Describe the uses, hardware components, implementation standards and media types for major topologies used in Local Area Networks

Topology

Describe the uses, hardware components, implementation standards and media types for major topologies used in Local Area Networks.

LAN components may be connected in various ways. The way the cable is laid and the way the signals are transmitted are known as network topologies. This learning outcome will explain the different topologies and standards in common use.

Describe the major Local Area Network topologies.

There is a distinction between the Physical, Electrical, and Logical Topologies of a network.

· Physical - describes what the user sees and is normally based upon seeing a single "cable".

· Electrical - describes the flow of data which travels along conductors within the physical cable.

· Logical - describes the users view of the network.

There are basically two types of Local Area Networks (LAN):

· bus network

· token ring network

Bus networks consist of various different topologies.

· bus topology

· star topology

· mesh topology

Bus Topology

A bus topology connects each computer to a single cable. At each end of the cable is a terminating resistor, or a terminator. A signal is passed back and forth along the cable past the workstations and between the two terminators.

The "bus" carries a message from one end of the network to the other. As the bus passes each workstation, the workstation checks the destination address on the message. If the address in the message matches the workstation's address, the workstation receives the message. If the address doesn't match, the bus carries the message to the next workstation, and so on.

One type of bus network, called a local bus, is shown below. A local bus uses a T-connector to connect the cable to the workstation's network adapter card. A terminator is connected to the last T-connector at each end of the network.

A local bus network connects the network cable directly to each computer using T-connectors.

A bus network, as shown below, uses drop cables to connect each workstation to the main "backbone" cable.

A regular bus network uses drop cables to connect each computer to the network cable.

Bus topology is a passive network, meaning that workstations only listen for data being sent on the network and are not responsible for moving data from one workstation to the next. If one workstation fails, it doesn't affect the entire LAN. On the other hand, if the connection to one workstation comes loose, or if a cable breaks, the entire cable segment (the length between two terminators) loses its connectivity, causing the entire segment to be nonfunctional until it can be repaired.

The following table lists some advantages and disadvantages of bus networks.

Advantages	Disadvantages
Failure of a single workstation doesn't affect the entire LAN	Cable break can affect large number of users
Easy cable connections; flexible	Limited cable length and number of workstations
Inexpensive cable and connectors	Difficult to isolate network cabling errors
	Performance degradation is not graceful

Star Bus Topology

In a network using a star configuration, each workstation is connected to a special unit called a hub. The hub provides a common connection so that all of the computers can communicate with one another.

Star bus topology uses signal splitters in the hub to send out signals in different directions on the cable connections. Both active and passive hubs are allowed. Active hubs can transmit a stronger signal to feed a longer cable and/or more signal splitters.

Each computer on a star network is connected to a hub. Hubs may be connected together to extend the network.

The following table lists some advantages and disadvantages of star bus topology.

Advantages	Disadvantages
Easy to add new workstations	Failure of hub cripples the workstations connected to the hub
Central monitoring and network management

Token-Ring Topology

On a token-ring network, workstations are situated on a continuous network loop on which a "token" is passed from one workstation to the next. Although the name token ring implies a ring, the token ring is physically implemented as a star and electrically a ring. Workstations are centrally connected to a hub called a Multi Station Access Unit (MSAU) and are wired in a star configuration. Workstations use a token to transmit data and must wait for a free token in order to transfer messages.

A computer communicates on a token-ring network by "grabbing" the token and sending it around an electric ring.

The token contains the address of the sender and the address of the recipient. When the recipient has copied the information from the received message, it returns the token to the originating workstation to verify that the message was received. The original sender then passes the token to the next workstation in the ring so that workstation can send information over the network.

The following table lists some advantages and disadvantages of a token-ring topology.

Advantages	Disadvantages
Cable failures affect a small number of users	Costly wiring and connections
Equal access for all workstations
Graceful performance degradation as the size of the network grows

Describe the typical uses for the various topologies and explain why they are used.

Each topology has advantages and disadvantages. When considering the way a topology may be used or the historical significance of a topology one can examine the following properties.

Impact of cable breaks on a network. Cable breaks can cause a serious problem for a badly designed network. In the case of a local bus, all workstations would immediately be off the network until such time as the break was located and repaired. However, if cable access is not a problem, on small networks, cost and ease of installation usually override this problem.

