Data Communications and Networking CSCS 311 Lecture 3 Amjad Hussain Zahid

Lecture Focus: Data Communications and Networking Lecture 3  Networking  Distributed Processing  Network Criteria  Physical Structures  Categories of Networks

Data Communications and Networking Lecture 3 NETWORKING: Physical Structures Type of connection:  A network is two or more devices connected through links.  A link is a communications pathway that transfers data from one device to another.  For visualization purposes, it is simplest to imagine any link as a line drawn between two points.  For communication to occur, two devices must be connected in some way to the same link at the same time.

Data Communications and Networking Lecture 3 NETWORKING: Physical Structures Type of connection: There are two possible types of connections: point- to-point and multipoint. 1.Point-to-Point 2.Multipoint

Data Communications and Networking Lecture 3 Point-to-point connection  A point-to-point connection provides a dedicated link between two devices.  The entire capacity of the link is reserved for transmission between those two devices.  Most point-to-point connections use an actual wire or cable to connect the two ends, but other options, such as microwave or satellite links, are also possible. When you change television channels by infrared remote control, you are establishing a point-to-point connection between the remote control and the television's control system. A transmission medium is point-to-point if, first, it provides a direct link between two devices and, second, those are the only two devices sharing the medium.

Data Communications and Networking Lecture 3 Point-to-point connection There is no central server.Used in small business, homes etc.

Data Communications and Networking Lecture 3 Multi-point connection  A multipoint (also called multidrop) connection is one in which more than two specific devices share a single link.  More than two devices share the same medium  In a multipoint environment, the capacity of the channel is shared spatially or temporally.  If several devices can use the link simultaneously, it is a spatially shared connection.  If users must take turns, it is a timeshared connection.  A central computer acts as a server and remaining computers are called clients in the network.

Data Communications and Networking Lecture 3 Multi-point connection Server

Data Communications and Networking Lecture 3 Physical Topology  Throughout the history of the networks, various techniques have been deployed to connect the end-user devices together.  The layouts of these cable systems are the network topologies that define the shape of a network or other communications system.

Data Communications and Networking Lecture 3 Physical Topology  The term physical topology refers to the way in which a network is laid out physically.  Two or more devices connect to a link;  Two or more links form a topology.  The topology of a network is the geometric representation of the relationship of all the links and linking devices (usually called nodes) to one another.  The topology refers to the physical arrangement of stations on a transmission medium.

Data Communications and Networking Lecture 3 Physical Topology  Traditional multipoint topologies are made possible when the terminals are only transmitting a fraction of the time.  In a multipoint configuration, the computer needs only a single I/O port, thereby saving hardware costs.  Only a single transmission line is needed, which also saves costs.  If each terminal has a point-to-point link to its computer, then the computer must have one I/O port for each terminal.  Also, there is a separate transmission line from the computer to each terminal.

Data Communications and Networking Lecture 3 Physical Topology Computer (Primary) T T T T T T T T T T Terminals (Secondaries) Point-to-point

Data Communications and Networking Lecture 3 Physical Topology Computer (Primary) T T T TT T Multi-point

Data Communications and Networking Lecture 3 Physical Topology  If there are only two stations, the link is point-to-point. If there are more than two stations, then it is a multipoint topology.  Traditionally, a multipoint link has been used in the case of a computer (primary station) and a set of terminals (secondary stations). In today's environments, the multipoint topology is found in local area networks.

Data Communications and Networking Lecture 3 Types of Topologies

Data Communications and Networking Lecture 3 Types of Topologies

Data Communications and Networking Lecture 3 BUS Topology  Bus topology is one of the earliest methods of connecting PCs together for a network. Linear Topology

Data Communications and Networking Lecture 3 BUS Topology  A bus topology uses one cable to connect multiple computers.  It is very easy to set up and install this type of network.  The cable is also called a trunk, a backbone, or a segment.  A bus topology is configured in a couple of common ways.  Most of the time, T-connectors are used to connect to the cabled segment. They are called T-connectors because they are shaped like the letter T. In a bus topology, all computers are connected on one linear cable. Linear Topology  Bus topology is one of the earliest methods of connecting PCs together for a network.

Data Communications and Networking Lecture 3 BUS Topology Linear Topology  Any computer can send data to any other computer.  For this purpose, coordination is required to decide which computer has to use the line at what time.  Data is sent to all computers on the trunk.  Each computer examines every packet on the wire to determine either this packet is for it or not, and accepts only messages addressed to them.

Data Communications and Networking Lecture 3 BUS Topology Linear Topology  The signal travels from one end of the bus to the other.  A terminator is required at each end of the bus to absorb the signal so it does not reflect back across the bus:  (i.e. signal is removed from the bus).  In a bus topology, signals are broadcast to all stations.  Bus is a multipoint topology.  A transmission from any station propagates the length of the medium in both directions.

