635.412 Class #2 1 Local Area Networks Topologies & Transmission Media

635.412 Class #2 1

Local Area Networks

Topologies & Transmission Media

635.412 Spring 2005 Class 2: More Basics 2

Topologies & Transmission MediaNetwork Topology Overview

The topology of a network refers to the way in which end systems attached to the network are interconnected

In many networks (especially today) there is a significant difference between the logical and physical topologies of the network (e.g. – virtual LANs or VLANs)

Different types of topologies: Bus and Tree Ring Star


LAN Topologies


Transmission on a Bus


Topologies & Transmission MediaBus and Tree Topologies

Requires the use of a multipoint medium The bus is a special case of the tree topology, with

one trunk and no branches All stations attach to the transmission medium and

can both send data on it and receive data from it Termination required at each end of the bus to

‘absorb’ the propagated signal, preventing reflections

The tree topology consists of a connected set of branches with no closed loops– The root of the tree is a device called the headend– The tree structures can be very complex, with large

numbers of nodes and branches


Topologies & Transmission MediaBus and Tree Topologies (continued)

Bus/Tree topologies present two significant problems:– Since a transmission is received by all stations, an

addressing scheme is necessary to denote who the intended recipient is

– There must be a mechanism for regulating access to the transmission medium

To solve these problems, data to be transmitted is broken into frames for transmission – frames include addressing as well as control information

If proper termination is present no special procedures are necessary to remove data from a bus/tree topology


Topologies & Transmission MediaRing Topology

A network consisting of a set of transmission nodes (repeaters) connected in a closed loop by a set of point-to-point links

Data transmission around the ring is unidirectional Like the bus/tree topology data is broken into

chunks and transmitted in frames Unless one station removes the frame from the ring,

it will circulate around the ring indefinitely (usually the transmitting station removes the frame as it returns to the station)

Requires a medium access control mechanism to control when stations transmit frames



Topologies & Transmission MediaStar Topology

Each station is connected by point-to-point links to a central node that regulates and controls transmission

The star topology has two variants depending on the role the central controller plays: physical star/logical bus and physical star/logical star– Physical star/logical bus: the central controller

broadcasts incoming frames out to all stations– Physical star/logical star: the central controller acts

as a frame switching device; only the destination node sees and receives the frame


Topologies & Transmission MediaChoice of Topology

Depends on a variety of factors:– Reliability– Expandability– Performance– Cost

The choice is not independent of other factors such as transmission medium, wiring, & access control methods

Although a bus topology can be used with different transmission media, only baseband coaxial cable has achieved widespread use

Ring topologies can create very high speed links, but a single repeater or link failure can disable the entire network

The star topology can take advantage of the natural layout of wiring within a building


Topologies & Transmission MediaChoice of Transmission Medium

Besides topology, a number of other factors must be considered in choosing an appropriate transmission medium

– Capacity– Reliability– Type of data supported– Environmental scope

General observations on the choice of transmission medium:

– Voice grade UTP (Category 3) cabling may already exist within a building, minimizing installation costs – however LANs it can support will have limited range and data rates

– Coaxial cable is more expensive than Cat 3 UTP but supports higher data rates


Topologies & Transmission MediaChoice of Transmission Medium (continued)

The current trend is toward Category 5 or 5e UTP, which supports high data rates but is less costly than coax

Optical fiber can a number of attractive characteristics, but its use in LANs has been limited because of high cost & specialized maintenance requirements

Relationship between Medium & Topology– For several reasons the choice between transmission

medium & network topology are not independent– Table 4.1: Medium versus Topology for LANs and MANs

Ring may use (TP, Baseband/Broadband Coax, Optical Fiber, and Wireless)

Bus may use (TP, Baseband/Broadband Coax, Optical Fiber, and Wireless)

Tree may use (Broadband Coax) Star may use (TP, Optical Fiber, and Wireless).


