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

Network Media and Layer 1 FunctionalityBSAD 146Dave Novak

Dean, Chapter 3, pp 93-124

Objectives Introduction to transmission media Basic cabling

CoaxialTwisted pair Optical fiber

Basic wireless Network Interface Card (NIC) Basic Physical Layer (layer 1) functionality

Background Different parts of the electromagnetic

frequency spectrum can be used for data transmission depending on the medium used and the communications standards being followed

Properties of specific media affectBandwidthAttenuation NoiseDistortion

Background Bandwidth – range of frequencies

occupied or used by a carrier wave Attenuation – strength of signal

decreases as it propagates Noise – unwanted electromagnetic

energy that degrades the signal (crosstalk, background interference)

Distortion – original shape or characteristic of waveform is altered

Transmission Computer communication at basic level

involves the transfer of signals over mediaLayer 1 functionalityExamples

• Electrical current over wire• Radio waves through air• Microwave through air• Infrared through air• Light rays through glass fibers

Source: http://mynasadata.larc.nasa.gov/images/EM_Spectrum3-new.jpg

Transmission Media Directed

Coaxial cableTwisted pairFiber-optic

UndirectedRadio Frequency (RF)

Coaxial Cable At one time, coax was the most widely used

copper networking cable

Coaxial Cable “Legacy Ethernet” LAN implementation

supports two IEEE 802.3 standardsRG 58 cable “Thinnet” supports 10Base2RG 8 cable “Thicknet “ supports 10Base5

Coaxial Cable RG 8 cable “Thicknet “ supports 10Base5

TransceiverAUI Vampire taps

Coaxial Cable RG 58 cable “Thinnet “ supports 10Base2

Twisted Pair Cable Currently the most widely used LAN cabling

Two physical copper circuits (wires) are twisted together to filter out EMI and reduce cross talk

Each wire carries an equal, but opposite signal

Twisted Pair Cable 1) UTP - unshielded 2) STP - shielded

Source: http://www.howtogeek.com/70494/what-kind-of-ethernet-cat-5e6a-cable-should-i-use/

Twisted Pair Cable


Twisted Pair Cable Cable reduces interference and supports

higher bandwidth through twisting and isolation

Cat 6 UTP has more twists/cmand has thicker sheathingthan Cat 5 UTP


Twisted Pair Cable


Twisted Pair Cable Different types of cable have different

performance characteristics Network Cable Specification Comparison

Specification CAT5 CAT5e CAT6 CAT6a CAT7

Frequency 100 MHz 100 MHz 250 MHz 500 MHz 600 MHz

Attenuation (Insertion Loss)* 24 dB 24 dB 19.8 dB 18.4 dB 20.8 dB

Characteristic Impedence 100 ohms ±15

100 ohms ±15

100 ohms ±15

100 ohms ±15

100 ohms ±15

NEXT * 27.1 dB 30.1 dB 44.3 dB 59 dB 62.1 dB

PS-NEXT * N/A 27.1 dB 42.3 dB 59.1 dB 59.1 dB

ELFEXT * 17.9 dB 17.4 dB 27.8 dB 43.1 dB Not Specified

PS-ELFEXT * 14.4 dB 14.4 dB 24.8 dB 41.8 dB Not Specified

Return Loss * 16 dB 20.1 dB 20.1 dB 32 dB 14.1 dB

Delay Skew * 50 ns 45 ns 45 ns 45 ns 20 ns

Twisted Pair Cable Different types of cable may be needed to

support different Ethernet standards10BASE-T, 100BASE-TX, 100BASE-FX,

1000BASE-T10GBASE-T

Cat5e supports most common standards One could potentially use Cat 6, 6a, or 7 –

why wouldn’t you?

Twisted Pair Cable Refers to copper conductor in wire pairs

Solid – uses single piece copper strand• Not flexible, but more durable

Stranded – uses multiple smaller copper strands woven together • Flexible

Twisted Pair Cable Most “common” UTP and STP cables use 8

pin RJ45 connectors

Optical Fiber Cable Supports high bandwidth, long distance

transmission by sending pulses of light over very thin, pure glass (or plastic) strands

Source: https://www.google.com/search?q=image+of+fiber+optic+cable&tbm=isch&tbo=u&source=univ&sa=X&ei=XijpUvbnCpStsQTQhYFg&sqi=2&ved=0CDwQsAQ&biw=1129&bih=842/

Optical Fiber Cable Two types of cable / transmission

1) Multi-mode2) Single-mode

Source: https://www.google.com/search?q=fiber+optic+cable&tbm=isch&tbo=u&source=univ&sa=X&ei=TAzoUviwLue0sAThgYHoDw&ved=0CFAQsAQ&biw=1129&bih=842/

Multi-Mode Fiber Mostly used for communication over short

distances – within building, around campus High bandwidth, but higher dispersionBandwidth-distance product limit is lower than

single mode Has larger core diameter (and larger wave

length) than single mode Less complex equipment than single mode

LED versus laser

Single-Mode Fiber

Has higher bandwidth than multi-mode and is effective over very large distancesHigh bandwidth, low dispersion

Carries only a single ray Equipment is complex and expensive

Specific types of lasers

Optical Fiber Connectors

Source: http://www.telegaertner.de/en/karl-gaertner/data-voice/office/artikel/images/lwl-patchkabel.jpg

Optical Fiber Connectors

Source: http://www.arcelect.com/fibercable%20connectors.gif

Wired Cable Design

Wireless Transmission

A WLAN (wireless LAN) is a LAN that does not rely on physical cabling to transmit data between devices

WLANs use radio frequencies (RF) to transmit data

The goal of wireless communication is to transmit large amounts of data as fast as possible without wires

Wireless Transmission Wireless communication follows the IEEE

802.11 specification which defines half-duplex transmission using the same frequency for send and receiveLet’s talk about this a bit

Wireless Transmission RF transmission does not require licensing

but it is regulated by the government! The regulating agencies are the FCC in the

US and the ETSI in Europe802.11 provides standards for RF operation

within FCC rules (frequencies and power levels)


There tend to be more difficulties achieving high speed transmission with RF than with wired technologies for a number of reasons


To send an RF signal, the use of a modulation technique is required

How did we define modulation?


