NETWORK TOPOLOGY

Computer networks may be classified according to the network topology upon which the network is based, such as Bus network, Star network, Ring network, Mesh network, Star-bus network, Tree or Hierarchical topology network, etc.

Network Topology signifies the way in which devices in the network see their logical relations to one another. The use of the term "logical" here is significant. That is, network topology is independent of the "physical" layout of the network. Even if networked computers are physically placed in a linear arrangement, if they are connected via a hub, the network has a Star topology, rather than a Bus Topology. In this regard the visual and operational characteristics of a network are distinct; the logical network topology is not necessarily the same as the physical layout.
In communication networks, a topology is a usually schematic description of the arrangement of a network, including its nodes and connecting lines. There are two ways of defining network geometry: the physical topology and the logical (or signal) topology.

The physical topology of a network is the actual geometric layout of workstations. There are several common physical topologies, as described below and as shown in the illustration.
In the bus network topology, every workstation is connected to a main cable called the bus. Therefore, in effect, each workstation is directly connected to every other workstation in the network.
In the star network topology, there is a central computer or server to which all the workstations are directly connected. Every workstation is indirectly connected to every other through the central computer.
In the ring network topology, the workstations are connected in a closed loop configuration. Adjacent pairs of workstations are directly connected. Other pairs of workstations are indirectly connected, the data passing through one or more intermediate nodes.
If a Token Ring protocol is used in a star or ring topology, the signal travels in only one direction, carried by a so-called token from node to node.
The mesh network topology employs either of two schemes, called full mesh and partial mesh. In the full mesh topology, each workstation is connected directly to each of the others. In the partial mesh topology, some workstations are connected to all the others, and some are connected only to those other nodes with which they exchange the most data.
The tree network topology uses two or more star networks connected together. The central computers of the star networks are connected to a main bus. Thus, a tree network is a bus network of star networks.
Logical (or signal) topology refers to the nature of the paths the signals follow from node to node. In many instances, the logical topology is the same as the physical topology. But this is not always the case. For example, some networks are physically laid out in a star configuration, but they operate logically as bus or ring networks.

NETWORK devices

All networks are made up of basic hardware building blocks to interconnect network nodes, such as Network Interface Cards (NICs), Bridges, Hubs, Switches, and Routers. In addition, some method of connecting these building blocks is required, usually in the form of galvanic cable (most commonly Category 5 cable). Less common are microwave links (as in IEEE 802.11) or optical cable ("optical fiber").

Network Interface Cards

 

A network card, network adapter or NIC (network interface card) is a piece of computer hardware designed to allow computers to communicate over a computer network. It provides physical access to a networking medium and often provides a low-level addressing system through the use of MAC addresses. It allows users to connect to each other either by using cables or wirelessly.

Repeaters

A repeater is an electronic device that receives a signal and retransmits it at a higher level or higher power, or onto the other side of an obstruction, so that the signal can cover longer distances without degradation. In most twisted pair ethernet configurations, repeaters are required for cable runs longer than 100 meters.

Hubs

A hub contains multiple ports. When a packet arrives at one port, it is copied to all the ports of the hub for transmission. When the packets are copied, the destination address in the frame does not change to a broadcast address. It does this in a rudimentary way, it simply copies the data to all of the Nodes connected to the hub.

Bridges

A network bridge connects multiple network segments at the data link layer (layer 2) of the OSI model. Bridges do not promiscuously copy traffic to all ports, as hubs do, but learns which MAC addresses are reachable through specific ports. Once the bridge associates a port and an address, it will send traffic for that address only to that port. Bridges do send broadcasts to all ports except the one on which the broadcast was received.
Bridges learn the association of ports and addresses by examining the source address of frames that it sees on various ports. Once a frame arrives through a port, its source address is stored and the bridge assumes that MAC address is associated with that port. The first time that a previously unknown destination address is seen, the bridge will forward the frame to all ports other than the one on which the frame arrived.

Bridges come in three basic types:

1. Local bridges: Directly connect local area networks (LANs)
2. Remote bridges: Can be used to create a wide area network (WAN) link between LANs. Remote bridges, where the connecting link is slower than the end networks, largely have been replaced by routers.
3. Wireless bridges: Can be used to join LANs or connect remote stations to LANs.

