Topology

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CHAPTER TEN (10)CHAPTER TEN (10)

LOCAL AREA NETWORK

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Learning ObjectivesLearning Objectives

After completing this chapter you will be able to:-– Components of LAN

- Topology– Describe Project 802.– Explain IEEE 802.3 (Ethernet), IEEE 802.4 (Token Bus),

IEEE 802.5 (Token Ring) network data transports and media specifications.

– Understand the access methods used in Ethernet and Token Ring.

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Chapter OutlineChapter Outline

IntroductionTopologyProject 802EthernetFast EthernetToken BusToken Ring

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IntroductionIntroduction

Computer Network – a set of devices (nodes) connected by communication links creating data routes where we share and exchange information.

Categories of Network: Local Area Network (LAN) Metropolitan Area Network (MAN) Wide Area Network (WAN) Internetworks (internet)

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MANMAN

Designed to extend over an entire city.It may be a single network (cable television

network) or it may be a means of connecting a number of LANs into larger network so that resources may be shared LAN-to-LAN or device-to-device.

Example: (Telekom, ASTRO)

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MANMAN

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WANWAN

Provides long-distance transmission of data, voice, image, and video information over large geographical areas that may comprise a country, a continent, or even the whole world.

WAN that is wholly owned by a single company is often referred to as an enterprise network.

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WANWAN

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internet vs. Internetinternet vs. Internet

internet ~ two or more networks connected by the use of internetworking devices such as routers or gateways.

Internet ~ specific worldwide network using TCP/IP protocol suite.

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Internetwork (internet)Internetwork (internet)

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LANLAN

A local area network (LAN) is a data communication system that allows a number of independent devices to communicate directly with each other in a limited geographic area.

It is usually privately owned within a single office, building or campus of up to a few kilometers in size.

It is widely used to connect personal computers and workstations in home, company offices and factories to share resources and exchange information.

The resources to be shared can include hardware (e.g. printers), software (e.g. application program) or data.

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CharacteristicsCharacteristics

LANs are distinguished from other types of networks by their size, transmission technology and topology.

They are restricted in size, which means that the worst-case transmission time is bounded and known in advance.

Traditionally, LANs have data rates in the 4 to 100 Mbps range, however, newer LANs may operate at higher speeds, up to gigabits/second.

Various topologies are possible for broadcast LANs, and the most common are bus, star and ring.

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TopologyTopology

Topology ~ refers to the way network is laid out, either physically or logically.

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 links and linking devices (nodes) to each other.

i.e. mesh, star, bus, ring, tree, hybrid

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Mesh TopologyMesh Topology

Every device has a dedicated point-to-point link to every other device.

Dedicated means that the link carries traffic only between the two devices it connects.

A fully connected mesh network has n(n-1)/2 physical channels to link n devices.

To accommodate that many links, every device on the network must have n –1 input/output ports.

I.e. telephone network

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Mesh Topology (example)Mesh Topology (example)

The Big Pond Company has a fully connected mesh network consisting of 5 devices. Calculate the total number of cable links needed and the number of ports for each device.

Solution:– # of links = n(n-1)/2 = 5(4)/2 = 10 links– # of ports per device = n-1 = 5-1 = 4 ports

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Mesh Topology AdvantageMesh Topology Advantage

Eliminating traffic problems ~ each connection can carry its own data load.

Robust ~ if one link fails, it does not anticipate the entire system.

Privacy or security ~ when every message sent travels along a dedicated link, only the intended recipient sees it.

Fault identification & isolation easy ~ traffic can be routed to avoid links with suspected problems.

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Mesh Topology DisadvantageMesh Topology Disadvantage

The amount cabling and I/O ports required ~ every device must be connected to every other device.

Installation and reconfiguration are difficult.Need a lot of space since bulk of wiring

required.Expensive ~ the hardware required to

connect each link (I/O ports and cable) are expensive.

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Star TopologyStar Topology

Each device has a dedicated point-to-point link only to a central controller/ concentrator, usually called a hub.