If cable access is difficult and/or expediency of repair is needed, a star network offers more protection from cable break network failures, and the breaks are also easier to locate.

Impact of adding or removing nodes. The addition and removal of nodes depends to a certain extent upon the location. If a node can simply be added or removed from the end of a local bus network, then this by far is the easiest. But if there is a likelihood that nodes might need to be added or removed indiscriminately throughout the network, then a starbus would be more appropriate.

Flow of messages and which nodes see the messages. On a bus network, both the local bus and starbus normally transmit all messages to all stations. If more control is needed over the flow of messages, a token ring could be used.

Ability to use nodes as repeaters. Normally the hub of a starbus is a repeater and further extensions to the network can easily be made by linking or cascading another hub.

With both networks types further extensions or combinations are possible by using various devices such as bridges, routers, switches etc.

Maximum physical size of the network. It is unusual to use a local bus network on medium to large networks as management can become nearly impossible.

Amount of cable used. There is a tradeoff with small networks in that although local bus networks use more cable in most cases over starbus, no hub is needed.

There is no single answer to which is the best topology and each situation has to be looked at on its own merits. The final choice could be either one or more likely a combination of topologies that depend upon, need, cost, speed, and reliability.

Identify the hardware components of the major topologies and their functions.

LAN nodes usually consist of PC workstations, file servers, and printers. These are normally connected with some sort of cable. To connect the cable to the nodes there are various devices.

Network Interface Cards (NIC)

A NIC is used to attach PC's to the network. In most cases, the cable is attached directly to the NIC. The NIC is responsible for receiving and transmitting data between the PC and the network.

In the case of 10Base5 however NIC is typically equipped with a female 15-pin connector and sliding latch for an AUI cable attachment.

BNC-Tee Connectors

"Bayonet Neill-Concelman Tee connector" - a type of connector used for attaching coax cable to a NIC in bus topology networks. They can be attached or detached quicker than connectors that screw. ThinWire Ethernet (IEEE 802.3 10BASE2) is used with BNC T connectors.

Terminator

For 10Base2 and 10Base5 a Type N 50 (50 ohm) terminating resistor is needed. The terminator is fitted to the final Tee in the segment.

Patch C
ables

Typically, UTP cables terminate at some sort of wall plate. This necessitates a cable to plug into the wall plate and connect to the network device. This patch cable should only be made from twisted-pair patch cables rated for use in twisted-pair Ethernet systems.

Hubs and MSAUs

A hub is a common wiring point for star-topology networks, and is a common synonym for concentrator (though the latter generally has additional features or capabilities). Arcnet, 10BaseT Ethernet and 10BaseF Ethernet and many proprietary network topologies use hubs to connect multiple cable runs in a star-wired network topology into a single network. Token-ring MSAUs (Multi-Station Access Units) can also be considered a type of hub. Hubs have multiple ports to attach the different cable runs. Some hubs (such as 10BaseT and active Arcnet) include electronics to regenerate and retime the signal between each hub port. Others (such as 10BaseF or passive Arcnet) simply act as signal splitters, similar to the multi-tap cable-TV splitters you might use on your home antenna coax (of course, 10BaseF uses mirrors to split the signals between cables). Token-Ring MSAUs use relays (mechanical or electronic) to reroute the network signals to each active device in series, while all other hubs redistribute received signals out all ports simultaneously, just as a 10Base2 multi-port repeater would.

Transceivers

Transceivers are used to connect nodes to the various Ethernet media. Transceivers, also known as Media Attachment Units (MAUs), attach to the Ethernet cable and provide an Application User Interface, or AUI, connector for the computer. The MAU is generally attached directly to the computer's AUI connector, or the MAU may be attached to the computer with a specially shielded AUI cable. In addition to an AUI connector, many computers and interface network cards also contain a built-in 10BASE-T or 10BASE2 MAU, allowing them to be connected directly to Ethernet without requiring an external MAU. The AUI connector consists of a 15 pin D-shell type connector, female on the computer side, male on the MAU side. Many Ethernet-compatible computers provide such an AUI connector.

The most popular attachment mechanism (MDI) for a 10BASE5 MAU is sold by AMP Corporation, and consists of a metal and plastic clamp that makes a direct physical and electrical connection to the coaxial cable. This clamp is also called a transceiver tap, since to install the clamp you must drill a hole into the thick coaxial cable in a process known as tapping the cable. Since this clamp may be installed while the network is active, it is also called a "non-intrusive" tap.

