Layer 1 Technologies

高雄大學資工系嚴力行

Network Classification byTransmission Technology

•Broadcast Networks–A single communication channel is shared by

all the machines on the network.–Addressing

•broadcasting: All the machines are receivers.•multicasting: A subset of the machines are

receivers.

•Point-to-point Networks–many connections between individual pairs of

machines

Broadcast Vs. Point-to-PointNetworks

BroadcastNetworks Point-to-point

Networks

Physical TopologiesPhysical topology is the actual layout of thewire or media

BusTopology

RingTopology

StarTopology

Extended StarTopology

HierarchicalTopology

MeshTopology

Bus Topology

A bus topology uses a single backbone segment(length of cable) that all the hosts connect to directly

Ring Topology

A ring topology connects one host to the next andthe last host to the first. This creates a physical ringof cable.

Token Ring

•Token Ring NIC and Hub (or MAU)

IBM Token Ring: The Star-Ring Architecture

bridge

Gateway

WAN

concentrator

Star Topology

A star topology connects all cables to a central pointof concentration. This point is usually a hub orswitch, which will be described later

Extended Star Topology

An extended star topology links individual starstogether by linking the hubs/switches, which willextend the length and size of the network

Mesh Topology

•each host has its own connections to all otherhosts

•used when there can be absolutely no break incommunications, for example the controlsystems of a nuclear power plant

Transmission Media

•Media: material through which datapackets travel

•Optical•Cooper

–Coaxial cable–Twisted Pair

•Space

Fiber Optical Cable

•Dielectric (free frominterference)

•Low Attenuation overdistance

•Small size, light weight•Good bandwidth (Up

to 3.3 GHz)•Support long distance:

6-8 km withoutrepeater

Coaxial Cable

•Bandwidth ~ 350 MHz•Data Rate ~ 500 Mbps•Repeater spacing 1~10 Km•Baseband Coaxial Cable, 50-ohm, used in

Ethernet•Broadband Coaxial Cable, 75-ohm, used

in Cable TV

Coaxial Cable

塑膠絕緣體

Twisted Pair

•Two insulated wires arranged in a spiral pattern•The wires are copper or steel coated with copper•A cable may have many pairs (Ethernet: 4 pairs)•Twisting minimizes electromagnetic interference

between pairs•Noise Immunity: Good at low frequency•Least expensive•High data error rate: 1 x 10-5

•Typical data rate: 10/100 Mbps (within 100m)•Used as a point-to-point link

Shielded Twisted Pair (STP)

reduced interference, expensive

Screened Twisted Pair (ScTP)

Unshielded Twisted Pair (UTP)

Category 3 UTP: up to 16MbpsCategory 5 UTP: up to 100Mbps with limited distances

UTP Cabling

Pin Out

•10BASET與100BASET只用到UTP 8蕊中的其中四蕊 (1, 2, 3, 6)

Pin 1Pin 2Pin 3Pin 4Pin 5Pin 6Pin 7Pin 8

NCNC

NCNC

TD+TD– TxRD+

RD–

Rx

10/100 Mbps只用到這四條線

Crossover

•用UTP連接兩端設備時要進行跳接 (13,26)。跳接可在設備端或線材上進行。

Pin 1Pin 2Pin 3Pin 4Pin 5Pin 6Pin 7Pin 8

NCNC

NCNC

TD+TD– TxRD+

RD–

Rx

Pin 1Pin 2Pin 3Pin 4Pin 5Pin 6Pin 7Pin 8

TD+TD–TxRD+

RD–

Rx

crossover

MDI/MDIX Interfaces

•MDI (Medium Dependent Interface)–沒有內部跳線的設備端interface (port)–PCs and routers use an MDI interface

•MDIX (Medium Dependent Interface Crossover)–有內部跳線的設備端interface (port)–hubs and switches use MDIX interfaces–通常會標示X

•AUTO MDI/MDIX–會自動選擇採用MDI或MDIX的介面

Three Types of UTP Cables

•Straight-through cable

–用以連接MDI至MDIX介面

•Crossover cable (線材跳線)

–用以連接MDI至MDI或MDIX至MDIX介面

•Roll-over cable

–當作console線(如將NB連至Router當Router的主控台)

Straight-through Cable

Pin 1 Pin 1Pin 2 Pin 2Pin 3 Pin 3Pin 4 Pin 4Pin 5 Pin 5Pin 6 Pin 6Pin 7 Pin 7Pin 8 Pin 8

SwitchPC

MDIX MDI

Crossover Cable

Switch Switch

Pin 1 Pin 3Pin 2 Pin 6Pin 3 Pin 1Pin 4 Pin 4Pin 5 Pin 5Pin 6 Pin 2Pin 7 Pin 7Pin 8 Pin 8

TD+

TD–

RD+

RD–RD+ TD+

TD– RD–

MDIX MDIX

Connector and Jack

•Connector •Jack

Cabling Standard: T568A/B

Straight-through Cable: 兩端皆使用T568A或T568BCrossover Cable: 一端使用T568A；另一端用T568B

T568B白橘、橘、白綠﹑藍﹑白藍﹑綠﹑白棕﹑棕

Transmission Techniques

•Baseband–Signals sent at their original form

•Broadband–Signals are modulated before transmission

and demodulated after receiving

Baseband

•In LAN, baseband usually refers to digitalsignaling

•In baseband coaxial cable (50 OHM)–500 meters–50-OHM Terminators at both end–Distance between two Taps - multiples of 2.5

m to ensure no reflection from adjacent Tap–At most 100 Taps

Broadband

•75-OHM coaxial cable uses broadband•Need modem : Modulation/Demodulation•Unidirectional transmission•Multiple/Single channel broadband•Cover a larger distance•Cascaded amplifiers can result in loss of

data integrity at high data rates

Broadband Coaxial (70 Ohm)