The controller acts as an exchange; if one device wants to send data to another, it sends data to the controller, which then relays the data to the other connected device.

Star topology does not allow direct traffic between devices.

I.e. 10BaseT

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Star Topology AdvantageStar Topology Advantage

Less expensive than a mesh topology ~ each device needs only one link and one I/O port to connect it to any number of others.

Easy to install and reconfigure ~ involve only one connection between that device and the hub.

Robustness ~ if one link fails, only that link is affected; all other links remain active.

Easy fault identification & isolation ~ hub can be used to monitor link problems and bypass defective links.

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Bus TopologyBus Topology

Multipoint configuration, where one long cable acts as a backbone to link all the devices in the network.

Backbone ~ the major transmission path in a network.

Nodes are connected to the bus cable by drop lines and taps.

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.

The end of the bus cable, must be terminated with a resistive load or terminating resistors.

I.e. 10Base2

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Bus Topology AdvantageBus Topology Advantage

Ease of installation ~ backbone cable can be laid along the most efficient path, then connected to the nodes by drop lines of various lengths.

Cheap ~ Less cabling is needed than mesh and star topology

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Bus Topology DisadvantageBus Topology Disadvantage

Difficult reconfiguration ~ difficult to add new device since require modification or replacement of the backbone.

fault isolation is difficult ~ a fault or break in the bus cable stops all transmission.

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Ring TopologyRing Topology

Each device has a dedicated point-to-point line configuration only with the two devices on either side of it.

A signal is passed along the ring in one direction, from one device to device, until it reaches its destination.

Each device in the ring incorporates a repeater; when a device receives a signal intended for another device, its repeater regenerates the bits and passes them along.

I.e. Token Ring (IEEE 802.5)

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Ring Topology AdvantageRing Topology Advantage

Easy to install and reconfigure ~ each device is linked only to its neighbors. To add or delete a device requires moving only two connections.

Easy fault identification & isolation ~ a device capable of issuing an alarm to alert the network operator to the problem and its location.

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Ring Topology DisadvantageRing Topology Disadvantage

Unidirectional traffic ~ a break in the ring (disable station) can disable the entire network.

Note: this weakness can be solved by using a dual ring or a switch capable of closing off the break.– I.e. FDDI (using dual ring)

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Wireless TopologyWireless Topology

There’s no topology for wireless network.

Nodes can be everywhere but it must be in the range that can be access between the access point.

Example: IEEE 802.11

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Project 802Project 802

In 1985, the Computer Society of the IEEE started a project, called Project 802, to set standards to enable intercommunication between equipment from a variety of manufacturers.

It covers the first two layers of the OSI model and part of the third level.

Project 802 has split the data link layer into two different sublayers:– Logical link control (LLC)– Media access control (MAC)

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Project 802 & OSI ModelProject 802 & OSI Model

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Logical Link Control (LLC)Logical Link Control (LLC)

IEEE 802.2 LLC is the upper sublayer of the data link layer.

The functions include the logical address, control information, and data.

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Medium Access Control (MAC)Medium Access Control (MAC)

MAC is the lower layer of the DLL layer.MAC sublayer resolves the contention for

the shared media.It contains the synchronization, flag, flow,

and error control specifications necessary to move information from one place to another, as well as the physical address of the next station to receive and route a packet.

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ArchitectureArchitecture

IEEE 802 are subdivided into several functions, identified by:– a number

• 802.1 ~ internetworking• 802.2 ~ LLC

– The MAC module• 802.3 ~ CSMA/CD (Ethernet)• 802.4 ~ Token Bus• 802.5 ~ Token Ring• Others (802.6 ~ DQDB, 802.11 ~ Polling)

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Project 802 ArchitectureProject 802 Architecture

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IEEE 802.1~ InternetworkingIEEE 802.1~ Internetworking

The section of project 802 devoted to internetworking issues in LANs and MANs.

