Physical and Logical Topologies

Chapter 5

Four standard physical topologies

• Bus• Star• Ring

• Hybrid

Bus Topology

• Single cable connecting all computers– Coaxial cable

• Easy to set up

Bus topology network

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• Signal is heard by all computers, but only the destination computer accepts the data

• Passive technology--no active electronics to amplify or regenerate the signal

Bus topology network

• Only one computer at a time can send data– Data collisions occurs if two computers

attempt to send data at the same time• Network must be

terminated at both ends to prevent signal bounce-back

Terminator

Advantages of a Bus

• Easy to set up• Inexpensive• Easy to add new users• Repeaters can be added to amplify

signal

Disadvantages of a Bus

• Data collisions may result in slower network traffic

• Each connection weakens the electrical signal

• Can be hard to troubleshoot• An unplugged cable

is not terminated and will take down the network

Ring network

• Each device is connected to the next device in the network

• Data is passed from one device to the next

• Data flows around the ring in one direction

Ring network• A token is passed around

the ring• To send data, a computer

intercepts the token• Packet of data is attached

to the token and addressed to a specific computer

• When data is received, message is sent to sending computer and token is placed back on the network

Data

Data

DestinationComputer

Sending Computer

Advantages of a Ring

• Avoids data collision• Equal access by all computers on the

network• Each station acts as a repeater

– Regenerates the signal

Disadvantages of a Ring

• Additional cost of network adapters and software

• Entire network can be affected by a single break in the cable or failure of one computer

• As more computers are added to network, response time slows

• It is more difficult to troubleshoot a ring network

Star network

Hub

• Each computer is connected to a central hub– Active hubs—amplify or regenerate signal– Passive hubs—pass signal, no amplification or

regeneration

Star network

Hub

• All data is sent to the hub.– Broadcast star--hub sends data out to all

attached devices (passive or active hubs)– Switched star--hub sends data to specific

device

Advantages of a Star

• A break or unplugged cable takes down only the unplugged computer

• Easy to add new computer to network

• Hub provides centralized location for diagnosing problems

Disadvantages of a Star

• If hub is down, entire network is down• Additional cost for hub and cabling• If passive or active hubs are used, data

collisions can occur slowing network traffic

Hybrid Networks

• Star-bus• Star-ring

Star-ring network• Computers are attached to a hub (MSAU)

but within the hub, a ring is implemented to pass data

Hub (MSAU) showing the internal ring and clockwise token path

Star-bus network

Hub Hub Hub

• Computers are attached to a hub (star). Hubs are attached to other hubs using bus topology.

Enterprise-wide Topologies

• Expand on the simple star and star-bus network

• Connects hubs, switches, and routers on a network

• Fiber-optic cable often used as backbone of enterprise-wide network

• Many different ways to implement depending upon needs of organization

Enterprise-wide Topologies

• Simplest implementation—series of hubs connect to each other– Creates a single collision domain

• All devices on the network have to look at all packets

• Hubs to Bridges/Switches– Segments the network

• Controls flow of traffic

• Routers—create subnetworks

Mesh Network

• Provides redundant physical links between network devices

• Used in small installations or where it is essential that there always be a path to critical devices

• Very fault tolerant• Difficult and expensive to install and

reconfigure

Mesh Network• True mesh–Each

device is connected to every other device

• Hybrid mesh–Redundant paths are created to critical devices

WAN Topology

• WAN—Wide Area Network– Connects geographically distinct

locations

• Topologies– Peer-to-Peer (Bus)– Ring – Star– Mesh– Tiered

Logical Topologies

• The way data will be transmitted between nodes

• Most popular logical topologies is:– Ethernet

• Other logical topologies include:– Token Ring– FDDI– ATM– LocalTalk

Switching

• Technique that determines how connections are made and how data is moved

• 3 types of switching– Circuit switching– Message switching– Packet switching

Circuit Switching

• Connects sender and receiver by a single physical path for the duration for the conversation

• Telephone call is an example of circuit switching

• Route selected from sender to receiver varies with each connection but once the connection is made the path stays the same

Advantages of Circuit Switching

• Guaranteed data rate– Once circuit is established there are no

delays waiting for an available channel

• Can be used for time-critical transmission such as audio and video

Message Switching

• Messages are stored and forwarded from one intermediate device to another until the message reaches the destination

• E-mail messages are an example of message switching– The message is sent from server to

server

• Sometimes referred to as “store-and-forward”

Message Switching Advantages

• Provides effective traffic management– Priority can be assigned to messages so

that you can assure that high priority messages are handled promptly

• Reduces network traffic– Message can be stored at intermediate

devices until a communications channel is available

• Provides asynchronous communications across time zones

Packet Switching

• Messages are broken into packets– Each packet contains header information including

source, destination, sequence number etc.• Individual packets may not follow the same

path from the sender to the receiver• The Internet is an example of a packet-

switched WAN

Advantages of Packet Switching

• If a particular path goes down during transmission, packets can be rerouted

• Better use of bandwidth because packets can be sent via different routes

Ethernet

• Most popular architecture in use today• Bus, star, or star-bus based technology• Uses baseband signaling• Uses CSMA/CD access method• 10Mbps or 100Mbps transfer rate

– Higher rates are now possible

• Cable: Thinnet, Thicknet, UTP/STP, Fiber

CSMA/CD

• Carrier sense multiple access with collision detection media access method

• Contention-based system• Only one workstation can use network at a

time• When a collision is detected, workstations wait

a random amount of time and retransmit packet

• More nodes on the network—the more collisions– Users complain of slow network response