Switches

A switch is a device that performs switching. 
Specifically, it forwards and filters OSI layer 2 datagrams (chunk of data communication) between ports (connected cables) based on the Mac-Addresses in the packets. This is distinct from a hub in that it only forwards the datagrams to the ports involved in the communications rather than all ports connected. Strictly speaking, a switch is not capable of routing traffic based on IP address (layer 3) which is necessary for communicating between network segments or within a large or complex LAN. Some switches are capable of routing based on IP addresses but are still called switches as a marketing term. A switch normally has numerous ports with the intention that most or all of the network be connected directly to a switch, or another switch that is in turn connected to a switch.
Switches is a marketing term that encompasses routers and bridges, as well as devices that may distribute traffic on load or by application content (e.g., a Web URL identifier). Switches may operate at one or more OSI layers, including physical, data link, network, or transport (i.e., end-to-end). A device that operates simultaneously at more than one of these layers is called a multilayer switch.
Overemphasizing the ill-defined term "switch" often leads to confusion when first trying to understand networking. Many experienced network designers and operators recommend starting with the logic of devices dealing with only one protocol level, not all of which are covered by OSI. Multilayer device selection is an advanced topic that may lead to selecting particular implementations, but multilayer switching is simply not a real-world design concept.

Routers

Routers are networking devices that forward data packets between networks using headers and forwarding tables to determine the best path to forward the packets. Routers work at the network layer of the TCP/IP model or layer 3 of the OSI model. Routers also provide interconnectivity between like and unlike media (RFC 1812). This is accomplished by examining the Header of a data packet, and making a decision on the next hop to which it should be sent (RFC 1812) They use preconfigured static routes, status of their hardware interfaces, and routing protocols to select the best route between any two subnets. A router is connected to at least two networks, commonly two LANs or WANs or a LAN and its ISP's network. Some DSL and cable modems, for home (and even office) use, have been integrated with routers to allow multiple home/office computers to access the Internet through the same connection. Many of these new devices also consist of wireless access points (waps) or wireless routers to allow for IEEE 802.11b/g wireless enabled devices to connect to the network without the need for a cabled connection.

OSI Model

         The OSI layer was introduced by the International Organization for Standardization (ISO) in 1984 in order to provide a reference model to make sure products of different vendors would interoperate in networks.

·         OSI is short for Open System Interconnection.

·         The OSI layer shows WHAT needs to be done to send data from an application on one computer, trough a network, to an application on another computer, not HOW it should be done.

·         A layer in the OSI model communicates with three other layers: the layer above it, the layer below it, and the same layer at its communication partner.

·         Data transmitted between software programs passes all 7 OSI layers.

·         The Application, Presentation and Session layers are also known as the Upper Layers.

·         The Data Link and Physical layers are often implemented together to define LAN and WAN specifications.

Data Encapsulation

·         Data Encapsulation is the process of adding a header to wrap the data that flows down the OSI model.

·         Each OSI layer may add it's own header to the data received from above. (from the layer above or from the software program 'above' the Application layer.)

·         The 5 Steps of Data Encapsulation are:

·         1. The Application, Presentation and Session layers create DATA from users' input.

·         2. The Transport layer converts the DATA to SEGMENTS

·         3. The Network layer converts the SEGMENTS to PACKETS (or datagrams)

·         4. The Data Link layer converts the PACKETS to FRAMES

·         5. The Physical layer converts the FRAMES to BITS.

·         At the sending computer the information goes from top to bottom while each layers divides the information received from upper layers in to smaller pieces and adds a header. At the receiving computer the information flows up the model discarding the corresponding header at each layer and putting the pieces back together.

(Although the data block shown in the animation below does not change size, it actually gets smaller as it passes down the OSI model until is goes in bits / electrical signals over the physical network cable.)

Application Layer (Layer 7)

·         Provides network services directly to applications. Type of software programs vary a lot: from groupware and web browser to Tactical Ops(video game). Software programs itself are not part of the OSI model.

·         Determines the identity and availability of communication partners, and determines if sufficient resources are available to start program-to-program communication.

·         This layer is closest to the user.

·         Examples of Application layer protocols are:

·         Telnet

·         SMTP

·         FTP

·         SNMP

·         NCP

·         SMB

·         Gateways operate at this layer.

·         Transmits Data.

Presentation Layer (Layer 6)

·         Defines coding and conversion functions.