It seeks to resolve the incompatibilities between network architectures without requiring modifications in existing addressing, access, and error recovery mechanisms, among others.

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802.2 ~LLC802.2 ~LLC

IEEE 802.2 LLC is the upper sublayer of the data link layer.

The functions include the logical address, control information, and data.

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IEEE 802.3IEEE 802.3

It supports a LAN standard originally developed by Xerox and later extended by a joint venture between Digital Equipment Corporation, Intel Corporation and Xerox.

This was called Ethernet and the access method used is CSMA/CD.

IEEE 802.3 therefore is known as Ethernet or CSMA/CD.

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IEEE 802.3 CategoriesIEEE 802.3 Categories

IEEE 802.3 defines two categories:– Baseband ~ base specifies digital signal– Broadband ~ broad specifies analog signal

IEEE divides the baseband category into several different standards:– 1Base5, 10Base5, 10Base2, 10BaseT, 100BaseT

IEEE defines only one specification for the broadband catergory:– 10Broad36

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IEEE 802.3 CategoriesIEEE 802.3 Categories

1Base5, 10Base2, 100BaseT, 10Broad36 The first number (1,10,100) indicates the data rate

in Mbps. The base or broad indicates type of signal. The last number or letter (1,2,5,T) indicates the

maximum cable length or the type of cable. The maximum cable length restriction can be

changed using network devices such as repeaters or bridges.

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IEEE 802.3 Access MechanismIEEE 802.3 Access Mechanism

Whenever multiple users have unregulated access to a single line, there is a danger of signals overlapping and destroying each other.

Such overlaps, which turn the signals into unusable noise, are called collisions.

As traffic increases on a multiple-access link, so do collisions.

A LAN therefore needs a mechanism to coordinate traffic, minimize the number of collision occur, and maximize the number of frames that are delivered successfully.

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The access mechanism used in an Ethernet is called Carrier Sense Multiple Access with Collision Detection (CSMA/CD).

CSMA/CD is the result of an evolution from Multiple Access (MA) to Carrier Sense Multiple Access (CSMA), and finally to CSMA/CD.

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Multiple Access (MA)Multiple Access (MA)

The original design was MA method in which every workstation had equal access to a link.

There was no provision for traffic coordination. Access to the line was open to any node at any time. Any station wishing to transmit did so, then relied on

acknowledgements to verify that the transmitted frame had not been destroyed by other traffic on the line.

The examples of algorithms for allocating a multiple access channel is ALOHA.

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CSMACSMA

In Carrier Sense Multiple Access (CSMA) system, any workstation wishing to transmit must first listen for existing traffic on the line.

Carrier Sense ~ a device listens by checking for a voltage. If no voltage is detected, the line is considered idle and the

transmission is initiated. CSMA cuts down on the number of collisions but does not

eliminate them. Collisions can still occur, if another station has transmitted

too recently for its signal to have reached the listening station, the listener assumes the line is idle and introduces its own signal onto the line.

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CSMA/CDCSMA/CD

In CSMA/CD the station wishing to transmit first listens to make certain the link is free, then transmits data, then listens again.

During the data transmission, the station checks the line for the extremely high voltages that indicate a collision.

If a collision is detected, the station quits the current transmission and waits a predetermined amount of time for the line to clear, then sends its data again.

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IEEE 802.3 Frame FormatIEEE 802.3 Frame Format

IEEE 802.3 specifies one type of frame containing seven fields:– Preamble ~ alert the receiving system of incoming frame

and synchronize input timing– Start Frame Delimiter (SFD) ~ starts signal the beginning

of the frame– Destination Address (DA) ~ physical address– Source Address (SA) ~ physical address– Length/type of PDU– 802.2 frame/PDU/Data and padding/payload– CRC ~ error detection

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MAC Frame FormatMAC Frame Format

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ImplementationImplementation

Although the Project 802 standard focuses on the data link layer of the OSI model, it also defines some physical specifications for each of the protocols defined in the MAC layer.

