LAN topologies and access techniques (with a focus on Ethernet)

BSAD 141Dave Novak

Chap 5 Network+ Guide to Networks, Dean

Topics Covered LAN Topologies

Three base topologies1. Bus2. Star 3. Ring

Hierarchical Star Mesh Wireless

1. Infrastructure2. Ad-hoc

Topics Covered Media access techniques

Three common techniques1. CSMA/CD2. Token passing3. CSMA/CA

Network backbone design Four basic designs

1. Serial2. Distributed3. Collapsed4. Parallel

Network Topologies

Topology – physical or logical arrangement or design used to connect devices to network medium – addresses logic of how packets move from one device to another

How is topology different from architecture?

Network Topologies The topology is related to the

networking technology (defined at Layer 2) and the specific standards that are being followed

Cannot adopt any standard or any networking technology to every topology

Can create separate networks using different standards and different topologies and connect them using bridges, switches, or routers

Network Topologies

Three base network topologies 1) Bus 2) Star 3) Ring

Bus topologyEach device is cabled directly to the device

next to it“Legacy” Ethernet standards support this

topology• Coaxial cable: Thicknet (10Base5) and Thinnet

(10Base2)

Bus topology Signal is sent along connecting media and all

attached devices receive the signal Signal travels in both directions along the bus

Ends of bus medium must be terminated Devices coordinate to ensure that only one

device transmits a signal at a time This is not a common used topology today –

the Ethernet standards that employ the bus topology are antiquated

Star topology All devices are attached to central access

or connection point (hub or switch) Separate cable connection from each

device to connection point

Star topology Most modern Ethernet standards (as well as

standards using other access technologies and layer 2 protocols) employ some variation of star topologyWidely used today

Can use different types of cable including various twisted pair and fiber

Ring topology Ring topology typically uses the token

passing access techniqueThe ring topology is a LOGICAL loop,

although it is implemented in a physical ring layout

Ring topology Ring topology physically looks like a star

topology The “ring” is a logical function of the

connection point (the MAU or hub) Packets passed to each computer

sequentially in round-robin fashion Each computer acts as a repeater Signal regenerated and forwardedPackets flow in one direction

Hierarchical Star / Star Bus Variation of the base star

Once all ports on switch are used, new switch is added to the network• Switches are connected to each other - one end

of cable plugged into the uplink port of one switch

Bus portion of network(bolded)

Star portion of network

Mesh topology

More theoretical than applied solution in terms of actual LAN / WAN topology

Not used in practice, but is a valid topologyEach device has dedicated physical wired

connection to all other devices on network

Mesh topology Not practical for most wired LANs

Number of connections required:

Number of NICs required

Mesh topology May be used as a model for WAN wiring

(internetworking)Mesh internetwork has multiple paths

between two destinations using redundant routers

Wireless topologies Term “topology” also used to describe

wireless communication structural patterns1) Ad hoc topology2) Infrastructure topology

Ad Hoc topology In wireless world, ad-hoc topology often

synonymous with P2P architectureGroup of wireless devices communicate

directly with one anotherInside the communication range of the

wireless technology• Free to communicate • Free to roam

Works for small number of devices in a small geographical setting

Infrastructure topology Wireless network is joined to a wired

network via a wireless access point (AP)Wireless devices communicate directly with

the AP – NOT directly with each other (as in ad hoc mode)• All communication between both wired and

wireless devices runs through the AP

Most wireless networks utilize the infrastructure topology

Locality of Reference LAN technology is inexpensive and widely

available Locality of reference principle:

• 1) Communicate most frequently with devices nearby

• 2) Communicate repeatedly and most frequently with the same subset of devices

Shared media LANs typically rely on shared media

Devices attached to network must coordinate use of the network• Coordination requires communication• Communication requires time• Time increases as distance between computers

growsShared networks with long delay are

inefficient• Spend more time coordinating actions than

sending data

Shared versus Dedicated? Shared – media / channel capacity is used

jointly by multiple users or applications as neededEnvision a typical toll road with many users

Dedicated – media / channel capacity is used exclusively by a single user or applicationEnvision a toll road where users pay to

reserve their own lane and no one else can use that lane while it is occupied

Access control techniques The method or process for how different

devices share the mediaData Link (2) Layer has 2 sublayers

• 1) LLC• 2) MAC

Access control defined by MAC mechanism Different networking technologies have

different MAC mechanisms

Access control techniques Access control defined by Data Link Layer

protocols (Layer 2)1) CSMA/CD2) CSMA/CA3) Token passing

Ethernet (IEEE 802.3) Most commonly used data link standard /

protocolsProvides many Physical Layer (1)

Specifications including (see Table 5.1 on page 121):• 100BaseTX – Cat 5 UTP, Star, 100 Mbps, 100 meters

• 1000BaseSX (160 MHz) – 62.5/125 multimode fiber, Star, 1000 Mbps, 220 meters

• 10Base2 – RG-58 coaxial, Bus, 10 Mbps, 185 meters

Ethernet (IEEE 802.3) Does NOT have a central control structure

controlling when devices can transmitEncapsulates data received from Network

Layer (Layer 3) into Ethernet frame (Layer 2) where standard IEEE 802.3 specifies:• MAC address• Size and format of frame• Physical layer specifications (Layer 1)

Ethernet (IEEE 802.3)