Data Communications and Networking Lecture 3 BUS Topology Linear Topology  Nodes are connected to the bus cable by drop lines and taps.  A drop line is a connection running between the device and the main cable.  A tap is a connector that either splices into the main cable or punctures the sheathing of a cable to create a contact with the metallic core.  As a signal travels along the backbone, some of its energy is transformed into heat. Therefore, it becomes weaker and weaker as it travels farther and farther. For this reason there is a limit on the number of taps a bus can support and on the distance between those taps.  This segment is limited to 500 meters in length.  The minimum separation between connections is 3 meters.

Data Communications and Networking Lecture 3 BUS Topology Linear Topology  Only one computer at a time can transmit a packet on a bus topology.  Computers in a bus topology “listen” to all traffic on the network but accept only the packets that are addressed to them.  Broadcast packets are an exception because all computers on the network accept them.  When a computer sends out a packet, it travels in both directions from the computer.  This means that the network is occupied until the destination computer accepts the packet.  The number of computers on a bus topology network has a major influence on the network’s performance.

Data Communications and Networking Lecture 3 BUS Topology  If the cable isn’t terminated, the packet (which is an electronic signal) will bounce back and forth along the cable and bring down the whole network.  To prevent packets from bouncing up and down the cable, devices called terminators must be attached to both ends of the cable.  A terminator absorbs an electronic signal and clears the cable so that other computers can send packets on the network.  If there is no termination, the entire network fails.  A bus is a passive topology.  Computers on a bus topology only listen or send data. They do not resend or regenerate data. So, if one computer on the network fails, the network is still up.

Data Communications and Networking Lecture 3 BUS Topology

Data Communications and Networking Lecture 3 BUS Topology

Data Communications and Networking Lecture 3 BUS Topology

Data Communications and Networking Lecture 3 BUS Topology

Data Communications and Networking Lecture 3 BUS Topology Segment Terminator

Data Communications and Networking Lecture 3 BUS Topology  A transmission from any station propagates throughout the medium and can be received by all other stations. Two problems present themselves in this arrangement.  First, because a transmission from any one station can be received by all other stations, there needs to be some way of indicating for whom the transmission is intended.  Second, a mechanism is needed to regulate transmission.  Consider that if two stations on the bus attempt to transmit at the same time, their signals will overlap and become garbled. OR  Consider that one station decides to transmit continuously for a long period of time.

Data Communications and Networking Lecture 3 BUS Topology  To solve these problems, stations transmit data in small blocks, known as frames.  Each frame consists of a portion of the data that a station wishes to transmit, plus a frame header that contains control information.  Each station on the bus is assigned a unique address, or identifier, and the destination address for a frame is included in its header.  Consider the following situation and the figure.

Data Communications and Networking Lecture 3 BUS Topology  Station C wishes to transmit a frame of data to A.  The frame header includes A's address. Segment A ****… CB A C transmits frame addressed to A

Data Communications and Networking Lecture 3 BUS Topology  As the frame propagates along the bus, it passes B, which observes the address and ignores the frame. Segment A ****… CB A Frame is not addressed to B; B ignores it.

Data Communications and Networking Lecture 3 BUS Topology  A sees that the frame is addressed to itself and therefore copies the data from the frame as it goes by. A copies frame as it goes by. Segment A ****… CB A

Data Communications and Networking Lecture 3 BUS Topology  So the frame structure solves the first problem mentioned above:  It provides a mechanism for indicating the intended recipient of data.  It also provides the basic tool for solving the second problem, the regulation of access.  In particular, the stations take turns sending frames in some cooperative fashion; this involves putting additional control information into the frame header.  With the bus topology, no special action needs to be taken to remove frames from the medium. When a signal reaches the end of the medium, it is absorbed by the terminator.

Data Communications and Networking Lecture 3 BUS Topology  Ease of installation.  Backbone cable can be laid along the most efficient path, then connected to the nodes by drop lines of various lengths.  Less Cabling: The network needs fewer wires than a star.  Requires less cable length.  Flexibility: Easy to connect a computer or peripheral.  Adding additional nodes is easy.  Cost: Inexpensive hardware.  Does not require additional hardware to interconnect the attached devices  It is good for a temporary, small (fewer than 10 people) network.  Durability: Coax cable is durable and performs well in harsh conditions.  If a computer fails, the network stays functional.