In the 802.3 standard, IEEE defines the types of cable, connections, and signals that are to be used in each Ethernet implementations.

All Ethernet LANs are configured as logical buses, although they may be physically implemented in bus or star topologies.

Each frame is transmitted to every station on the link but read only by the station to which it is addressed.

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10BASE5: Thick Ethernet10BASE5: Thick Ethernet

The 1st of the physical standards defined in the IEEE 802.3 model is called 10Base5 or Thick Ethernet or Thicknet.

The name derives from the size of the cable, which is roughly the size of garden hose and too stiff to bend with your hands.

It’s a bus topology LAN that uses baseband signalling and has a maximum segment length of 500 meters.

Using networking devices such as repeaters or bridges the total length of the bus should not exceed 2500 meters (5 segments) to reduce collisions.

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10BASE5 (cont…)10BASE5 (cont…)

The standard demands that each separated from each neighbor by 2.5 meters (200 stations per segment and 1000 stations total)

The physical connectors and cables utilized by 10Base5 include coaxial cable, network interface cards, transceivers, and attachment unit interface (AUI) cables and terminators.

Coaxial cable– RG-8 (Radio Government) cable is a thick coaxial

cable that provides the backbone.

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10BASE5: Thick Ethernet10BASE5: Thick Ethernet

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Transceiver– Each station is attached by an AUI cable to an

intermediary device called medium interface unit (MAU) or transceiver (transmitter-receiver).

– It performs the CSMA/CD function of checking for voltages and collisions on the line and may contain a small buffer.

– It also serves as the connector that attaches a station to the thick coaxial cable itself via a tap.

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AUI Cables– Each station is linked to its corresponding transceiver by an

attachment unit interface (AUI), also called a transceiver cable.

– An AUI is a 15-wire cable with plugs that performs the physical layer interface functions between the station and the transceiver.

– Each end of an AUI terminates in a DB-15 (15 pin) connector.– One connector plugs into a port on the NIC, the other into a

port on the transceiver.– AUIs are restricted to a maximum length of 50 meters,

allowing for some flexibility in placement stations relative to the 10Base5 backbone cable.

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Transceiver Tap– Each transceiver contains a connecting mechanism called a

tap because it allows the transceiver to tap into the line at any point.

– The tap is a thick cable-sized well with a metal spike in the center.

– The spike is attached to wires inside the transceiver.– When the cable is pressed into the well, the spike pierces the

jacket and sheathing the layers and makes an electrical connection between the transceiver and the cable.

– This kind of connector is often called the vampire tap because it bites the cable.

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Terminator– Terminators are required for bus topologies where

one main cable acts as a backbone with branches to several devices.

– Located at the end in bus cabling. – If the main cable is left unterminated, any signal

transmitted over the line echoes back and interferes with the original signal.

– A terminator absorbs the wave at the end and eliminates echo-back.

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10Base2: Thin Ethernet10Base2: Thin Ethernet

The 2nd Ethernet implementation defined by the IEEE 802 series called 10Base2, thin Ethernet, thinnet, cheapnet, cheapernet, thinwire Ethernet.

It provides an inexpensive alternative to 10Base5 Ethernet, with the same data rate.

Like 10Base5, 10Base2 is also bus topology LAN. The advantages of thinnet are reduced cost and ease

of installation. (The cable is lighter weight and more flexible than that used in Thicknet)

The disadvantages are shorter range (185 meters) and smaller capacity (30 stations/segment).

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10Base2 (cont…)10Base2 (cont…)

The connectors and cables utilized are NICs, thin coaxial cable, BNC-T connectors and terminators.

NIC– The NICs in a thinnet system provide all of the same

functionality as those in the thicknet system, plus the functions of the transceivers.

– It does not only provide the station with an address but also checks for voltages on the link.

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Thin coaxial cable– The cable required to implement the 10Base2 standard is

RG-58.

– These cables are relatively easy to install and move around.

BNC-T– The BNC-T connector is a T-shaped device with three ports:

one for the NIC and one each for the input and output ends of the cable.

