Communication Networks Lecture Week 3 Local Area Network.
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Transcript of Communication Networks Lecture 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
CC2009NI - Communication Networks, Saroj S. Regmi
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
CC2009NI - Communication Networks, Saroj S. Regmi
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
CC2009NI - Communication Networks, Saroj S. Regmi
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
CC2009NI - Communication Networks, Saroj S. Regmi
LAN Topology
• The connection configuration
• Different configuration• Bus• Token Ring• Star
CC2009NI - Communication Networks, Saroj S. Regmi
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
CC2009NI - Communication Networks, Saroj S. Regmi
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
CC2009NI - Communication Networks, Saroj S. Regmi
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
CC2009NI - Communication Networks, Saroj S. Regmi
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
CC2009NI - Communication Networks, Saroj S. Regmi
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
CC2009NI - Communication Networks, Saroj S. Regmi
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
CC2009NI - Communication Networks, Saroj S. Regmi
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
CC2009NI - Communication Networks, Saroj S. Regmi
LAN Transmission Medium
• Twisted pair cable
• Coaxial cable (thin coax)
• Optical fiber
• Wireless
CC2009NI - Communication Networks, Saroj S. Regmi
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
CC2009NI - Communication Networks, Saroj S. Regmi
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)
CC2009NI - Communication Networks, Saroj S. Regmi
LAN Standards
• Standard defined as a 3-layer protocol hierarchy:• Logical Link Control (LLC)• Medium Access Control (MAC)• Physical Layer
LLC
MAC
Physical Layer
CC2009NI - Communication Networks, Saroj S. Regmi
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)
CC2009NI - Communication Networks, Saroj S. Regmi
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)
CC2009NI - Communication Networks, Saroj S. Regmi
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
CC2009NI - Communication Networks, Saroj S. Regmi
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
CC2009NI - Communication Networks, Saroj S. Regmi
Physical Layer
• 802 Layers - Physical• Encoding/ decoding• Preamble generation/ removal• Bit transmission/ reception• Transmission medium and topology
CC2009NI - Communication Networks, Saroj S. Regmi
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
CC2009NI - Communication Networks, Saroj S. Regmi
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
CC2009NI - Communication Networks, Saroj S. Regmi
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
CC2009NI - Communication Networks, Saroj S. Regmi
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
CC2009NI - Communication Networks, Saroj S. Regmi
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
CC2009NI - Communication Networks, Saroj S. Regmi
Ethernet (CSMA/CD)
• Carrier Sense Multiple Access with Collision Detection
• Xerox – Ethernet
• IEEE 802.3
CC2009NI - Communication Networks, Saroj S. Regmi
IEEE 802.3 Medium Access Control
• Random Access• Stations access medium randomly
• Contention• Stations content on time on Medium
CC2009NI - Communication Networks, Saroj S. Regmi
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)
Baseband (Manchester)
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
CC2009NI - Communication Networks, Saroj S. Regmi
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
CC2009NI - Communication Networks, Saroj S. Regmi
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
CC2009NI - Communication Networks, Saroj S. Regmi
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
CC2009NI - Communication Networks, Saroj S. Regmi
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
CC2009NI - Communication Networks, Saroj S. Regmi
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