Data Communications and Networking Lecture 3 BUS Topology  Connectors used with Coaxial cable  BNC Connector  BNC T- Connector  BNC Terminator BNC = Bayone-Neill-Concelman

Data Communications and Networking Lecture 3 BUS Topology

Data Communications and Networking Lecture 3 BUS Topology

Data Communications and Networking Lecture 3 BUS Topology

Data Communications and Networking Lecture 3 BUS Topology DISADVANTAGES  Entire network is disabled when a cable cut occurs.  Entire network shuts down if any break in the main cable.  Terminators are required at both ends.  Difficult to identify the problem if the entire network shuts down.  Performance: Coax technology is usually limited to a maximum of 10mbs  Not used for large networks  Performance degrades as more computers are added to the bus.

Data Communications and Networking Lecture 3 BUS Topology DISADVANTAGES  Difficult fault isolation and troubleshooting:  A fault or break in the bus cable stops all transmission.  When the network goes down, it is usually due to a break in the cable segment. With a large network, this problem can be tough to isolate.  The heavier the traffic, the slower the network.  Adding new devices may require modification or replacement of the backbone.  The damaged area reflects signals back in the direction of origin, creating noise in both directions.  Less popular now  Scalability:  Difficult to make changes easily within the size and layout of network  The bus topology is not very scalable.

Data Communications and Networking Lecture 3 STAR Topology  All computers are directly attached to a central point, which is sometimes called the “Hub” or concentrator.  Data on a star network passes through the hub or concentrator before continuing to its destination.  The hub or concentrator manages and controls all functions of the network.

Data Communications and Networking Lecture 3 STAR Topology  These networks are not physically like stars but they are logically like stars.  It means that their shape does not look like a star but their connections are just like a star. The above diagram is idealized.  Here is shown a star network in practice in the figure below: Hub

Data Communications and Networking Lecture 3 STAR Topology  Hub & Switch  A Hub/Switch is a central device used on star network topology that repeats or amplifies signals, allowing the network to be expanded with additional stations.

Data Communications and Networking Lecture 3 STAR Topology  Hub & Switch  The hub offers a common connection for all stations on the network.  Each station has its own direct cable connection to the hub. In most cases, this means more cable is required than for a bus topology. However, this makes adding or moving computers a relatively easy task; simply plug them into a cable outlet on the wall.  Typically, each station is attached to a central node, referred to as the star coupler, via two point-to-point links, one for transmission and one for reception.

Data Communications and Networking Lecture 3 STAR Topology  Hub & Switch  In a star topology, each device has a dedicated point-to-point link only to a hub.  The devices are not directly linked to one another. A star topology does not allow direct traffic between devices. The controller acts as an exchange:  If one device wants to send data to another, it sends the data to the controller, which then sends the data to the other connected device.  The hub:  Manages and controls all network functions  Acts as a repeater for the data flow

Data Communications and Networking Lecture 3 STAR Topology  In general, there are two alternatives for the operation of the central node.  One approach is for the central node to operate in a broadcast fashion.  A transmission of a frame from one station to the node is retransmitted on all of the outgoing links. In this case, although the arrangement is physically a star, it is logically a bus; a transmission from any station is received by all other stations, and only one station at a time may successfully transmit.  Another approach is for the central node to act as a frame switching device. An incoming frame is buffered in the node and then retransmitted on an outgoing link to the destination station.  Operation of the Hub

Data Communications and Networking Lecture 3 STAR Topology

Data Communications and Networking Lecture 3 STAR Topology

Data Communications and Networking Lecture 3 STAR Topology

Data Communications and Networking Lecture 3 STAR Topology ADVANTAGES  Modern networks normally use a star topology.  Easy to install and wire.  Flexibility: Devices can be added or removed without affecting the other devices on the network  Easy to detect faults and to remove parts.  A star topology is less expensive than a mesh topology.  In a star, each device needs only one link and one I/O port to connect it to any number of others. This factor also makes it easy to install and reconfigure. Far less cabling needs to be housed, and additions, moves, and deletions involve only one connection: between that device and the hub.

Data Communications and Networking Lecture 3 STAR Topology ADVANTAGES  Robustness:  If one link fails, only that link is affected. All other links remain active. This factor also lends itself to easy fault identification and fault isolation. As long as the hub is working, it can be used to monitor link problems and bypass defective links.  Performance:  Greater performance with speeds capable of 10mbps to 100mbps or more  Centralized management and monitoring of network traffic:  Centralizing network components can make an administrator’s life much easier in the long run. With a star configuration, it is also easy to add or change configurations because all the connections come to a central point.  Performance is greater with speeds capable of 10mbps to 100mbps or more

Data Communications and Networking Lecture 3 STAR Topology DISADVANTAGES  Requires more cable length than a linear topology.  If the hub or concentrator fails, nodes attached are disabled.  The dependency of the whole topology is on one single point, the hub. If the hub goes down, the whole system is dead.  More expensive than linear bus topologies because of the cost of the concentrators.  Although a star requires far less cable than a mesh, each node must be linked to a central hub. For this reason, often more cabling is required in a star than in some other topologies (such as ring or bus).