Terminator– 50-ohm is used to absorbs the wave at the end and

eliminates the echo-back.

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10BaseT: Twisted Pair Ethernet10BaseT: Twisted Pair Ethernet

The most popular standard defined in the IEEE 802.3 series.

Physically a star-topology LAN, but logically is a bus-topology.

It uses unshielded twisted pair cable (UTP CAT-3 or CAT-5)

It supports data rate of 10Mbps and has a maximum length (hub to station) of 100 meters.

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10BaseT (cont…)10BaseT (cont…)

It places all of its networking operations in an intelligent hub with a port for each station.

Stations are linked into the hub by four-pair RJ-45 cable (8-wire UTP) terminating at each end in a male-type connector much like telephone jack.

The hub sends out any transmitted frame to all of its connected stations.

Logic in the NIC assures that the only station to open and read a given frame is the station to which that frame is addressed.

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10BaseT (cont…)10BaseT (cont…)

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Other Ethernet NetworksOther Ethernet Networks

During the last decade, there has been an evolution in Ethernet Networks.

Several new schemes have been devised to improve the performance and the speed of Ethernet LANS.

Some of the efforts are:– Switched Ethernet– Fast Ethernet– Gigabit Ethernet

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Switched EthernetSwitched Ethernet

An attempt to improve the performance of 10BaseT Ethernet.

10BaseT is a shared media network, which means that the entire media is involved in each transmission.

This is because the topology, though physically a star, is logically a bus.

When a station sends a frame to a hub, the frame is sent out all ports (interfaces) and every station will receive it.

In this situation, only one station can send a frame at any time.

If two stations try to send frames simultaneously, there is a collision.

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10BaseT Example10BaseT Example

Station A is sending a frame to station E. The frame is received by the hub and is sent to every station. All of the cabling in the system involved in this transmission. Another way to think about this is that one transmission uses the entire capacity of 10Mbps; if one station uses it, no other station can.

A

E

DC

F

B

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Switched Ethernet (cont…)Switched Ethernet (cont…)

If we replace the hub with a switch, a device that can recognize the destination address and can route the frame to the port to which the destination station is connected, the rest of the media are not involved in the transmission process.

This means that the switch can receive another frame from another station at the same time and can route this frame to its own final destination.

In this way, theoretically, there is no collision. Using a switch, instead of hub, we can theoretically increase

the capacity of a network with N devices to N x 10 Mbps because the two pairs of UTP for full-duplex communications.

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Switched Ethernet ExampleSwitched Ethernet Example

Station A is sending a frame to station E. The frame is received by the hub and sent to station E. Only station E receives the frame, so the rest of the media is free for another transmission. Station B can also send a frame to station D without any collision.

A

E

DC

F

B

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Fast Ethernet (IEEE 802.3u)Fast Ethernet (IEEE 802.3u)

Fast Ethernet is a version of Ethernet operates at 100 Mbps data rate.

There is no change in the frame format and the access method (CSMA/CD).

The only two changes in the MAC layer are the data rate and the collision domain.

The data rate is increased by a factor of 10; the collision domain is decreased by a factor of 10.

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Fast Ethernet (cont…)Fast Ethernet (cont…)

In the physical layer, the specification developed for Fast Ethernet is a star topology similar to 10Base-T; however, to match the physical layer to different resources available, IEEE has designed two categories of Fast Ethernet:– 100Base-X (2 cables between the station to hub)

• 100Base-TX (2 pairs of UTP or STP)• 100Base-FX (2 optical fibers)

– 100Base-T4 (4 pairs of UTP)

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100Base-TX100Base-TX

It uses two category 5 UTP or STP cables to connect a station to the hub.

One pair is used to carry frames from the station to the hub and the other to carry from the hub to the station (1 pair for transmission, 1 pair for reception).

The distance between the station and the hub (or switch) should be less than 100 meters.

