretary architecture.
Network Software PROTOCOL HIERARCHIES • Network architecture - • A set of layers and protocols. It contains details on what happens in the layer and what the layers says to its peer. • Functional interfaces and implementation details are not part of the spec, since that's not visible outside the machine. • Protocol stack - • A list of protocols used by a system, one protocol per layer.
Network Software DESIGN ISSUES FOR THE LAYERS o Addressing o Number of logical channels per connection (for priority purposes) o Error control. (garbled or missing.) o Preservation of message ordering. o Flow control. o Breaking up messages into a smaller chunks (and reassembly.) o Multiplexing messages on same connection. o Routing - how to get from one host to another. That word “Multiplexing”:
CONNECTION-ORIENTED / CONNECTIONLESS SERVICES Network Software • Connection oriented service - • Like the phone system. The system establishes a connection, uses it, and closes it. Acts like a tube. Data comes out the other end in the same order as it goes in. • Connection Setup • Data Transfer • Connection Termination • Connectionless service - • Like the post office. Each message has the entire address on it. Each • message may follow a different route to its destination. Ordering not • maintained. • Data Transfer
CONNECTION-ORIENTED / CONNECTIONLESS SERVICES Network Software Quality of service - Will the message arrive?? A reliable connection-oriented service guarantees success. It is implemented by having the rxer acknowledge the receipt of each msg . It is appropriate for FileTransfer( the owner of the file wants to b sure that all the bits arrive correctly and in the same order) Has 2 variations: o Message sequence - message boundaries and order are maintained. o Byte streams - messages are broken up or combined; flow is bytes.
CONNECTION-ORIENTED / CONNECTIONLESS SERVICES Network Software Datagram Service –(Unreliable Connectionless) Like junk mail. It's not worth the cost to determine if it actually arrived. Needs a high probability of arrival, but 100% not required. Connectionless. No acknowledgment. Acknowledged datagram service - As above, but improved reliability via acknowledgment. Request-reply service - The sender transmits a single datagram containing a request; The reply contains the answer Summarized in this Table.
Service Primitives • Five service primitives for implementing a simple connection-oriented service.
Service Primitives • Packets sent in a simple client-server interaction on a connection-oriented network.
Services to Protocols Relationship • The relationship between a service and a protocol.
Services & Protocol Service: • Is a set of primitives (operations) that a layer provides to the layer above it. • It defines what operations the layer is prepared to perform. • It say nothing about how these operations are implemented. • It relates to an i/f between 2 layers with the lower layer being the service provider and the upper layer being the service user.
Protocol: • Is a set of rules governing the format and meaning of the packets. • Entities use protocols to implement their service definitions. • They r free to change their protocol at will, provided they do not change the service visible to their users.