Presentation on theme: "Data and Computer Communications"— Presentation transcript:
1Data and Computer Communications Chapter 1 – Data Communications, Data Networks, and the InternetNinth Editionby William Stallings
2Data Communications, Data Networks, and the Internet “The fundamental problem of communication is that of reproducing at one point either exactly or approximately a message selected at another point”- The Mathematical Theory of Communication,Claude ShannonMessage
31-1 DATA COMMUNICATIONSThe term telecommunication means communication at a distance. The word data refers to information presented in whatever form is agreed upon by the parties creating and using the data. Data communications are the exchange of data between two devices via some form of transmission medium such as a wire cable.1.#
4Figure 1.1 Five components of data communication 1.#
5Figure 1.2 Data flow (simplex, half-duplex, and full-duplex) 1.#
61-2 NETWORKSA network is a set of devices (often referred to as nodes) connected by communication links. A node can be a computer, printer, or any other device capable of sending and/or receiving data generated by other nodes on the network.1.#
7Figure 1.3 Types of connections: point-to-point and multipoint 1.#
8Technological Advancement Driving Forces Development of new servicesAdvances in technologyTraffic growth at a high & steady rate
9Changes in Networking Technology * Emergence of high-speed LANs* Corporate WAN needs* Digital electronics
11Communications Tasks Transmission system utilization Addressing InterfacingRoutingSignal generationRecoverySynchronizationMessage formattingExchange managementSecurityError detection and correctionNetwork managementFlow control
12Transmission Lines Capacity The basic building block of any communications facility is the transmission line.ReliabilityThe business manager is concerned with a facility providing therequired capacity,with acceptable reliability,at minimum cost.CostTransmissionLine
13Transmission Mediums Wireless transmissions Two mediums currently drivingthe evolution of data communications transmission are:Fiber optic transmissionsandWireless transmissions
14NetworkingAdvances in technology have led to greatly increased capacity and the concept of integration, allowing equipment and networks to work simultaneously.VoiceDataImageVideoThe number of computers in use worldwide is in the hundreds of millions, with pressure from users of these systems for ways to communicate among all these machines being irresistible. Advances in technology have led to greatly increased capacity and the concept of integration, allowing equipment and networks to deal simultaneously with voice, data, image, and even video.
15Network Hardware Local Area Networks Metropolitan Area Networks Classifying networks based on their scale:Local Area NetworksMetropolitan Area NetworksWide Area NetworksWireless NetworksHome NetworksInternetworks
17LANs and WANs There are two broad categories of networks: Local Area Networks (LAN)Wide Area Networks (WAN)Networks are currently classified into two broad categories: Local Area Networks (LANs) and Wide Area Networks (WANs).
18Wide Area Networks (WANs) Span a large geographical areaRequire the crossing of public right-of-waysRely in part on common carrier circuitsTypically consist of a number of interconnected switching nodesWide area networks generally cover a large geographical area, require the crossing of public right-of-ways, and rely at least in part on circuits provided by a common carrier. Typically, a WAN consists of a number of interconnected switching nodes. A transmission from any one device is routed through these internal nodes to the specified destination device. These nodes are not concerned with the content of the data; rather, their purpose is to provide a switching facility that will move the data from node to node until they reach their destination.
19Wide Area Networks Alternative technologies used include: Circuit switchingPacket switchingFrame relayAsynchronous Transfer Mode (ATM)Traditionally, WANs have been implemented using one of two technologies: circuit switching and packet switching. More recently, frame relay and ATM networks have assumed major roles.
20Circuit Switching Uses a dedicated communications path Connected sequence of physical links between nodesLogical channel dedicated on each linkRapid transmissionThe most common example of circuit switching is the telephone network
22Packet SwitchingData are sent out in a sequence of small chunks called packetsPackets are passed from node to node along a path leading from source to destinationPacket-switching networks are commonly used for terminal-to-terminal computer and computer-to-computer communications
23Asynchronous Transfer Mode (ATM) Referred to as cell relayCulmination of circuit switching and packet switchingUses fixed-length packets called cellsWorks in range of 10’s and 100’s of Mbps and in the Gbps rangeData rate on each channel dynamically set on demand
24Local Area Networks (LAN) Smaller scope, typically a single buildingLANs are usually owned by the same organization that owns attached devicesInternal data rates greater than WANsMost common configurations are switched LANs and wireless LANs
25Metropolitan Area Networks (MAN) Covers a geographic area such as a town, city or suburbMiddle ground between LAN and WANSupports both data and voicePrivate or public network
27Network Hardware Wireless Networks System interconnectionBluetooth a short-range wireless network. Allows system components together, digital cameras, headsets, scanners, and other devices to connect to a computer by merely being brought within range.Wireless LANsEvery computer has a radio modem and antenna with which it can communicate with other systems.Standard for wireless LANs: IEEE , which most systems implement and which is becoming very widespread.
28Network Hardware Wireless Networks Wireless WANs (cont.)Wireless LANs an operate at rates up to about 50 Mbps over distances of tens of meters. While cellular systems (Wireless WANs) operate below 1 Mbps, but the distance between the base station and the computer or telephone is measured in kilometers rather than in meters.High-bandwidth wide area wireless networks are also being developed (IEEE ).
