Communication Networks Lecture Week 3 Local Area Network.

Communication NetworksLecture Week 3

Local Area Network

CC2009NI - Communication Networks, Saroj S. Regmi

Local Area Networks (LANs)

• Owned by organizations for inter-networking in-house systems

• Cover limited geographical area:• Department• Building• Cluster of buildings

• Higher data transmission capacity than WANs


LAN Applications

• Personal computer LANs• Low cost• Limited data rate

• Back end networks• Interconnecting large systems (mainframes and large

storage devices)• High data rate• High speed interface• Distributed access• Limited distance• Limited number of devices


LAN Applications

• Storage Area Networks (SANs)• Separate network handling storage needs• Detaches storage task from specific servers• Shared storage facility across high speed networks• Hard disks, tape libraries, CD arrays• Improved client-server storage access• Direct storage to storage communication for backup

• High speed office networks• Desktop image processing• High capacity local storage

• Backbone LANs• Interconnect low speed local LANs• Reliability• Capacity• Cost


Typical Large LAN Organization• Thousands to tens of thousands of devices

• Desktop systems links 10Mbps to 100Mbps• Into layer 2 switch

• Wireless LAN connectivity available for mobile users

• Layer 3 switches at local network’s core• From local backbone• Interconnect at 1 Gbps• Connect to layer 2 switches at 100 Mbps to 1 Gbps

• Servers connect directly to layer 2 or layer 3 switches at 1 Gbps

• Lower cost software based routers provide WAN connection

• Separate LAN subnetworks (circles in diagram)

• MAC broadcast frame to own subnetwork


LAN Topology

• The connection configuration

• Different configuration• Bus• Token Ring• Star


Bus LANs

• Central cable, to which all devices are attached through a hardware interface (a transceiver)

• Transmission from a device travels along the bus in both directions• Can be picked up by any other

device attached

• At end of bus, terminators absorb, remove the signal


CSMA/CD

• Carrier Sense Multiple Access with Collision Detection

• Originally created by Xerox as the propriety, baseband Ethernet standard

• Simple, easily implemented algorithm

• Bus LANs are based on CSMA/CD algorithm


CSMA/CD Algorithm

1. IF ready to signal AND no traffic on the bus, THEN transmit

2. IF traffic on bus, WAIT until it ceases; THEN transmit immediately

3. IF a collision detected on the bus, STOP transmission immediately afterwards

4. WAIT a random period of time

5. GOTO 1


Frame Transmission on Bus LAN

B

B


Token Ring LANs

• Devices attached to a closed loop (ring) through a set of repeaters.

• A token (electronic signal) circulates around the ring continuously in one direction only; either clockwise or anti-clockwise

• A station wishing to transmit:• Grabs the token• Marks it as “busy”• Fills it with data (now a frame)• Adds address of receiving station

• Frames move around the ring until it reaches destination

• Token ring LAN is based on Fiber Distributed Data Interface (FDDI) algorithm


Star LAN

• Each station attached to a central node

• Central node acts either as:• A broadcaster: Incoming frame transmitted on all

outgoing lines• A switching device: incoming frames switched to one

outgoing line


Choice of Topology

• Depends on:• Performance• Reliability• Scalability

(expandability)

• Needs considering in context of:• Medium• Wiring layout• Access control

Bus Ring Star

Popular, versatile Higher speed, scalability up to some point

Only suitable for small configurations and depends on integrity of central switch


LAN Transmission Media

• Distinction between baseband and broadband signaling

Baseband:• Uses digital signal• Used on bus networks• Consume entire bandwidth of cable• Signal weakens (attenuates) quickly

so repeaters used to boost signals

Broadband:• Uses analog signal• Digital signal modulated into analog

waveform• Attenuation less of a problem, hence

greater data transmission speed possible


LAN Transmission Medium

• Twisted pair cable

• Coaxial cable (thin coax)

• Optical fiber

• Wireless


LAN Transmission Media

• Twisted pair• Early LANs used voice grade

cables• Didn’t scale for fast LANs• Not used in bus LANs now• Very easy to use with star

topology

• Baseband coaxial cable• Uses digital signaling• Original Ethernet• Not often used in new

installations

• Broadband Coaxial cable• As in cable TV systems• Analog signals at radio

frequencies• Expensive, hard to install

and maintain• No longer used in LANs

• Optical fiber• Expensive taps• Better alternatives available• Not used in bus LANs


LAN Standards

• Initial standards for communication on a LAN produced by IEEE• Institute of Electrical & Electronics Engineers• IEEE 802 family• IEEE 802.11 for wireless LAN

• Adopted by American National Standard Institute (ANSI) and International Organization for Standardization (ISO)


LAN Standards

• Standard defined as a 3-layer protocol hierarchy:• Logical Link Control (LLC)• Medium Access Control (MAC)• Physical Layer

LLC

MAC

Physical Layer


Logical Link Control

• Upper sub-layer of data link layer

• Deals with addressing, data link control

• Independent of the topology, transmission medium, MAC techniques

• Specifies mechanism for controlling data exchange between users:• Connectionless service (i.e. datagram)• Connection-mode service (i.e. virtual circuit, with

some flow and error control)


Media Access Control

• Defines how devices gain access to the transmission medium

• Two main techniques:• Bus LAN (Carrier Sense Multiple Access with

Collision Detection)• Token ringing LAN (Fiber Distributed Data Interface)