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100Base-FX100Base-FX

It uses two optical fibers (either multimode fiber or single mode fiber), one to carry frames from the station to the hub, and the other from the hub to the station (1 for transmission, 1 for reception).

The distance between the station and the hub should be less than 412 meters for multimode half duplex, 2 km for multimode full duplex and 20 km for single mode fiber.

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100Base-T4100Base-T4

It uses 4 pairs of UTP between the station to the hub (2 pairs for transmission and two pairs for reception).

A length of 4-pair UTP must not more than 100 meters between the NIC in the station and the port in the 100Base-T4 hub.

The physical layer implementation for 100base-T4 varies significantly from the physical layer definition of 10baseT.

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100Base-T4 (cont…)100Base-T4 (cont…)

It achieves its speed by dividing a 100 Mbps data stream into three 33 Mbps streams.

These 3 streams are sent over 3 pairs of UTP.A 4th pair is used for collision detection.No data is sent on the 4th pair; instead, a hub

uses it to signal a station when a collision occurs.

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Gigabit EthernetGigabit Ethernet

The migration from 10 Mbps to 100 Mbps encouraged the IEEE 802.3 committee to design gigabit Ethernet, which has data rate of 1000 Mbps or 1 Gbps.

The MAC layer and the access method remain the same, but the collision domain is reduced.

The physical layer ~ the transmission media and the encoding system, however, changes.

It is mainly designed to use optical fiber, although the protocol does not eliminate the use of twisted pair cables.

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Gigabit Ethernet (cont…)Gigabit Ethernet (cont…)

It usually serves as a backbone to connect Fast Ethernet networks.

Four implementation has been designed for Gigabit Ethernet:– 1000Base-LX (multimode FO, max 550 meters)

– 1000Base-SX (multi, max 550 m or single, 5 km)

– 1000Base-CX (STP, max 25 meters)

– 1000Base-T (UTP, max 25 meters)

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TOKEN BUS (IEEE 802.4)TOKEN BUS (IEEE 802.4)

It combines features of Ethernet and Token Ring. It combines the physical configuration of Ethernet (a bus

topology) and the collision free (predictable delay) feature of Token Ring.

Token Bus is a physical bus that operates as a logical ring using tokens.

Stations are logically organized into a ring. A token is passed among stations, if a station wants to

send data, it must wait and capture token. Token bus is limited to factory automation and process

control and has no commercial application in data communication.

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Token Ring (IEEE 802.5)Token Ring (IEEE 802.5)

A LAN using a ring topology and token-passing method.

A control station creates a special entity on the network called a token and passes it around the network ring.

A token is a simple placeholder frame that is passed from station to station around the ring.

A station may send data only when it has possession of the token.

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Token Ring: Access MethodToken Ring: Access Method

Token Ring use the access method called token passing.

Token passing is an example of deterministic protocol. It solves collision problem in CSMA/CD. 2 types of token

- Free token (no data)

- Busy token (with data) Token moves in one way circling the ring and only one

token at a time.

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A token is passed from NIC to NIC in sequence until it encounters a station with data to send.When the token reaches a station on the network that has something to transmit, it “captures” (keep the token in its network board) the token, changes the status of the token to busy.It then sends data frame.This data frame proceeds around the ring, being regenerated by each station.

Token Ring: Access Method (cont…)Token Ring: Access Method (cont…)

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Each intermediate station examines the destination address, finds that the frame is addressed to another station, and relays it to its neighbor.

The intended recipient recognizes its own address, copies the message, checks for errors, and changes four bits in the last byte of the frame to indicate address recognized and frame copied.

The full packet then continues around the ring until it returns to the station that sent it.

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The sender receives the frame and recognizes itself in the source address field.It then examines the address-recognized bits.If they are set, it knows the frame was received.The sender then discards the used data and releases the token back to the ring.


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ExerciseExercise

Complete the figure below with 10BaseT configuration.

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SolutionSolution

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ReviewReview

IntroductionTopologyProject 802EthernetFast EthernetToken BusToken Ring

Topology

Technology

Transcript of Topology