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Chapter 2
Network Topologies and Network Control Devices
1.1 Introduction
Physical Structures: 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. There are two possible types of connections: point-to-point and multipoint.
Point-to-Point 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 length of 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. E.g.: -
- PC to modem.
- Workstation to workstation.
- Server or mainframe to workstation.
- PC to printer.
- Microwave to microwave.
- TV to remote control. When a point-to-point subnet is used important interconnection topology. It should look like in Fig (a)
Multipoint (multi-drop) A multipoint (also called multi-drop) connection is one in which more than two specific devices share a single link. In a multipoint environment, the capacity of the channel is shared, either 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.
Fig. Multi Drop Network
Topology
It defines physical or logical arrangement of links in network. Topology is physical layout of computers, cables and other connected devices on a network. The topology of a network is the geometric representation of the relationship of all the links and linking devices (called node) to each other. There are two types of topologies-
- Physical Topology
- Logical Topology
Physical topology The complete physical structure of transmission media is called physical topology. This refers to the layout of cabling, location of nodes and interconnection between the nodes and cabling.
Logical Topology The logical topology is refers to how data is actually transferred in a network. This represents the way that data passes through the network from one device to another.
Selection Criteria for Topologies -
- Size (no. of nodes) of the system.
- Cost of the components and service required.
- Management of network.
- Architecture of network.
- Cable type.
- Expandability of the network.
- The desired performance and reliability of entire system. Different types of topologies are: a. Bus Topology b. Ring Topology c. Star Topology d. Mesh Topology e. Tree and Hybrid Topology
Advantages of bus topology
- The bus is simple, reliable in very small network, easy to use and easy to understand.
- It is easy for installation.
- The bus network requires less cables and is therefore less expensive.
- It is easy to extend- (by using BNC Barrel connector) two cables can be joined into one longer cable.
- A repeater can also be used to extend a bus - A repeater boosts the signal and allows it to travel a longer distance.
- If one node fails others are not affected.
Demerits of bus topology
- In case of failure of the backbone cable, the whole network will be affected.
- Heavy network traffic can slow a bus considerably, because only one PC can transmit at any time resulting in wasting a lot of bandwidth as they interrupt each other instead of communicating. The problem can get worse when more PC’s are connected to a network.
- Each barrel connector weakens the electrical signal and too many may prevent the signal from being correctly received along the bus.
- It is difficult to troubleshoot faults - as bus cable breaks or malfunctioning computer.
- A cable break or loose connection also cause reflection and bring down the whole network and causing all the network activity to stop.
- Difficult for reconfiguration-E.g.: Adding new devices may therefore require modification or replacement of the backbone.
Ring Topology In Ring topology each node is connected to the two nearest nodes so the entire network forms a circle. Rings are used in high performance network. A signal is passed along the ring in one direction, from device to device, until it reaches its destination. Each device in the ring works as a repeater. When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along. Since each PC retransmits what it receives a ring is an active network.
Token Ring - Some ring networks use token passing. Token is a short message. A token is passed around the ring until a PC wishes to send information to another PC. That PC modifies the token adds an electronic address and data and sends it around the ring. Each PC in sequence receives the token and the information and passes them to the next PC until either the electronic address of computer matches or the token returns to its origin the receiving PC returns a message to originator that the message has been received. The sending PC than creates another token and begins transmitting the token. The token is circulated until the station is ready to send. E.g.: - FDDI is a fast fiber optic networks based on ring topology. FDDI (Fiber Distributed data interface)
Advantages of ring topology i. A ring is relatively easy to install and configure (for fix number of devices). ii. Fault isolation is simplified - generally in a ring a signal is circulating at all time if any device does not receive a signal within the specified period. It can issue an alarm. Alarm alerts the network operator to the problem of its location. iii. To add or delete a device requires moving only two connections. iv. Time to send data is known: that is package delivery time is fixed and guaranteed because every PC is given to the token. No one PC can monopolies network. v. No data collisions.
Disadvantages of ring
- A single node failure leads to the collapse of the full network.
- Unidirectional traffic can be disadvantage in a simple ring. A break in the ring can disable the entire network; using dual ring can solve the weakness.
- Expansion to the network can cause network disruption
Disadvantages of star topology
- If the central hub fails the whole network fails to operate.
- Many star networks requires a devices at the central point to rebroadcast or switched network traffic.
- It cost more to cable a star networks because all the network cables must be pulled to one central point requiring more cable than other networking topologies.
Mesh topology In a mesh topology every device has dedicated point-to-point link to every other device. The term dedicated means that the link carries only between the two devices it connects. A fully connected mesh network has n (n-1)/2 physical connections to link devices. To accommodate that many links every device on the network must have (n-1) output ports because each device requires an interface for every other on the network. Mesh topology are not usually practical. In addition unless each station frequently sends signal to all the other stations and excessive amount of network bandwidth is wasted. Mesh gets unmanageable beyond a very small number of devices. Most mesh topology networks are not true mesh networks.
Mesh installation Mesh topology N/w become more difficult to install as the no. of devices increases because of the sheer quantity of connections that must be made. A true mesh topology of seven devices would require 21 connections and six I/O ports.