Media Access Control

• Assembly of data into frame with address and error detection fields

• Disassembly of frame• Address recognition• Error detection

• Govern access to transmission medium• Not found in traditional layer 2 data link control

• For the same LLC, several MAC options may be available


Physical Layer

• Defines the characteristics for different transmission media options for each MAC e.g.• For IEEE 802.3 (CSMA/CD):

• Baseband coaxial cable• Shielded twisted pair• Optical fiber

• For IEEE 802.5 (Token Ring):• Shielded twisted pair


Physical Layer

• 802 Layers - Physical• Encoding/ decoding• Preamble generation/ removal• Bit transmission/ reception• Transmission medium and topology


Hubs

• Active central elements of star layout

• Each station connected to hub by two lines• Transmit and receive

• Hubs act as repeater

• When single station transmits, hub repeats signal on outgoing line to each station

• Line consists of two unshielded twisted pairs

• Limited to about 100 m

• Optical fiber may be used for longer distances (500 m max.)

• Physically star, logically bus

• Transmission from any station received by all other stations

• If two stations transmit at the same time, collision


LAN Protocols in Context


Layer 2 Switches

• Central hub acts as switch

• Incoming frame from particular station switched to appropriate output line

• Unused lines can switch other traffic

• More than one station transmitting at a time

• Multiplying capacity of LAN


Layer 2 Switch Benefits

• No change to attached devices to convert bus LAN or hub LAN to switched LAN.

• For Ethernet LAN, each device uses Ethernet MAC protocol

• Device has dedicated capacity equal to original LAN• Assuming switch has sufficient capacity to keep up with all

devices

• Layer 2 switch scales easily• Additional devices attached to switch by increasing

capacity of layer2


Router Problems

• Routers do all IP-level processing in software• High speed LANs and high performance layer 2 switches

pump millions of packets per second• Software based router only able to handle well under a

million packets per second

• Solution: Layer 3 switches• Implement packet forwarding logic of router in hardware

• Two categories• Packet by packet• Flow based


High-speed LANs

• Extensions made to older LAN standards to get higher data transmission speeds:• Fast Ethernet – 100 Mbps for Bus networks• Gigabit Ethernet – 1 Gbps• FDDI – 100 Mbps for Token Rings• ATM LANs


Ethernet (CSMA/CD)

• Carrier Sense Multiple Access with Collision Detection

• Xerox – Ethernet

• IEEE 802.3


IEEE 802.3 Medium Access Control

• Random Access• Stations access medium randomly

• Contention• Stations content on time on Medium


10 Mbps Specifications (Ethernet)

10BASE5 10BASE2 10BASE-T 10BASE-FP

Transmission medium

Coaxial cable (50 ohm)

Coaxial cable (50 ohm)

Unshielded twisted pair

850-nm optical fiber pair

Signaling technique

Baseband (Manchester)



Manchester/on-off

Topology Bus Bus Star Star

Maximum segment length (m)

500 185 100 500

Nodes per segment 100 30 — 33

Cable diameter (mm)

10 5 0.4 to 0.6 62.5/125 µm


100 Mbps Fast Ethernet

• Use IEEE 802.3 MAC protocol and frame format

• 100 BASE-X use physical medium specifications from FDDI• Two physical links between nodes

• Transmission and reception• 100BASE-TX use STP or Cat.5 UTP

• May require new cable• 100BASE-FX uses optical fiber• 100BASE-T4 can use Cat.3, voice grade UTP

• Uses four twisted pair lines between nodes• Data transmission uses three pairs in one direction at a time

• Star-wire topology• Similar to 10BASE-t


Gigabit Ethernet Configuration

Gigabit Ethernet – Differences:• Carrier Extension

• At least 4096 bit-times long (512 for 10/100)

• Frame bursting• Not needed if using a

switched hub to provide dedicated media access


Gigabit Ethernet - Physical

• 1000Base-SX• Short wavelength, multimode fiber

• 1000Base-LX• Long wavelength, multi or single mode fiber

• 1000Base-CX• Copper jumpers < 25 m, shielded twisted pairs

• 1000Base-T• 4 pairs, Cat. 5 UTP

• Signaling – 8B/10B


10Gbps Ethernet Uses

• High speed, local backbone interconnection between large-capacity switches

• Server farm

• Campus wide connectivity

• Enables Internet Service Providers (ISPs) and Network Service Providers (NSPs) to create very high speed links at very low cost

• Allow construction of (MANs) and WANs• Connect geographically dispersed LANs between campuses or points of

presence (PoPs)

• Ethernet competes with ATM and other WAN technologies

• 10 Gbps Ethernet provides substantial value over ATM


10 Gbps Ethernet Advantages

• No expensive, bandwidth consuming conversion between Ethernet packets and ATM cells

• Network is Ethernet, end to end

• IP and Ethernet together offer QoS and traffic policing approach ATM

• Advanced traffic engineering technologies available to users and providers

• Variety of standard optical interfaces (wavelengths and link distances) specified for 10 Gb Ethernet

• Optimizing operation and cost for LAN, MAN or WAN


10 Gbps Ethernet Advantages


Fiber Channel Applications


Thank You!!

Communication Networks Lecture Week 3 Local Area Network.

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