·         Ensures that information sent from the application layer of one system is readable by the application layer of another system.

·         Includes common data representation formats, conversion of character representation formats, common data compression schemes, and common data encryption schemes, common examples of these formats and schemes are:

·         MPEG, QuickTime

·         ASCII, EBCDIC

·         GIF, TIFF, JPEG

·         Gateways operate at this layer.

·         Transmits Data.

Session Layer (Layer 5)

·         The session layer establishes, manages, maintains and terminates communication channels between software programs on network nodes.

·         Provides error reporting for the Application and Presentation layer.

·         Examples of Session layer protocols are:

·         NFS

·         SQL

·         RPC

·         Zone Information Protocol (ZIP)

·         Gateways operate at this layer.

·         Transmits Data.

Transport Layer (Layer 4)

·         The main purpose of this layers is making sure that the data is delivered error-free and in the correct sequence.

·         Establishes, maintains and terminates virtual circuits.

·         Provides error detection and recovery.

·         Is concerned with reliable and unreliable transport. When using a connection-oriented, reliable transport protocol, such as TCP, acknowledgments are send back to the sender to confirm that the data has been received.

·         Provides Flow Control and Windowing.

·         Provides multiplexing; the support of different flows of data to different applications on the same host.

·         Examples of Transport layer protocols are:

·         TCP (connection-oriented, reliable, provides guaranteed delivery.)

·         UDP (connectionless, unreliable, less overhead, reliability can be provided by the Application layer)

·         Gateways operate at this layer.

·         Transmits Segments.

Network Layer (Layer 3)

·         Defines logical addressing for nodes and networks/segments.

·         Enables internetworking, passing data from one network to another.

·         Defines the logical network layout so routers can determine how to forward packets trough an internetwork.

·         Routing occurs at this layer, hence Routed and Routing protocols reside on this layer.

·         Routed protocols are used to encapsulate data into packets. The header added by the Network layer contains a network address so it can be routed trough an internetwork.

·         Examples of Network layer Routed protocols are:

·         IP

·         IPX

·         AppleTalk

·         Routing protocols are used to create routing tables; routing tables are used to determine the best path / route. Routing protocols provide periodic communication between routers in an internetwork to maintain information on network links in a routing table.

·         Examples of Network layer Routing protocols are:

·         OSPF

·         IGRP/EIGRP

·         RIP

·         BGP

·         NLSP

·         Transmits Packets.

·         Routers operate at this layer.

Data Link Layer (Layer 2)

·         Defines psychical addressing, network topology, and is also concerned with error notification, sequencing of frames and flow control.

·         Examples of network topologies are:

·         Star

·         Bus

·         Ring

·         Physical addresses are also known as hardware and BIA's (Burned In Addressess) but most commonly as MAC addresses.

·         Examples of Data Link LAN specifications are:

·         Ethernet

·         FastEthernet

·         Token Ring

·         FDDI

·         Examples of Data Link WAN specifications are:

·         Frame Relay (operates also on the Physical layer)

·         PPP (operates also on the Physical layer)

·         X.25 (operates also on the Physical and Network layer)

·         Transmits Frames.

·         Bridges and Switches operate at this layer.


The Data Link layer consists of two sublayers:

·         LCC (Logical Link Control) Layer

·         Manages communication between devices over a single link of a network.

·         Enables multiple higher-layer protocols to share a single physical data link.

·         MAC Layer

·         Manages protocol access to the physical network medium.

·         Determines hardware addresses.

Physical Layer (Layer 1)

·         The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems.

·         Physical layer specifications define characteristics such as:

·         voltage levels

·         timing of voltage changes

·         physical data rates

·         maximum transmission distances

·         physical connectors

·         Physical layer implementations can be categorized as either LAN or WAN specifications.

·         Examples of LAN specifications are:

·         Ethernet

·         FastEthernet

·         Token Ring

·         FDDI

·         Examples of WAN specifications are:

·         HSSI

·         V.24

·         V.35

·         BRI

·         SLIP

·         RS-232

·         Transmits bits. (bitstream)

·         Repeaters operate at this layer.

Modem

A modem is like a telephone for a computer to communicate with other computers over telephone lines. Broadly, a modem’s performance depends on the characteristics of the modem and those of the telephone line it is working with. The condition of telephone lines varies widely across the country. So how your modem works will depend to a large extent on the telephone line.