All 802.3 standards use the Carrier Sense Multiple Access (CSMA) access techniqueUses activity on cable to determine statusDevice checks for activity before transmittingIf activity, the device waitsChecking for activity (in use or idle?) is

called Carrier Sensing

CSMA/CD Demonstrate sharing on legacy Ethernet

(think about a bus topology for conceptual reasons)

One device has exclusive use of cable during transmission

After one is done, another can transmit

CSMA/CD Collision detection (CD) – most important

phase of transmission Electrical and fiber Collisions do not harm hardware

• Result in data being destroyed or corrupted

Devices detect collisions using collision detection Once a collision is detected, any device currently

transmitting stops and sends jamming signal After collision occurs, devices wanting to transmit

must wait for the wire to become idle again

Binary exponential back off In the event of a collision devices using

CSMA/CD use the binary exponential back off algorithm to retransmit

CSMA/CD Collisions are normal part of Ethernet

operation More devices and more activity result in

more collisions Collisions result in retransmission and delay Ethernet does not perform well when

heavily utilizedInstall switch, bridge, router

Wireless (IEEE 802.11) Generally considered to be slower and less

reliable than wired technologies IEEE 802.11 provides Physical Layer (1)

specifications that support different modulation techniques at Physical Layer• 1) Direct Sequence Spread Spectrum (DSSS) • 2) Frequency Hopping Spread Spectrum (FHSS)

Wireless (IEEE 802.11) IEEE 802.11 also provides Data Link (2)

Layer specificationsFrame - use standard 802.3 frameCSMA/CA (collision avoidance) as opposed

to CD (collision detection)• CD requires full duplex – not available in wireless• Performs layer 2 error detection on incoming

packets – if no errors, sends ACK indicating no collisions

• If sender does not receive ACK, assumes a collision and retransmits

Token Passing (IEEE 802.5)

Far less common than Ethernet Hardware is generally more expensive Single shared media Historically uses a ring topology Passive MAU – packets are forwarded to

single device at a time in order

Token Passing (IEEE 802.5)

IEEE 802.5 provides NO Physical Layer (1) specificationsOriginal IBM Token Passing employed a

variety of cable specificationsModern 802.5 generally use Cat 5/5e/6 and

RJ45 connectors

Token Passing (IEEE 802.5)

IEEE 802.5 provides Data Link Layer (2) specificationsToken Ring Frames

• 4 different frame formats used in communication (Ethernet employs only 1): 1) Data Frame, 2) Token Frame, 3) Command Frame, and 4) Abort Delimiter Frame

Token Passing (IEEE 802.5)

Devices must wait for the token before transmitting

Device can only transmit if it has token When sending device is finished, the token

is passed to the next device on the ring

Token Passing (IEEE 802.5)

Logical operation of MAU

Token Passing (IEEE 802.5)

Considered more efficient than CSMA/CD because access technique works well even under heavy load

Provides each device with equal opportunity to transmit

Collision free environment

FDDI / CDDI Prior to Fast Ethernet, FDDI was only data

link protocol to offer 100 Mbps transmissionProvides redundancy to avoid failure

• Two rings• Only one used when network is functioning properly

• Counter rotating• Data flow in opposite directions on the two rings

• Self healing• Hardware can detect failure and recover automatically• Failure is bypassed

FDDIProvides unique Layer 1 and 2 specifications - frame format different from token ring, although access method (token passing) the same

Public versus Private? Public – users pay fees to access a shared

network Often a “pay-as-you-go” approachParts of the physical infrastructure network

are available to the general public for sharing Private – users pay fees to obtain a

dedicated portion of the network Often a “flat fee” approachParts of the physical infrastructure network

are dedicated just for the private user

Public versus Private? Concepts of differentiated service /

differentiated pricing

Point-to-Point and PSTN In many cases we are talking about a

“classification” or a stratification of the same physical infrastructure network…

For example, AT&T infrastructure networks can provide users with both:Dedicated / private serviceShared / public service

Topology and Access methods Three basic topologies

Bus Star Ring

Three basic access methods CSMA/CD CSMA/CA Token passing

http://www.youtube.com/watch?v=Cl6PerDT_ew

Mapping Topologies do not map to access methods

on a one-to-one basis

CSMA/CD

Token Passing

CSMA/CA

Topology

Star, bus

Star, busAd hocinfrastructure

Any

Architecture

Any

Any

Any

LAN technology

802.3 Ethernet

802.11 Wireless,Localtalk

802.5 HSTR802.4 Token bus

Backbone Network Design Serial Distributed Collapsed Parallel

Serial Backbone Simplest backbone consisting of two or more devices

connected directly to each other via a single medium – daisy chain

Dean, Network+ Guide to Networks (2013)

Distributed Backbone Hierarchy consisting of multiple intermediate devices

connected to one or more central devices

Dean, Network+ Guide to Networks (2013)

Distributed Backbone Provides the ability to segregate specific workgroups or

functional areas

Dean, Network+ Guide to Networks (2013)

Collapsed Backbone Single router or switch is central connection point for

multiple LANs or subnetworks

Dean, Network+ Guide to Networks (2013)

Parallel Backbone Most robust backbone consisting of redundant

connections from all central routers to all switches and network segments

Dean, Network+ Guide to Networks (2013)

Lecture Summary

LAN topologies

Media access techniques

Network backbone