Advantages:- The use of dedicated links guaranties that connections can carry its own data load. Thus eliminating the traffic problem that can occur when links must be shared by multiple devices. Mesh topology is Robust (strong) if one link becomes unusable. It doesn’t incapacitate the entire n/w. Another advantage is privacy and security when every message sent travels along a dedicated line only the intended recipients sees it. Physical boundaries prevent other users from gaining access to message. Point to point link make fault identification and fault isolation easy. Traffic can be routed to avoid links with respected problems. This facility enables the n/w manager to discover the precise location of the fault and aids it finding its cause and solution. Extremely fault tolerant. It is more reliable compare to other topologies. In case of heavy traffic data can be routed around busy root.
Disadvantages As it involves a lot of connection. The total no. of physical links and the no. of I/O ports require to connect will be more and hence is prohibitively expensive. Difficult to install and reconfigure specially as no. of devices increases. Hardware required to connect each device is highly expensive. The sheer bulk of the wiring can be greater than the available space (walls, ceiling and floors) can accommodate. For these reasons a mesh topology is usually implemented in a limited fashion.
Hybrid Topology A network can be hybrid. For example, we can have a main star topology with each branch connecting several stations in a bus topology as shown in Figure
Star Bus (Tree Topology) Star bus topology combines the bus and the star linking several stars hubs together with the bus trunk. If one computer fails, the hub can detect the fault and isolate the PC.
Question Bank:- What is topology? Describe
- Bus topology with diagram and state it’s any two merits and demerits.
- Star topology with diagram and state it’s any two merits and demerits.
- Ring topology with diagram and state it’s any two merits and demerits.
- Mesh topology with diagram and state it’s any two merits and demerits. BTE.Q Describe n/w topology. Draw star topology.(4) State whether bus is active or passive n/w. Justify.(4) Compare bus with ring topology on the basis of cable use and fault tolerance.(4) State four adv. Of bus topology.(4) You are considering n/wing topologies for a n/w for a telemarketing firm under what circumstances would a ring be less appropriate than star. State any two merits of ring topology. Describe token. State whether ring topology is broadcast or point to point n/w? Describe n/w topology. Draw star, bus, topology connecting three star networks each star n/w consist of 4 Pc’s. State any two adv. And disadvantage of star topology. State merits of bus topology. Explain the use of BNC barrel connector in bus topology. You are installing a new n/w for a company that is growing rapidly the current design calls for 40 computers with expansion to 100 in the next six months because of the speed at which the n/w is expected to grow. You want to make sure that trouble shooting will be easy as possible. Considering this factors which topology should be used in the new n/w. Justify your answer. State whether star is active or passive n/w. Justify? Give two advantages of star topology.
2.2 Network control devices
To expand a single network without breaking it into new pass or connecting it through another different network. All networks require devices to provide connectivity and functionality.
Purpose of Using Network Devices. Allow a greater number of nodes to be connected to the network. Extend the distance over which a network can extend. Localize traffic on the network. Can merge existing networks. Isolate network problems so that they can be diagnosed more easily.
Devices and the layers at which they operate
You can usually use one of the following devices. Connectors Hubs Repeaters Bridges Switches Routers Modem, etc.
1. Connectors:
Que. List the different types of connector used in communication? State its uses. To connect cable between two computers. Connectors are of different type such as –
- Twisted Pair cable
- Co-axial Cable
- Fibre optic cable. Connectors are type such as-
- Jacks
- Plugs
- Sockets and ports Example: RS232 and V35 for serial interface RJ45 and BNC connectors for Ethernet. SC or ST connectors for fibre optic
ST (Straight Tip) and SC (Subscriber Connector or Standard Connector) Fibre network segments always require two fibre cables: one for transmitting data, and one for receiving. Each end of a fibre cable is fitted with a plug that can be inserted into a network adapter, hub, or switch. In the North America, most cables use a square SC connector (Subscriber Connector or Standard Connector) that slides and locks into place when inserted into a node or connected to another fibre cable, Europeans use a round ST connector (Straight Tip) instead.
USB (Universal Serial Bus)
Universal Serial Bus, or USB, is a computer standard designed to eliminate the guesswork in connecting peripherals to a PC. It is expected to replace serial and parallel ports. A single USB port can be used to connect up to 127 peripheral devices, such as mice, modems, keyboards, digital camera's, printers, scanners, MP3 players and many more. USB also supports Plug-and-Play installation and hot plugging.
2. Repeaters
A repeater or regenerator is an electronic device that operates on only the physical layer of the OSI MODEL. Signal that carry information within a network can travel a fix distance before attenuation enlarges the integrity of data. A repeater installed on a link receives the signal before it becomes too weak, and put the refresh copy back on the link.
The two sections (segments) connected by the repeater in fig are in reality one network.
Fig.Repeater Advantages: A repeater allows us extending the physical length of a network. The repeaters allow stations to receive the true (regenerated) copy of the frame. It connects two segments of the same network. Types of repeaters are Single port, multi-port repeaters.