29The Channel Allocation Problem To allocate a single broadcast channel among competing users, we can use:Static Channel Allocation in LANs and MANsDynamic Channel Allocation in LANs and MANs
30Static Channel Allocation in LANs and MANs Frequency Division Multiplexing (FDM) is an example of static channel allocation where the bandwidth is divided among a number of N users.When there is only a small and constant number of users, each of which has a heavy (buffered) load of traffic (e.g., carriers' switching offices), FDM is a simple and efficient allocation mechanism.However, when the number of senders is large and continuously varying or the traffic is bursty, FDM presents some problems.1) when fewer than N users are currently interested in communicating, a large piece of valuable spectrum will be wasted.2) when more users wants to communicate, those who have not been assigned a frequency will be denied permission.3) even assuming that the number of users could somehow be held constant at N, each user traffic usually changes dynamically over time.
31Multiple Access Protocols ALOHACarrier Sense Multiple Access ProtocolsCollision-Free ProtocolsLimited-Contention ProtocolsWavelength Division Multiple Access ProtocolsWireless LAN Protocols
35Pure ALOHAThe basic idea of an ALOHA system is simple: let users transmit whenever they have data to be sent. There will be collisions, of course, and the colliding frames will be damaged. If the frame was destroyed, the sender just waits a random amount of time and sends it again.How the channel know that there is a collision:Due to the feedback property of broadcasting, a sender can always find out whether its frame was destroyed by listening to the channel, the same way other users do. With a LAN, the feedback is immediate; with a satellite, there is a delay of 270 msec before the sender knows if the transmission was successful.If listening while transmitting is not possible for some reason, acknowledgements are needed.
36In pure ALOHA, frames are transmitted at completely arbitrary times. The throughput of ALOHA systems is maximized by having a uniform frame size rather than by allowing variable length frames.
37Other protocolsSlotted ALOHA: It assumed the time is divided into discrete intervals.The station can send at the beginning of the next time interval whenever it have data ready after the start of the current time interval.1- Persistent CSMA: When a station has data to send, it first listens to the channel to see if anyone else is transmitting at that moment.if the channel is idle, it start transmission.If the channel is busy, the station waits until it becomes idle. When the station detects an idle channel, it transmits a frame. If a collision occurs, the station waits a random amount of time and starts all over again.Nonpersistent CSMA: same as 1-persistent except that the station does not continually sense the channel when it finds it busy, rather it waits a random period of time and then sense the channel again. When the channel becomes idle it transmit.p-Persistent CSMA: same as Nonpersistent CSMA but the station transmit with probability p when the channel is idle.
41Wireless LAN Protocols A system of notebook computers that communicate by radio can be regarded as a wireless LANA common configuration for a wireless LAN is an office building with base stations (also called access points) strategically placed around the building.All the base stations are wired together using copper or fiber.A simplifying assumption that all radio transmitters have some fixed range will be used follow.When a receiver is within range of two active transmitters, the resulting signal will generally be garbled and useless.
42Wireless LAN Protocols (2) A naive approach to using a wireless LAN might be to try CSMA: just listen for other transmissions and only transmit if no one else is doing so.The trouble is, this protocol is not really appropriate because what matters is interference at the receiver, not at the sender.A wireless LAN. (a) A transmitting. (b) B transmitting.
43Wireless LAN Protocols (3) When A is transmitting to B (previous figure part a)If C senses the medium, it will not hear A because A is out of range, and thus falsely conclude that it can transmit to B.If C does start transmitting, it will interfere at B, wiping out the frame from A.The problem of a station not being able to detect a potential competitor for the medium because the competitor is too far away is called the hidden station problem.
44Wireless LAN Protocols (4) When B transmitting to A (previous figure part b)If C senses the medium, it will hear an ongoing transmission and falsely conclude that it may not send to D, when in fact such a transmission would cause bad reception only in the zone between B and C, where neither of the intended receivers is located.This is called the exposed station problem.
50Wireless LAN Protocols (5) The problem is that before starting a transmission, a station really wants to know whether there is activity around the receiver.An early protocol designed for wireless LANs is MACA (Multiple Access with Collision Avoidance) (Karn, 1990).The basic idea behind it is for the sender to stimulate the receiver into outputting a short frame, so stations nearby can detect this transmission and avoid transmitting for the duration of the upcoming (large) data frame.
51Wireless LAN Protocols (6) Let us now consider how A sends a frame to B.- A starts by sending an RTS (Request To Send) frame to B. This short frame (30 bytes) contains the length of the data frame that will eventually follow.- Then B replies with a CTS (Clear to Send) frame. The CTS frame contains the data length (copied from the RTS frame). Upon receipt of the CTS frame, A begins transmission.
52Wireless LAN Protocols (7) The MACA protocol. (a) A sending an RTS to B.(b) B responding with a CTS to A.
53Topics discussed in this section: UNICAST ROUTING PROTOCOLSA routing table can be either static or dynamic. A static table is one with manual entries. A dynamic table is one that is updated automatically when there is a change somewhere in the Internet. A routing protocol is a combination of rules and procedures that lets routers in the Internet inform each other of changes.Topics discussed in this section:Optimization Intra- and Interdomain RoutingDistance Vector Routing and RIPLink State Routing and OSPFPath Vector Routing and BGP
61Summary Trends challenging data communications: Transmission mediums traffic growthdevelopment of new servicesadvances in technologyTransmission mediumsfiber opticwirelessNetwork categories:WANLANInternetevolved from the ARPANETTCP/IP foundation