Computer Network An interconnected collection of autonomous computers. Two computers are said to be interconnected if they are able to exchange information. A system with one control unit and many slaves is not a network.
Computer Network(Cont.) SystemsDistributed Computer NetworkThe existence of multiple autonomous User must explicitlycomputers is transparent to the user. do everything.Allocation of jobs to processor and files to disksand all other system functions must beautomatic.Distributed system is a software system builton top of a network.Overlap between distributed systems andComputer NetworkExample:More files around System can involve the Usermovement.
Computer Network (Cont.) Uses of Computer NetworkCompanies People Social IssuesResource Sharing Access to remote News-groups informationGeography Person To Person Bulletin Boards communication & e- mailHigh reliability: Interactivereplication EntertainmentSaving money on the flowClient-server modelScalability: Ability toincrease systemperformance graduallyas the workload grows.
A Communications Model Source Generates data to be transmitted Transmitter Converts data into transmittable signals Transmission system Carries data Receiver Converts received signal into data Destination Takes incoming data
Key Communications Tasks Transmission system utilization Interfacing Signal generation Synchronization Exchange management Error detection and correction Addressing and routing Recovery Message formatting Security Network management
Network Hardware Transmission Technology Broadcast Network Point – To – Point NetworkSingle communication channel that Many connections betweenis shared by all the machines on individual pairs of machinesthe network.All the others receive “Packets” in A packet may have to visit onecertain contexts, sent by any or more intermediate machine.machine.An address field within the packet Routing algorithms play anspecifies for whom it is intended. important role in PTP networks.Multicasting: transmission to asubnet of the machines.
Local Area Networks Smaller scope Building or small campus Usually owned by same organization as attached devices Data rates much higher Usually broadcast systems Now some switched systems and ATM are being introduced
Local Area Networks (Cont.) NETWORKS LAN MAN WAN INTERNET LAN CHARACTERISTICS Size Transmission Technology Topology Restricted in Single Cable BUS (Ethernet) Size 10 to 100 Mbps Ring (Token ring) Low delay (ms) Very few Errors Megabits/Sec. (Unit)
MANMetropolitan Area NetworkSupport data and voiceNo switching elementsStandard: DQDB (Distributed Queue Dual Bus)Two unidirectional buses to which all thecomputers are connected.Each bus has a head-end, a device that initiatestransmission activity.Traffic that is destined for a computer to the right ofthe sender uses the upper bus, traffics to the leftuses the lower one.
Wide Area Networks Large geographical area Crossing public rights of way Rely in part on common carrier circuits Alternative technologies Circuit switching Packet switching Frame relay Asynchronous transfer mode (ATM)
Wide Area Networks (Cont.) Host (end system). Subnet (communication subnet). WANs typically have irregular topologies. WAN CONSISTS OFTransmission Lines:- Circuits, Switching Elements:-Channels or Tanks Specialized computers used to connect two or more transmission lines.
Internet Collection of interconnected networks. Example: A collection of LAN’s connected by a WAN. WAN : (router + hosts). SUBNET : (only routers).
Circuit Switching Dedicated communications path established for the duration of the conversation E.G. Telephone network
Packet Switching Data sent out of sequence Small chunks (packets) of data at a time Packets passed from node to node between source and destination Used for terminal to computer and computer to computer communications
Frame Relay Packet switching systems have large overheads to compensate for errors Modern systems are more reliable Errors can be caught in end system Most overhead for error control is stripped out
Asynchronous Transfer Mode ATM (cell relay) Evolution of frame relay Little overhead for error control Fixed packet (called cell) length Anything from 10mbps to Gbps Constant data rate using packet switching technique Offers a constant data rate channel
Integrated Services Digital Network ISDN Designed to replace public telecom system Wide variety of services Entirely digital domain First generation ( narrowband ISDN ) 64 kbps channel is the basic unit Circuit-switching orientation Contributed to frame relay Second generation ( broadband ISDN ) 100s of mbps Packet-switching orientation Contributed to ATM ( cell relay )
Protocols Used for communications between entities in a system Must speak the same language Entities User applications E-mail facilities Terminals Systems Computer Terminal Remote sensor
Protocol Hierarchies Organized as a series of layers or levels. The purpose of each layer is to offer certain services to the higher layers. Layer n on one-machine carries on a conversation with layer n on another machine. Protocol: is an agreement between the communicating parties on how communication is to proceed. Peers communicate using the protocol. In reality, no data directly transferred from layer n on one machine to layer n on another machine.
Protocol Hierarchies (Cont.)Each layer passes data and control information tothe layer immediately below it.Between each pair of adjacent layers there is an“interface”.The design of layers helps in: Minimizing the amount of information that must be passed between layers Make it simpler to reduce the implementation of one layer with a completely different oneProtocol stack:A list of protocol used by a certain system, oneprotocol per layer.