The external MAU is equipped with a male 15-pin AUI connector that has two locking posts, providing an attachment point for a sliding latch connector. The MAU is powered from the Ethernet interface. The specifications state that a MAU may draw as much as 0.5 AMP (1/2 AMP) of 12 volt DC power.

MAU

Medium Access Unit, an IEEE term for a transceiver. MAU is also commonly [mis]used to describe a Token-Ring Multi-Station Access Unit (MSAU).

AUI Cable

This cable can be used to provide the connection between an external MAU and the Ethernet interface. The MAU is equipped with a male 15-pin connector with locking posts, and the workstation is equipped with a female 15-pin connector.

The AUI cable, in turn, has a female 15-pin connector on one end that is attached to the MAU. The other end of the AUI cable has a male 15-pin connector that is attached to the workstation.
The AUI cable carries three data signals between the workstation and MAU:

· transmit data (from the Ethernet interface to the network),

· receive data (from the network to the interface), and

· collision presence signal (from the network to the interface).

Each signal is sent over a pair of wires. Another pair of wires are used to carry 12 volt DC power from the workstation to the MAU.

Ethernet Switches

Ethernet switches increase overall network throughput by dividing a large single network into smaller segments. The switch filters unnecessary traffic from individual segments, thus making them faster and more efficient. When a packet is received by the switch, the switch determines the destination and source segments. If the segments are the same, the packet is dropped; if they are different, then the packet is forwarded. It is important to remember that all of the forwarding and filtering is done at the MAC layer, independent of any concern for network protocols

Repeater

http://www.kmj.com/fast/cot4.html

http://www.kmj.com/fast/cot4.htmlRepeaters are used to connect two or more Ethernet segments of any media type. A repeater acts on a purely electrical level to connect to segments. All it does is amplify and reshape (and, depending on the type, possibly retime) the analog waveform to extend network segment distances. It does not know anything about addresses or forwarding, thus it cannot be used to reduce traffic as a bridge can.

Repeaters also monitor all connected segments for basic characteristics necessary for Ethernet to run correctly. When these conditions are not met on a particular segment, for example when a break occurs, all segments in an Ethernet may become inoperable. Repeaters limit the effect of these problems to the faulty section of cable by "segmenting" the network, disconnecting the problem segment and allowing unaffected segments to function normally. A segment malfunction in a point to point network will generally only disable a single computer, where the same problem in a bus topology would disable all nodes attached to that segment.

Bridge

A bridge will connect to distinct segments (usually referring to a physical length of wire) and transmit traffic between them. This allows you to extend the maximum size of the network while still not breaking the maximum wire length, attached device count, or number of repeaters for a network segment.

There are also learning bridges that monitor MAC (OSI layer 2) addresses on both sides of its connection and attempts
to learn which addresses are on which side. It can then decide when it receives a packet whether it should cross the bridge or stay local (some packets may not need to cross the bridge because the source and destination addresses are both on one side). If the bridge receives a packet that it doesn't know the addresses of, it will forward it by default.

Remote bridges are as described above but have an Ethernet interface on one side and a serial interface on the other. It would connect to a similar device on the other side of the serial line. Most commonly used in WAN links where it is impossible or impractical to install network cables. A high-speed modem and intervening telephone lines or public data network would be used to connect the two remote bridges together.

Bridge capabilities are defined in IEEE specification 802.1D. In addition to store-and-forward filtering and forwarding, bridges also support the Spanning Tree Algorithm for dealing with potential bridging loops between networks. This allows a configuration of redundant return paths. The Spanning Tree is a technique whereby all of the bridges communicate information on the topology of the network to insure that there are no bridging "loops" present (see following section on "Spanning Tree" for greater detail). The distinction between multiport bridges and switches which support 802.1 D features sometimes is blurred in product descriptions these days.

Routers

Routers work much like bridges, but they pay attention to the upper network layer protocols (OSI layer 3) rather than physical layer (OSI layer 1) or (OSI layer 2) protocols.This means that they divide the network logically instead of physically. A router will decide whether to forward a packet by looking at the protocol level addresses (for instance, TCP/IP addresses) rather than the MAC address. Because routers work at layer 3 of the OSI stack, it is possible for them to transfer packets between different media types (i.e., leased lines, Ethernet, token ring, X.25, Frame Relay and FDDI). Many routers can also function as bridges.