Ethernet Networks

• 10Base2 (Thinnet)• 10Base5 (Thicknet)• 10BaseT (Twisted-pair)• 10BaseF (Fiber)• 100BaseT (twisted-pair)• 100BaseFX (Fiber)• Gigabit Ethernet

(Fiber or twisted-pair)

10Mbps bandwidth

100Mbps bandwidth

1000Mbps bandwidth

10Base2 Ethernet (Thinnet)

• 10Mbps transmission speed; baseband• Maximum segment length--185 meters• Maximum segments--5• Maximum repeaters--4

– Repeater--device that regenerates network signal to extend the distance signal can travel

• Maximum segments with nodes--3• Maximum nodes per segment--30• Maximum overall length with repeaters--

925 meters

10Base2 Ethernet

• Specification is 50-ohm RG-58A/U or RG-58C/U coaxial cable– TV cable is RG-59

• Uses BNC connectors• Transceiver is built into NIC cards--no

separate vampire tap transceiver– Connected to NIC by BNC T connector

• Terminator must be installed at both ends of network

10Base5 Ethernet (Thicknet)

• 10Mbps transmission speed; baseband• Maximum segment length--500 meters• Minimum distance between taps--2.5

meters• Maximum number of taps/segment--100• Maximum number of segments--5• Maximum number of repeaters--4• Maximum number of segments with

nodes--3

10Base5 Ethernet

• Maximum 300 nodes• Maximum overall length with

repeaters--2.5 kilometers (2500 meters)

• Maximum AUI cable length--50 meters– Distance from transceiver (vampire

tap) to the NIC)

10Base2 & 10Base5 Rule

Repeater 1

Repeater 2

Repeater 3

Repeater 4

Trunk Segment 1

Trunk Segment 2

Trunk Segment 3

Trunk Segment 4

Trunk Segment 5

• The 5-4-3 rule; 5 segments, 4 repeaters, and 3 populated segments

10BaseT Ethernet (Twisted-Pair)

• 10Mbps transmission; baseband• Physically wired as a star

– each device connected directly to a hub– hub acts a repeater

• Cable: Category 3, 4, or 5 UTP• Maximum segments--1024

10BaseT Ethernet

• Maximum segment length--100 meters• Maximum nodes per segment--2• Maximum nodes per network--1024• Maximum hubs in a chain—4• RJ-45 connectors

100BaseT Ethernet (Fast Ethernet)

• 100BaseTX– Uses 2 pairs cable– Category 5 UTP, Cat 5E or higher

• 100BaseT4– Uses 4 pairs cable– categories 3, 4, & 5 twisted pair

100BaseT Ethernet

• 100Mbps• Specification for maximum segment

length, # of segments, #of nodes per segment, etc. are the same as 10BaseT

10BaseF and 100BaseFX

• 10Mbps or 100Mbps throughput• Cable: Fiber—multimode• Maximum segment length

– 10BaseF—1000-2000meters (1-2 kilometers)

– 100BaseF—400meters

• Star-bus topology

Gigabit Ethernet

• Typically used for backbone– Hub to hub; hub to switch; hub to router

• Fiber-optic cable (typically)– Can be implemented for UTP

• Speeds in excess of 1000Mbps (1Gbps)

Token Ring

• Robust, reliable network architecture developed by IBM

• Addressed by 802.5 standard• Physically may look like a star but

logically works like a ring– Star-ring topology

• Workstations connect to a MAU, MSAU or SMAU– MSAU--MultiStation Access Unit

Token Ring Specifications• Bandwidth--4Mbps or 16Mbps

– All cards on the network must be configured the same

– 100Mbps token ring is available but not widely implemented• Called HSTR (High-Speed Token Ring)

• Cable--UTP, STP, fiber• Maximum MSAU--33• Maximum nodes per MSAU--8 (IBM)• Maximum nodes—255 (STP) 72 (UTP)

Adding hubs while maintaining the logical ring

Ring in

Ring out

Ring inRing out

Ring inRing out

Ring inRing out

Ring inRing out

FDDI(Fiber Distributed Data Interface)FDDI(Fiber Distributed Data Interface)

• Networking standard originally specified by ANSI in the 1980s and later refined by ISO

• Uses a dual fiber-optic ring to transmit data at speeds of 100 Mbps– Dual counter-rotating rings

provide fault tolerance

• Used to connect LANs located in multiple buildings, e.g., a large campus

FDDI (Fiber Distributed Data Interface)

• Cable lengths can extend 100 km (62 miles) with repeaters every 2 km

• Access to network is controlled using a token passing scheme

• Can attach up to 500 nodes• Computers can be attached to only the primary

ring or to both rings– Computers attached to both rings are called Class A

stations (dual-attached stations)– Computers attached to only the primary ring are

called Class B stations (single-attached stations)

ATM(Asynchronous Transfer Mode)ATM(Asynchronous Transfer Mode)

• WAN transmission that relies on a fixed packet size to achieve data transfer rates from 25 to 622 Mbps

• Cell—the fixed-length packet in ATM– Exactly 53 bytes– 48 bytes of data and a 5 byte header

48 bytes of data

5 byte header

53 bytes

1000 bytes of data

LocalTalk

• Specially for Macintosh networks• Maximum throughput—230Kbps• Cannot support non-Mac equipment• Use CSMA/CA media access

– Carrier sense, multiple access with collision avoidance• NIC signals its intent to send data before

it actually sends the data in order to avoid collisions