Topologies & Transmission MediaBus & Tree Topologies

While the bus & tree topologies are being replaced with star and ring topology LANs, they still enjoy a large installed base and are important in the CATV arena

Characteristics of the Bus & Tree Topologies– Requires medium access control, since multiple

stations could transmit simultaneously Both centralized & distributed schemes can be used Distributed schemes most commonly implemented

– Also needs signal balancing across all possible transmitters and receivers Must be done between all pairs; scales quickly to a

difficult problem A common solution is subdividing a network into smaller

segments using repeaters


Topologies & Transmission MediaCoaxial Cable for Bus & Tree LANs

For bus & tree topology LANs coaxial cable is a very popular choice for the transmission medium

– Two choices of transmission techniques using different types of coaxial cable are generally available: baseband and broadband

Baseband Coaxial Cable– The most popular transmission medium for bus & tree

topologies; uses baseband digital signaling (usually Manchester or differential Manchester)

– Baseband signaling prevents multiple channels and the attenuation limits network size to around 1-2 kilometers

– Transmission is bi-directional necessitating termination resisters at each end and a bus topology (Cannot have trees/branches)



Topologies & Transmission MediaBaseband Coaxial Cable

A special 50 ohm coaxial cable is used instead of 75 ohm CATV coax to minimize noise and reflections from tap insertions

– Two choices for baseband coaxial cable are defined in the 802.3 standard: 10BASE-5 and 10BASE-2 [Table 4.3]

– The thinner 10BASE-2 cable allows for easier and cheaper installations at the expense of total LAN size

Repeaters may be used with either cable type to extend the length of the LAN and allow more stations to connect

– Repeaters must connect together LANs of the same type– Repeaters do no buffering of data, transparently passing

digital signals in both directions– To prevent interference, only one path of segments and

repeaters are allowed between any two station


Topologies & Transmission MediaBroadband Coaxial Cable

Uses analog signaling – RF signaling and FDM is possible allowing multiple data and/or video channels

Most components can be used in either bus or tree topologies

Comparison of baseband and broadband [Table 4.2] Dual and split broadband configurations

– Instead of using expensive bi-directional amplifiers, systems use two different cables: upstream & downstream

– These two data paths are connected together at a point called the headend: all stations transmit upstream to the headend and signals from the headend are transmitted downstream to all stations

– Stations usually send & receive on the same frequency but on different cables


Topologies & Transmission MediaBroadband Coaxial Cable

The split system uses different frequency portions of one cable for the upstream & downstream paths

– Stations send and receive on different frequencies– Bi-directional amplifiers are necessary to amplify both the

upstream and downstream data paths– The headend provides frequency conversion to change

upstream to downstream signals; may also include remodulation to clean up the downstream signals


Topologies & Transmission MediaRing Topologies

Description– A ring consists of a number of repeaters each connected to

two others by unidirectional transmission links to form a single closed loop

– Repeaters are the attachment point for stations on the ring– Proper ring operation requires repeaters to perform three

functions: data insertion, data reception, and data removal As a frame passes by the station will examine the address

of the frame, if it matches it will copy the frame into its buffer for the station

A frame must be removed from the ring or it will continue to circulate around the ring indefinitely

– The frame could be removed by the addressed repeater– A better alternative is to have the transmitting repeater

remove the frame; allowing for automatic acknowledgements & multicasting


Topologies & Transmission MediaRing Operation

The Ring consists of a number of repeaters connected by unidirectional point-to-point links in a closed loop

To accomplish its primary functions repeaters are designed to operate using three different states: listen, transmit, and bypass


Topologies & Transmission MediaRing Operation – Listen State

In the listen state the repeater is examining frame addresses and control bits while retransmitting the frame to the next station

In addition the repeater copies bits of ‘interesting’ frames to the attached station

The repeater may modify a bit in the frame as it passes (e.g.- acknowledgements)


Topologies & Transmission MediaRing Operation – Transmit State

Here the repeater ‘breaks’ the ring and begins transmitting a frame from the attached station onto the ring

Meanwhile, bits may be received -- depending on the bit length of the ring two things may happen:

– The bit length of the ring < the frame transmitted: the frame has circulated around the ring and is sent back to the station to check it for successful reception

– The bit length of the ring > the frame transmitted: other frames may be received during transmission and must be buffered for retransmission


Topologies & Transmission MediaRing Operation – Bypass State

This state completely isolates the repeater from the ring, allowing signals to propagate past the repeater with no delay

– Not absolutely required for ring operation– Helps improve the reliability of the ring– Improves performance by reducing repeater delay; a result

of bypassing stations not active on the network


Topologies & Transmission MediaRing Benefits & Problems

Ring Benefits– The ring is a multi-access network like the bus/tree

topology allowing multicast & incremental cost growth– Uses point-to-point links, which have several benefits:

– Allows the ring to span greater distances with cheaper electronics

– Fiber optic links can be used allowing very high data rates

– Maintenance is easier than multipoint links

Potential Ring Problems– Cable vulnerability: can be a single point of failure if

the ring isn’t designed for redundancy– Repeater failure: again a single point of failure


Topologies & Transmission MediaPotential Ring Problems

Perambulation: locating a failure requires logically working around the ring to find the fault; can take a long time with large rings

Installation headaches: installation of a new repeater can present difficulties including locating adjacent nodes and installing cabling

Initialization and recovery: all stations must cooperate smoothly when initialization & recovery procedures are necessary (e.g. – loss of the token)

Size limitations: the ring’s size is constrained by the aggregate delay of the ring and accumulated timing jitter


Topologies & Transmission MediaPotential Ring Problems

Timing jitter– An effect caused by the fact that the stations pass

clocking via the data encoding scheme to maintain the synchronization necessary to properly recover data

– The delay distortion in the transmitted bits causes a deviation in the recovered clocking

– The recovered clocking is used to retransmit the signal, meaning that clocking errors are passed on (shows up as the gain or loss of bits on the ring)

– Ways to minimize timing jitter: Use a PLL in each repeater to allow more accurate

reception of clocking Use a jitter buffer & master clock at to provide clean

clocking at each repeater


Example: Ring Network Throughput

A ring network with N segments, each has a link of length L (meters) and a repeater. The average packet size is P (bits) and the data rate is R (bps).

The token size is TK (bits). Find the Ring Throughput and Efficiency. The generation time is T1= P/R (transmisstion) + N/R

(N repeaters). Average time for token arrival (assuming network is

free) is T2=(TK +N/2)/R. Overall packet time is T1+T2 The throughput is P/(T1+T2) (bps) The efficiency is E= P/R(T1+T2), where E is between

0 and 1.


Topologies & Transmission MediaStar-Ring Topologies

Most of the benefits derived from the ring topology are related to its logical characteristics, while several of its major drawbacks have to do with the physical topology

This realization led to the development of the Star-Ring architecture; a physical star topology overlaid by a logical ring [Figure 4.8]

A central concentrator is located in a central closet and incorporates the ring bypass function

This architecture eliminates several problems with ring architectures:

– Easier fault isolation and installation of new stations.– Equalizes the distance between repeaters minimizing signal

level reception issues– Allows the development of a switched-ring architecture


Topologies & Transmission MediaMultiple Rings & Bridges

Bridges will allow multiple rings to be used to overcome the limitations of a single ring or star-ring architecture

– More stations can be attached to the network– The network can span larger distances & provide higher

throughput– A failure will not disable the whole network

Bridges provide connectivity between rings plus:– Input filtering & buffering– Switching (a bridge may connect more than two rings)– Output buffering & transmission

Challenges to using multiple rings and bridges– Inter-ring traffic loses automatic acknowledgement feature – Performance suffers if the network is not designed to

minimize inter-ring traffic


Topologies & Transmission MediaStar Topologies

Twisted Pair & Fiber Optic Star-based LANs– Twisted pair does not currently enjoy any

performance benefits over coaxial cable, but it has a huge installed base and spare pairs can be used for a LAN (minimizing cost)

– Such twisted pair cabling is typically in a star topology homed out of a central closet; the hub is then located in the closet to create the central station

– Star-wired fiber optic cable is used to provide higher speeds and longer links While physically a star, it is a logical bus Many star network topologies use a mixed

configuration of fiber and UTP; fiber is usually used for backbone links




Topologies & Transmission MediaTwisted Pair & Fiber Optic Star-based LANs

Multiple levels of hubs can be employed as long as there are no closed loops and full connectivity; both data frames & a collision presence signal must be propagated throughout the LAN in order to preserve the logical bus

Twisted pair LANs commonly operate at a 10 or 100-Mbps speed though faster networks up to 1Gbps have been developed requiring refinements to the cabling systems– Higher performance cable (Cat 5 or better) must be

used– Installations require more rigorous design and

installation standards


Topologies & Transmission MediaMulti-level Star Topologies

It is very common to build hierarchical star topologies as long as one ‘header’ hub acts as the central point of the star