There’re several different types of noise that can happen either individually or in combination in a telephone line. Different modems react differently to these. And there’s no way you can identify which of these conditions affect your line. So nobody can correctly predict which modem will work the best on your line.
Test it on your phone line
There’s no other way out. You’ll have to convince your vendor to test the modem at your place, and to take it back if it does not work properly.

Internal or external?
If you’re buying a modem separately, go for an external fax-modem. External ones are easier to install. They cost Rs 2,000 upward. Buy the cheapest one that works okay with your line, giving you a steady high-speed connect for at least an hour.
If you get an internal modem card pre-installed when you buy a new PC, that’s okay, if it’s a 56 kbps fax-modem. An internal card does have one advantage: it reduces clutter. Again, it helps to check that it’s working okay on your line, else you may need to change it
Other modem features
True plug-and-play with USB
If you’re looking for swift installs and convenience, go for a USB modem. They cost about Rs 4,000.
The answering machine modem
If you’re looking for a modem that’s also a digital answering machine and a fax machine, go for a message modem. Message modems can store voice messages and faxes even while the PC is switched off. Once the computer is switched on, these messages are transferred to it and can then be heard or viewed. Some also let you retrieve the messages from a remote location—all you have to do is call up your message modem.

Ethernet

Cables

Cable is the medium through which information usually moves from one network device to another. There are several types of cable which are commonly used with LANs. In some cases, a network will utilize only one type of cable; other networks will use a variety of cable types. The type of cable chosen for a network is related to the network's topology, protocol, and size. Understanding the characteristics of different types of cable and how they relate to other aspects of a network is necessary for the development of a successful network. The following sections discuss the

types of cables used in networks and other related topics.

Unshielded Twisted Pair (UTP) Cable

Twisted pair cabling comes in two varieties: shielded and unshielded. Unshielded twisted pair (UTP) is the most popular and is generally the best option for school networks (See fig. 1).

Fig.1. Unshielded twisted pair

The quality of UTP may vary from telephone-grade wire to extremely high-speed cable. The cable has four pairs of wires inside the jacket. Each pair is twisted with a different number of twists per inch to help eliminate interference from adjacent pairs and other electrical devices. The tighter the twisting, the higher the supported transmission rate and the greater the cost per foot. The EIA/TIA (Electronic Industry Association/Telecommunication Industry Association) has established standards of UTP and rated five categories of wire.

Categories of Unshielded Twisted Pair

Type Use Category 1 Voice Only (Telephone Wire) Category 2 Data to 4 Mbps (LocalTalk) Category 3 Data to 10 Mbps (Ethernet) Category 4 Data to 20 Mbps (16 Mbps Token Ring) Category 5 Data to 100 Mbps (Fast Ethernet)

Buy the best cable you can afford; most schools purmyoung Category 3 or Category 5. If you are designing a 10 Mbps Ethernet network and are considering the cost savings of buying Category 3 wire instead of Category 5, remember that the Category 5 cable will provide more "room to grow" as transmission technologies increase. Both Category 3 and Category 5 UTP have a maximum segment length of 100 meters. In Florida, Category 5 cable is required for retrofit grants. 10BaseT refers to the specifications for unshielded twisted pair cable (Category 3, 4, or 5) carrying Ethernet signals. Category 6 is relatively new and is used for gigabit connections.

Unshielded Twisted Pair Connector

The standard connector for unshielded twisted pair cabling is an RJ-45 connector. This is a plastic connector that looks like a large telephone-style connector (See fig. 2). A slot allows the RJ-45 to be inserted only one way. RJ stands for Registered Jack, implying that the connector follows a standard borrowed from the telephone industry. This standard designates which wire goes with each pin inside the connector.

Fig. 2. RJ-45 connector

Shielded Twisted Pair (STP) Cable

A disadvantage of UTP is that it may be susceptible to radio and electrical frequency interference. Shielded twisted pair (STP) is suitable for environments with electrical interference; however, the extra shielding can make the cables quite bulky. Shielded twisted pair is often used on networks using Token Ring topology.

Coaxial Cable

Coaxial cabling has a single copper conductor at its center. A plastic layer provides insulation between the center conductor and a braided metal shield (See fig. 3). The metal shield helps to block any outside interference from fluorescent lights, motors, and other computers.