Key Elements of a Protocol Syntax Data formats Signal levels Semantics Control information Error handling Timing Speed matching Sequencing
Design Issues for the Layers Addressing. Data transfer. Simplex communication. Half-duplex communication. Full-duplex communication. Number and priorities of the logical connection channels. Many networks provide at least two logical channels per connection, one for normal data and one for urgent data. Error control. Error detecting code. Error correcting code.
Design Issues (Cont.) How to receive data in order (sequence no.). How to keep a fast sender from swamping a slow receiver with data (flow control). Size of the message: disassembling >transmitting >reassembling messages. Routing: multiple paths between source and destination.
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
A Three Layer Model Network access layer Transport layer Application 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
Interfaces and Services Active elements in each layer are called ENTITIES. Entity. Software [example: process.]. Hardware [example: intelligent I/O chip.]. The entities in layer n implement a service used by layer n+1. Layer n called service provider. Layer n + 1 called service user. Services are available at sap’s (service access points). Each SAP has an address that uniquely identifies it.
Interfaces and Services (Cont.) IDU: interface data unit. ICI: interface control info. SDU: service data unit. At a typical interface, the layer n + 1 entity passes an IDU to the layer n entity through the SAP. In order to transfer the SDU, the layer n entity may have to fragment it into several pieces, each of which is given a header and send to as a separate PDU (protocol data unit) such as a packet.
Addressing Requirements Two levels of addressing required Each computer needs unique network address Each application on a (multi-tasking) computer needs a unique address within the computer The service access point or SAP
Protocol Data Units (PDU) At each layer, protocols are used to communicate Control information is added to user data at each layer Transport layer may fragment user data Each fragment has a transport header added Destination SAP Sequence number Error detection code This gives a transport protocol data unit
SERVICES Connection Oriented ConnectionlessModeled after the telephone system Modeled after posted systemEstablish a connectionUse the ConnectionRelease the connectionActs like a tube: receive data by the Messages could be received insame order was sent different order than it was sent withReliable connection oriented service Unreliable connectionless service (not acknowledged)
Request reply service Sender transmits a single datagram containing a request, the reply contains the answer. Used to implement communication in the client-server model.
Service Primitives A service is formally specified by a set of primitives (operations) available to a user or other entity to access the service. Primitive tells the service to Perform some action OR Report an action by a peer entity. Example: Connection oriented service with 8 service primitives. CONNECT.request – Request a connection to be established. CONNECT.indication – Signal the called party.
Example (Cont.) CONNECT.response – Used by the caller to accept/reject calls. CONNECT.confirm – Tell the caller whether the call was accepted. DATA.request – Request the data be sent. DATA.indication – Signal the arrival of data. DISCONNECT.request – Request that a connection be released. DISCONNECT.indication – Signal the peer about the request. Service Could be. • Confirmed (Example: CONNECT). • Unconfirmed (Example: DISCONNECT).
Relationship of Services toProtocols Service: is a set of primitives (operations) that a layer provides to the layer above it. Protocol. A set of rules governing the format and meaning of the frames, packets, or messages that are exchanged by the peer entities within a layer. Entities use protocols in order to implement their service definitions. Entities are free to change their protocols, provided they do not change the service visible to their users. REFERENCE MODELS OSI References Model TCP/IP Reference Model
TCP/IP Protocol Architecture Developed by the US defense advanced research project agency (DARPA) for its packet switched network (ARPANET). Used by the global internet. No official model but a working one. Application layer. Host to host or transport layer. Internet layer. Network access layer. Physical layer.
Physical Layer Physical interface between data transmission device (e.G. Computer) and transmission medium or network Characteristics of transmission medium Signal levels Data rates Etc.
Network Access Layer Exchange of data between end system and network Destination address provision Invoking services like priority
Internet Layer (IP) Systems may be attached to different networks Routing functions across multiple networks Implemented in end systems and routers
Transport Layer (TCP) Reliable delivery of data Ordering of deliveryApplication Layer Support for user applications e.g. http, SMPT
OSI Model Open systems interconnection Developed by the international organization for standardization (ISO) Seven layers A theoretical system delivered too late! TCP/IP is the de facto standard
OSI References Model International Standards Organization. OSI (Open Systems Interconnection). Reference model: deals with connecting open systems that are; Open for communication with other systems.
Principles A layer should be created where a different level of abstraction is needed. Each layer should perform a well-defined function. The function of each layer should be chosen with an eye toward defining internationally standardized protocols. The layer boundaries should be chosen to minimize the information flow across the interfaces. The number of layers should be large enough that distinct functions need not be thrown together on the same layer out of necessity.