低速數據通道

交換聲音/數據通道

高速數據通道

視訊通道

保留

保留

保留

10MHz

25MHz

55MHz

75MHz

175MHz

210MHz

240MHz

310MHz

Analog signaling with FDM(Frequency Division Multiplexing)

In CATV, each channelhas 6 MHz bandwidth

Encoding Schemes

•Encoding Schemes for basebandtransmissions–RS-232C encoding–Zero-Complemented Encoding–Manchester Encoding–Differential Manchester Encoding

RS-232C andZero Complemented

1 0 1 1 1 0 1 1+ 6 V

0 V

0 0 0 1 1 0 0 1

位元時間

0 1 1 1 0 1 0 1+ V

- V

Start Bit Data bits

0 1 2 3 4 5 6 7

Stop Bit

RS-232C

Zero Complemented

0: 反相1: 不變

Manchester and DifferentialManchester

0 1 0 0 1 1 0 1 0 0+ V

- V

位元時間

1 0 1 1 0 0 1 0 1 1+ V

- V

位元時間

Manchester

DifferentialManchester

0 1 0 0 1 1 0 1 0 0+ V

- V

取樣時間

Sampling atReceiver side

0: 和上個相同1: 反相

0: highlow1: lowhigh

Switching Technologies

•Circuit Switching–need to setup an end-to-end path before any

data can be sent

•Store-and-Forward Switching–Message Switching

•no limit on block size

–Packet Switching–Virtual Circuit

Circuit Switching

Physical copperconnection set upwhen a call is made

switch

This is a dedicatedchannel; Not sharablewith other connections

Packet Switching

1

2

3

1

2

3

1

2

3

1

2

31

23

11

No dedicated path is establishedbefore data can be sent.

Links are shared

Circuit and Packet Switching: AComparison

ItemDedicated “cooper”pathBandwidth availablePotentially wasted bandwidthStore-and-forward transmissionEach packet follows the same routeCall setupWhen can congestion occurCharging

Circuit-switchedYes

FixedYesNo

YesRequired

At setup timePer minute

Packet-switchedNo

DynamicNo

YesNo

Not neededOn every packet

Per packet

Timing of Events

A B C DA B C D A B C D

Circuit Switching

Call RequestSignal

PropagationDelay

Hunting for anoutgoing trunk

CallProcessing

Call AcceptSignal

Data

Message Switching

QueuingDelay

Packet Switching

Virtual Circuit

A path is establishedwhen a connectionis made.

Physical links are shared.

Packets queued up for subsequent transmission

Crossbar Switches

•In a switch with n input and n output lines–n2 intersections (crosspoints)

0 1 2 3 4 5 6 7

0

1234567

0 41 72 6

Inpu

ts

Outputs

Space Division Switches

nN

nN

nN

nN

n* k

n* k

n* k

n* k

k* n

k* n

k* n

k* n

nN

nN

nN

nN

nN

nN

n* k

n* k

n* k

n* k

k* n

k* n

k* n

k* n

Multistage switches with many fewer crosspoints

crossbarsnN

crossbarsnN

crossbarsnN

crossbarsnN

kcrossbars

kcrossbars

Shared Media Environments

•multiple hosts have access to the same medium

Coaxial cable

When one host sends, all receive

The max length is limited

LAN Extension by Repeater/Hub

•Repeaters/hubs can extend the environment sothat it can accommodate more hosts or longercable distances

repeater

hub

Repeaters

•Physical Layer devices.•Protocol Independent.•Interconnect two or more network segments

(with the same type) to be a larger networksegment.

•Regenerates the received digital signals.Does not process the received frames.

Repeater: digital signal -> new, clean digital signalAmplifier: analog signal -> amplified analog signal

Hub

•Could be considered a multi-port repeater(sometimes called concentrator)

•Three types–Passive: a physical connection point that does

not boost or clean signal (no power needed)–Active: needs power to repeat the signal–Intelligent: active hub with microprocessor

and diagnostic capabilities

Collisions

•Devices attached to a hub receive all traffictraveling through the hub.

•The more devices there are attached to the hub,the more likely there will be collisions.

•A collision occurs when two or moreworkstations send data over the network wire atthe same time.

•In half-duplex Ethernet, all data are corruptedwhen that occurs

Collision Domain

•Two devices are said to be in the same collisiondomain if collisions are possible when theytransmit at the same time

A collision domain

Collision Detection in a Half-DuplexEthernet NIC

Collision Detection on a CoaxialCable (10Base2 or 10Base5)

•waveforms from different stations overlap,canceling some parts of the signal out andreinforcing or doubling other parts

•The signal amplitude on the media increases

Collision starts. 1st stops transmitting

Collision Detection on a UTP Cable

•a collision is detected on the localsegment only when a station detects asignal on the RX pair at the same time it issending on the TX pair

•Since the two signals are on different pairs,there is no characteristic change in thesignal.