Although coaxial cabling is difficult to install, it is highly resistant to signal interference. In addition, it can support greater cable lengths between network devices than twisted pair cable. The two types of coaxial cabling are thick coaxial and thin coaxial.

Thin coaxial cable is also referred to as thinnet. 10Base2 refers to the specifications for thin coaxial cable carrying Ethernet signals. The 2 refers to the approximate maximum segment length being 200 meters. In actual fact the maximum segment length is 185 meters. Thin coaxial cable is popular in school networks, especially linear bus networks.

Thick coaxial cable is also referred to as thicknet. 10Base5 refers to the specifications for thick coaxial cable carrying Ethernet signals. The 5 refers to the maximum segment length being 500 meters. Thick coaxial cable has an extra protective plastic cover that helps keep moisture away from the center conductor. This makes thick coaxial a great choice when running longer lengths in a linear bus network. One disadvantage of thick coaxial is that it does not bend easily and is difficult to install.

Coaxial Cable Connectors

The most common type of connector used with coaxial cables is the Bayone-Neill-Concelman (BNC) connector (See fig. 4). Different types of adapters are available for BNC connectors, including a T-connector, barrel connector, and terminator. Connectors on the cable are the weakest points in any network. To help avoid problems with your network, always use the BNC connectors that crimp, rather than screw, onto the cable.

Fig. 4. BNC connector

Fiber Optic Cable

Fiber optic cabling consists of a center glass core surrounded by several layers of protective materials (See fig. 5). It transmits light rather than electronic signals eliminating the problem of electrical interference. This makes it ideal for certain environments that contain a large amount of electrical interference. It has also made it the standard for connecting networks between buildings, due to its immunity to the effects of moisture and lighting.

• Outer insulating jacket is made of Teflon or PVC.
• Kevlar fiber helps to strengthen the cable and prevent breakage.
• A plastic coating is used to cushion the fiber center.
• Center (core) is made of glass or plastic fibers.

Fiber Optic Connector

The most common connector used with fiber optic cable is an ST connector. It is barrel shaped, similar to a BNC connector. A newer connector, the SC, is becoming more popular. It has a squared face and is easier to connect in a confined space.

Ethernet Cable Summary

Specification	Cable Type	Maximum length
10BaseT	Unshielded Twisted Pair	100 meters
10Base2	Thin Coaxial	185 meters
10Base5	Thick Coaxial	500 meters
10BaseF	Fiber Optic	2000 meters
100BaseT	Unshielded Twisted Pair	100 meters
100BaseTX	Unshielded Twisted Pair	220 meters

Wireless LANs

Not all networks are connected with cabling; some networks are wireless. Wireless LANs use high frequency radio signals, infrared light beams, or lasers to communicate between the workstations and the file server or hubs. Each workstation and file server on a wireless network has some sort of transceiver/antenna to send and receive the data. Information is relayed between transceivers as if they were physically connected. For longer distance, wireless communications can also take place through cellular telephone technology, microwave transmission, or by satellite.

Wireless networks are great for allowing laptop computers or remote computers to connect to the LAN. Wireless networks are also beneficial in older buildings where it may be difficult or impossible to install cables.

The two most common types of infrared communications used in schools are line-of-sight and scattered broadcast. Line-of-sight communication means that there must be an unblocked direct line between the workstation and the transceiver. If a person walks within the line-of-sight while there is a transmission, the information would need to be sent again. This kind of obstruction can slow down the wireless network.

Scattered infrared communication is a broadcast of infrared transmissions sent out in multiple directions that bounces off walls and ceilings until it eventually hits the receiver. Networking communications with laser are virtually the same as line-of-sight infrared networks.

Wireless LANs have several disadvantages. They provide poor security, and are susceptible to interference from lights and electronic devices. They are also slower than LANs using cabling.

Installing Cable - Some Guidelines

When running cable, it is best to follow a few simple rules:

• Always use more cable than you need. Leave plenty of slack.
• Test every part of a network as you install it. Even if it is brand new, it may have problems that will be difficult to isolate later.
• Stay at least 3 feet away from fluorescent light boxes and other sources of electrical interference.
• If it is necessary to run cable across the floor, cover the cable with cable protectors.
• Label both ends of each cable.
• Use cable ties (not tape) to keep cables in the same location together.