Spread Spectrum technique used to modulate data by “spreading” the bandwidth available for transmissionSignal is transmitted on bandwidth larger than

the frequency content of the transmission• Security; increased resistance to noise,

interference, and jamming… • Lower power per frequency band• Redundancy

Spread Spectrum Examples

Direct Sequence Spread Spectrum (DSSS)An RF modulation technique where data are

multiplied by a pseudo noise (PN) signal to “spread” the signal, the modified signal is transmitted over various frequencies according to the PN sequence, original signal reconstructed by receiver

As long as PN sequence is different than the PN sequence of other transmissions the signal to noise ratio is positive

Spread Spectrum Examples

Frequency Hopping Spread Spectrum (FHSS)Subdivides bandwidth on a particular channel

(say channel 1) into separate 1 MHz channels Switch rapidly (pseudo-randomly) between

the different 1 MHz frequency bands (min of 75 frequencies every 30 sec) broadcasting for only a few milliseconds over any particular band

Electromagnetic Spectrum

FCC controls the available spectrum and WLANs can operate in only 3 ranges: 1) 900 MHz, 2) 2.4 GHz, or 3) 5 GHz


900-MHz band (902 – 928 MHz) 2.4 GHz band (2.4 – 2.4835 GHz)

Most widely used frequency range used in WLANs

Used by 802.11 (base, b, g, n) standards11 channels, each 22 MHz wide


5 GHz band (complicated regs vary by country)23 non-overlapping channelsUsed by 802.11 (a, h, j, n, ac) standards


If you are interested see http://en.wikipedia.org/wiki/List_of_WLAN_channels

http://en.wikipedia.org/wiki/List_of_WLAN_channels

http://en.wikipedia.org/wiki/List_of_WLAN_channels


2.4 GHz band11 overlapping channels1, 6, and 11 non overlapping


Potential drawbacks to wireless ReliabilityIEEE 802.11 technologies have historically

been slower than wire-based technologiesLimited rangeSecurity concerns

Network Interface Card

Provides a physical connection between a networked device and the network medium

Also provides an interface between layers 1 and 2 of the OSI

Separate cards are needed to support different layer 2 networking technologies (Ethernet, Wi-Fi, dialup, etc.)

Network Interface Card

Network Interface Card Generally plugs right into PCI (Peripheral

Component Interconnect) slots Most modern NICs are PCI

Have different network cable connectors depending on types of cable supportedRJ-45 connector most common

Basics of data transfer Data move through devices along paths

called busesSeveral paths side by side (parallel)

• Data moves in lateral groups as opposed to single (serial) data stream

• 32-bit bus means 32 bits moving along side by side

Data move over network channel in a single streams called serial transmissionOne lane highway with data ALWAYS

traveling in same direction

Network Interface Card 1) Data transmission and reception

Primary function of NIC is to generate and transmit signals of the appropriate type over network and to receive incoming signals

Signal type depends on network medium and Data Link Layer (2) protocol used

On legacy Ethernet LAN, every computer receives all packets transmitted over network NIC must examine MAC destination address in each packet to see if it is intended for the local host

Network Interface Card 2) Signal encoding and decoding

Converts binary data encapsulated in a frame into electrical voltage, light pulses, or other signal

3) Data bufferingSince NIC transmits and receives data in

serial manner, it must be able to store data until outgoing frames can be sent and incoming frames can be processed by CPU

Network Interface Card 4) Data encapsulation

Building frame around data in preparation for transmission

Read contents of incoming frames, strips off frame, and pass data up to Network Layer (3)

5) Serial / parallel conversionUsually 64 bits at a time

6) Media access controlRegulates how device access network

medium

Layer 1 hardware functionality Recognize / work with basic signals Little to no management capabilities Examples

RepeatersHubs

Does not understand frames or MAC addresses

Does not understand network level addresses and routing

Layer 1 hardware functionality Hubs –vs- repeaters

I’ll discuss this from a functionality / historical perspective as no one really makes or uses “layer 1 hubs” anymore

The concept of how a “layer 1 hub” works is important with respect to our discussion of legacy versus switched Ethernet though…

Layer 1 hardware functionality Center discussion on hubs

Used to connect devices to a star or ring LANSend copies of electrical signals to next

segment, do not reconstruct frames• Regenerate and forward ALL electrical signals• Multiple ports

• Do not understand frame formats• Do not understand physical address• Do not understand higher level network addresses

Token Ring MAU Look like Ethernet hubs/switches, but are

NOT the same Passive device

Acts like a ring, even if topology is a starDo not retransmit incoming traffic out other

ports simultaneously Transmits to each connected device in round-

robin manner

Token Ring MAUPC on port 5 transmits packet

Hub receives packet and forwards a copy out each port

MAU receives incoming packet from port 5 and transmits out port 6 MAU waits for packet to return through port 6 before transmitting packet out port 7

Summary Introduction to transmission media Basic cabling

CoaxialTwisted pair Optical fiber

Basic wireless Network Interface Card (NIC) Basic Physical Layer (layer 1) functionality

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

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