When selecting between a router and a bridge, your network layout, type and amount of hosts and traffic, and other issues (both technical and non-technical….some protocols are non-routable i.e., NetBIOS, DEC LAT, etc.) must be considered. Routing would always be preferable to bridging except that routers are slower and usually more expensive (due to the amount of processing required to look inside the physical packet and determine which interface that packet needs to get sent out).

Brouters is a term sometimes used to describe devices which have both bridging and routing capability, however switches and bridges frequently have some router-like features such as selective protocol filtering.

Identify standards which relate to the implementation of major network topologies

With the invention of Ethernet, there was a need to try to standardise the system. The IEEE was assigned to come up with a set of standards. They formed a group to work on the problem, this group is known as the IEEE802 group. Two of the standards that have come out of that group that are of primary interest to this course are the:

· IEEE 802.3 - Ethernet Standards

· IEEE 802.5 - Token Ring Standards

The IEEE 802 standards are a complex set of standards that define the required properties of the LAN including Communications Media on which we will be focusing.

The IEEE 802.3 specifications describe rules for the maximum number of repeaters that can be used in a configuration. The maximum number of repeaters that can be found in the transmission path between two nodes is four; the maximum number of network segments between two nodes is five, with a further restriction that no more than three of those five segments may have other network stations attached to them (the other segments must be inter-repeater links which simply connect repeaters).

These rules are determined by calculations of maximum cable lengths and repeater delays. Networks which violate these rules may still be functional, but they are subject to sporadic failures or frequent problems of an indeterminate nature. Additionally, using repeaters simply extends a network to a larger size. As this occurs, available bandwidth on the network could become a problem.

An excellent resource regarding the ethernet standards can be found at the University of Texas.

A brief description of the two standards can be here.

100BASE-T

A new set of standards is currently under development. It is a set of proposals to the IEEE 802.3 for 100Mbps Ethernet. It consists of Unshielded Twisted Pair Cat. 5 cable. Each cable segment can only be connected to two nodes, and has a maximum length between nodes of 100m.

More information regarding 100 BaseT standards can also be found at the University of Texas site.

Identify media types commonly associated with particular network topologies

Ethernet Bus Networks

Common cabling for ethernet bus networks is coaxial cable. There are two types of ethernet based coaxial cable.

· 10BASE2

A wiring scheme which uses a type of (thin) coaxial cable for use in Ethernet. Primarily used in office environments. It has advantages over the general purpose UTP when wiring systems on one floor in an open environment. Thin wire cabling provides greater distance (185 metres), allows daisy chaining and offers higher noise immunity.

· Transmission Rate - 10 Mbps.

· Maximum Length - 185 metres/segment.

· Impedance - 50 ohm RG58, conductor diameter - 0.9 mm.

· BNC connections.

· 0.5 metre between consecutive connections.

· Up to 30 nodes per segment.

· 10BASE5

Standard Ethernet baseband coaxial cable which serves as a backbone transmission medium for the LAN. Primarily used for facility-wide installations. It is installed as a physical bus linking one communication closet to another communication closet.

· Transmission Rate - 10 Mbps.

· Maximum Length - 500 metres/segment.

· Impedance - 50 ohm, conductor diameter - 2.17 mm.

· Uses transceivers and AUI (Attachment Unit Interface) cable.

· Up to 100 nodes per segment.

· Total Maximum Extended Length (by Repeaters) - 1500 metres.

Ethernet Star Networks

Common cabling for ethernet star networks is Unshielded Twisted Pair cable or 10BASET cable. Attributes of such cable are:

· Transmission Rate - 10 Mbps for Category 3, up to 100Mbps for Category 5.

· Maximum Length - 100 metres/segment.

· Cable connectors used for this type of cabling are called RJ45

Token Ring Networks

Token ring networks can be designed using various cabling options, the specifications for each appear below:

Cable Type	4 Mbs	16 Mbs
Type 1 or Type 9: (STP)	375 Metres	145 Metres
Level 4 and 5: (UTP)	300 Metres	120 Metres
Type 3: (UTP)	170 Metres	100 Metres