Disadvantages: The repeater does not change the functionality of the network in any way. The repeater doesn’t have the intelligence to keep the frame from passing to the right side when it is meant for a station on the left.
Que: Describe repeater? State situations under which it is necessary in network?
3. Hub
Que. What is hub? State how they are classified?
Networks using a Star topology require a central point for the devices to connect. Originally this device was called a concentrator since it consolidated the cable runs from all network devices. The basic form of concentrator is the hub.
As shown in Figure; the hub is a hardware device that contains multiple, independent ports. Most common hubs interconnect Category 3 or 5 twisted-pair cable with RJ- 45 ends. Hubs offer an inexpensive option for transporting data between devices, but hubs don't offer any form of intelligence. Hubs can be active or passive.
4. Bridges
A bridge is used to join two network segments together, it allows computers on either segment to access resources on the other. They can also be used to divide large networks into smaller segments. Bridges have all the features of repeaters, but can have more nodes, and since the network is divided, there is fewer computers competing for resources on each segment thus improving network performance.
Bridges can also connect networks that run at different speeds, different topologies, or different protocols. But they cannot, join an Ethernet (star) segment with a Token Ring segment, because these use different networking standards. Bridges operate at both the Physical Layer and the Data Link layer. Bridges read the MAC header of each frame to determine on which side of the bridge the destination device is located, the bridge then repeats the transmission to the segment where the device is located.
Fig. Bridge
The bridges read the physical location of the source and destination computers from the addresses and store it to a table.
The process works like this For learning, bridges receives all signals from both the segments. The bridge reads the address and discards (filters) all signals from segment1 that are addressed to segment1 because they don’t need to cross the bridge.
The figure shows the messages or signals which do not need to cross the bridge (Message from computer-A to Computer- D) and, other half part shows the messages that needs to pass through the bridge (Message from computer-A to Computer-G). Bridges also provide security through this portioning of traffic. There are basic two types of bridges. Transparent bridges Keeps a table of addresses in memory to determination where to send the data. Also called learning bridges Build a table of MAC addresses as frames arrive. Ethernet networks use transparent bridge Duties are : Filtering frames , forwarding and blocking
Source routing bridge Requires the entire rule to be included in the transmission and don’t rout packets intelligently. IBM token ring n/w uses this type of bridges. If a segment on n/w is been used only 60% then consider, using bridges will improve performance. Used in Token Ring networks Frame contains not only the source and destination address but also the bridge addresses.
Reasons to go for bridges To divide the big n/w like university. Organization may geographically spread over multiple buildings. To split an n/w logical. Single LAN is adequate (sufficient but physical distance is too great). For reliability bridges can be placed at critical nodes. For security insert bridges at various places and being careful not to forward sensitive traffic.
Types of Switch es:
➤ Cut-through switch — In a cut-through switching environment, the packet is forwarded as soon as it is received. This method is very fast, but creates the possibility of errors being propagated through the network, as there is no error checking.
➤ Store-and-forward - In a store-and-forward switching environment, the entire packet is received and error is checked before being forwarded. The advantage of this method is that errors are not propagated through the network. The disadvantage is that the error checking process takes a relatively long time, and store-and-forward switching is considerably slower as a result.
➤ Fragment Free - It combines both cut through switching and Store and forward switching. Fragment Free-switching environment, enough of the packet is read so that the switch can determine whether the packet has been involved in a collision. As soon as the collision status has been determined, the packet is forwarded.
6.Routers
Routers are used to create larger networks by joining two or more network segments. A router can be a dedicated hardware device or a computer system with more than one network interface and the appropriate routing software. All modern network operating systems include the functionality to act as a router. A router derives its name from the fact that it can route data it receives from one network onto another. When a router receives a packet of data, it reads the header of the packet to determine the destination address. Once it has determined the address, it looks in its routing table to determine whether it knows how to reach the destination and, if it does, it forwards the packet to the next hop on the route. The next hop might be the final destination, or it might be another router. As you can see from bellow figure, routing tables play a very important role in the routing process. They are the means by which the router makes its decisions. For this reason, a routing table needs to be two things. o It must be up-to-date, and o it must be complete.
There are two ways that the router can get the information for the routing table through static routing or dynamic routing.
Static Routers: In environments that use static routing , routes and route information are entered into the routing tables manually. Not only can this be a time-consuming task, but also errors are more common. Additionally, when there is a change in the layout, or topology, of the network, statically configured routers must be manually updated with the changes. Again, this is a time consuming and potentially error-laden task. For these reasons, static routing is suited to only the smallest environments with perhaps just one or two routers. A far more practical solution, particularly in larger environments, is to use dynamic routing.
Dynamic Routers:
In a dynamic routing environment, routers use special routing protocols to
communicate.
The purpose of these protocols is simple; they enable routers
to pass on information about themselves to other routers so that other routers can build routing tables.
There are two types of routing protocols used the older distance vector protocols
and the newer link state protocols.
Advantages of routers: o Routers can connect different types of network. o Routes can choose the best path across the network.
o Routers reduce network traffic because they do not retransmit network
broadcast traffic.