OSI Layers Application Presentation Session Transport Network Data link Physical
The Physical Layer Deals with transmitting raw bits over a communication channel. How many volts for 1 or 0. How many microseconds a bit lasts. Mechanics, electrical and procedural interfaces.
Data link Layer Break the input data up into data frames. Process the acknowledgement frames sent back by the receiver. Insert the frame delimiter. Solve the problems caused by damaged, lost and duplicate frames. Flow control. Full duplex transmission (piggybacking) Medium access sub layer deals with how to control access to the shared channel in broadcast networks.
Network Layer Routing packets from source to destination. Routes can be static or dynamic Bottleneck, congestion Connect heterogeneous networks (different addressing method, larger packet service). In broadcast networks, routing problem is simple, so the network layer is thin.
Transport Layer Accept data from the session layer, split it up into smaller units if needed, pass these to the network layer and ensure that the all pieces arrive correctly at the other end Under normal conditions, the transport layer creates a distinct network connection for each transport connection required by the session layer If the transport connection requires a high throughput, the transport layer might create multiple network connections, dividing the data among the network connections to improve throughput
Transport Layer (Cont.) Transport layer determines what type of service to provide the session layer with and ultimately, the users of the entire network The transport layer is a true end-to-end layer, from source to destination Multiple connections will be entering and leaving each host. There is a need to tell which message belongs to which connection (transport header) Establishing and deleting connections across the network Flow control between hosts (as oppose between routers) so fast host cannot overrun a slow one
Session Layer Allows users on different machines to establish sessions between them A session might be used to allow a user to log into a remote timesharing system or to transfer a file between two machines Example: token management. Only the side holding the token may perform the critical operation. Synchronization: insert a checkpoint. Example: sending file for 20 hours. After a crash the portion after the checkpoint will be resend again.
Presentation Layer Concerned with the syntax and semantics of the information transmitted. A typical example of a presentation service is encoding data in a standard agreed upon way. [Character strings, integers, floating-point numbers…].
Application Layer The application layer contains a variety of protocols that are commonly needed. Example: incompatible terminal type. One way to solve this problem is to define an abstract network virtual terminal that editor can be written to deal with. To handle each terminal type, a piece of s/w must be written to map the functions of the network virtual terminal onto the real terminal. Other application is file transfer(ftp).
Example Of Networks Novell NETWARE. Client-server model. IPX/SPX. Network layer runs IPX (internet packet exchange). IPX uses 10 byte address (IP uses 4 bytes) flat addressing. Transport protocol. • NCP (network core protocol). • Transport service & other services. • SPX (sequenced packet exchange): • Just transport service.
Example Of Networks (Cont.) The application can choose between NCP & SPX Transport control field counts how many networks the packet has traversed. About once a minute, each server broadcasts a packet giving its address and telling what services it offers. SAP (Service Advertising Protocol) is used for broadcasting Routers run some kind of special agent processes to construct databases of which servers are running. When a client is booted, it sends a request for a server. The agent on the local router machine sees this request, and matches up the request with the best server.
Example Of Networks (Cont.)The APRANET. Packet switched network, consisting of subnet and host computers. IMPS (interface message processors) connected by transmission lines. Each IMP would be connected to at least two other imps. Each node consists of IMP and a host. Host sends messages of up to 8063 bits to its IMP. IMP breaks the message into packets of at most 1008 bits and forwards them independently toward the destination. 56-kbps lines leased from telephone companies interconnect the IMPS. By 1990, the ARPANET had been overtaken by newer networks.
Example Of Networks (Cont.) NSFNET By 1984 NSF Fig 1.26(the U.S. national science Foundation) began designing a high-speed successor to the ARPANET that would be open to all university research groups. By 1995 the NSFNET backbone was no longer needed to interconnect the NSF regional networks because numerous companies were running commercial IP Networks.
Example Of Networks (Cont.) The Internet. In 1992, the internet society was set up, to promote the use of the internet. Four main applications. Email. News. Remote login: telnet, rlogin. File transfer: FTP.
Example Of Networks (Cont.) Gigabit TESTBEDS. The backbones operate at megabit speeds. Gigabit networks provide better bandwidth but not always much better delay. Example: sending a 1-kbit packet from NYC to san Francisco at (1 mbps) take. 1 msec to pump the bits out and 20 msec for the delay, for a total of 21 msec. A 1-Gbps network can reduce this to 20.001 msec. For some applications, bandwidth is what counts, and these are the applications for which gigabit networks will make a big difference. Examples:- telemedicine & virtual meeting.
Example DataCommunication Services SMDS X.25 FRAME RELAY BROADBAND ISDN AND ATM