Three-level Star Topology

Stat io n Stat io nStat io n

Stat io n

Stat io n

Stat io n

Stat io n

I-Hub

I-Hub

I-Hub

Header Hub


Topologies & Transmission MediaHubs and Switches in Star Topologies

To boost LAN performance it has become popular to replace hubs in star topologies with switches

Instead of repeating an incoming frame out to all other stations a switch only forwards a frame out the line attached to the appropriate destination

Scales well & allows ‘in-place’ upgrades Switches all but eliminate collisions and allow each device

to have dedicated capacity equal to the original LAN


Topologies & Transmission MediaLAN Switch Modes

LAN switches can operate in two modes: store-and-forward and cut-through– Store-and-forward switches buffer the entire frame

before transmitting it on the outbound line– Cut-through switches read the MAC address in the

frame and as soon as it determines the outbound line it begins to transmit the frame

– Cut-through yields the highest possible throughput (with the lowest latency) but will transmit bad frames while store-and-forward switches will verify the CRC on frames before forwarding

– The most intelligent switches have an adaptive algorithm for switching between the two modes based on the actual error rate seen on incoming frames


Topologies & Transmission MediaLAN Switch Modes

Explanation of half-duplex & full-duplex modes– In half-duplex operation a station is connected to the

switch in a two-node Ethernet– With full-duplex switching each station is effectively

connected to the switch with two Ethernet LANs, theoretically allowing total transfer speeds of 20-Mbps

– More about LAN Switching & Ethernet in upcoming classes!


Topologies & Transmission MediaStructured Cabling Systems

Overview– To aid in the design and installation of LANs and

associated infrastructure, standards have been issued specifying the cabling types and layout for commercial and residential buildings

– Such standards are referred to as Structured Cabling Systems and have the following characteristics: Refers to all data, voice, & video wiring within a building The cabling layout and cable specs are independent of

vendor and active end-user equipment The cabling layout is designed to reach all areas of the

building minimizing the need for future cable installation Should ‘future-proof’ the building against the need to

rewire to accommodate new technologies


Topologies & Transmission MediaStructured Cabling Systems Overview

The core standards for commercial building structured cabling systems are the EIA/TIA-568 (United States) and the ISO 11801 (International)

BICSI is another important SCS standards development organization

General layout in these standards are based on a hierarchical star-wired cable layout [Figure 4.16]– Interbuilding cables terminate in a main closet,

usually in the basement– Backbone cabling connects together the main closet

with other telecommunications closets and equipment rooms

– Horizontal cabling connects end-user equipment to the nearest closet


Topologies & Transmission MediaStructured Cabling Systems Overview

Maximum distances for each type of cabling are specified in the standards

Many vendors have implemented proprietary or standards-compliant structured cabling systems:– Lucent/Avaya Systimax– Ortronics– Hubble


Topologies & Transmission Media Structured Cabling Systems Hierarchy

tex t

tex t

tex t tex ttex t tex t tex t

X

X

X X X X X

Underg roundCab le

ER

ER

TC TC TC TC TC

W A W A W A W A W A W A

Horizonta l Cabling

Backbone Cabling

Backbone Cabling

M C

IC

HC HC HC HC HC

ER: equipment roomMC: Main cross-connectIC: Intermediate cross-connectX: cross-connectHC: Horizontal cross-connectWA: working area



Topologies & Transmission MediaAppendix: Characteristic Impedance

One of the most important parameters associated with communications cable is its characteristic impedance

– Any transmission line has a distributed level of capacitance & inductance which can be modeled by an equivalent circuit

– The characteristic impedance is a function of the line construction and is independent of whatever signal is transmitted over the line!

Characteristic impedance is most important for power matching

– If a transmission line is terminated with a load or circuit matching its characteristic impedance no power is reflected back from the junction between the two

– When matched all power is transferred across the junction– Reflections are to be avoided because they will cause

interference and errors


KNOCKOUT SWITCH


PILED SWITCH


TENDOM BANYAN SWITCH


TREE SWITCH


DILATED SWITCH


PIPELINED SWITCH

Documents

635.412 Class #2 1 Local Area Networks Topologies & Transmission Media