Computer Networking: LAN and WAN Technologies

LAN and WAN Technologies

Prepared By Adugna A.


Categorizing Network Technologies

• Transmission Medium Wired vs Wireless

• Link ConfigurationPoint-to-point vs Point-to-multipoint

• User Mobility Fixed vs mobile

• Types of UsersAccess vs core (backbone)

• Coverage Area


Selecting the Transmission Medium

• Wiredo No interference from others ! high data rates, easy too upgrade capacityo Small, predictable delayo Expensive to install in hard to access locationso Device locations are fixed

• Wirelesso No physical connection ! mobility, convenienceo Interference, varying channel conditions ! pooro performanceo Licenses often requiredo Hard to add more capacityo Physical security is difficult


Access vs Core Network


Area Coverage

In centimeters; people, objects– Body Area Network– Personal Area Network– Infrared, Bluetooth, ZigBee, IEEE 802.15.4, . . .

In meters; homes, offices, buildings– Local Area Network– Home Area Network– Storage Area Network– IEEE 802.3 (Ethernet), IEEE 802.11 (WiFi), Fibre– Channel . . .

In kilometers; cities, countries, between countries– Metropolitan Area Network– Wide Area Network– PDH, SDH, ATM, Frame-Relay, WiMax, satellite, . . .

In megamergers; globe, between planets– Global Area Network– The Internet, interplanetary networks


Topology Design Requirements

• Station should be able to communicate with any other station

• Dedicated point-to-point links are better than shared multipoint links

• Use as few links as possible• Scales well: adding a new node requires little effort• Fault-tolerant: failure of a link still allows other nodes to

communicate; failure of a device doesn't prevent other nodes from communicating

• Fault-detection: a fault can be automatically detected by network


Network Topologies Links

• Links: point-to-point, point-to-multipoint– Mesh: every station has point-to-point link to every

other station– Bus: every station connected via a multipoint link– Ring: point-to-point links between pairs of stations, or

via special link, to form ring– Star: every station has point-to-point link to central

device– Hybrid: combination of 2 or more of above, e.g. tree

is combination of star and bus topologies


Full Mesh Topology

• Used in small WANs; becomes too complex as number of nodes increase

• Each pair of nodes have dedicated point-to-point link


Partial Mesh Topology

• Selection of node pairs have point-to-point link• Some pairs can not communicate, unless nodes can

forward data.• Used in WANs


Bus Topology

• Used in early (Ethernet) LANs, but replaced by star• point to multipoint link(one node transmit the others receives)• Transmission propagates throughout medium and is heard by all

stations• Terminator absorbs frames at end of medium/cable• Pros: easy installation• Cons: require protocols to share medium


Ring Topology

• Unidirectional point-to-point links to form loop• Stations attach to repeaters• Pros: simple to install and reconfigure; easy to identify faults• Cons: require protocols to share medium; traffic flows in

one direction• Usage: old LANs (e.g. IBM/IEEE 802.5 Token Ring) • Dual ring is still common in MANs and WANs


Star Topology

• Traffic between stations goes via the central node• Usually two point-to-point links between station and central

node (or duplex link)• Pros: easy to install; fault tolerance for links• Cons: depends on central node• Usage: Most LANs today


Point-to-Multipoint Links

• Multiple users share a point-to-multipoint link• Typical for wireless systems (WiFi, mobile phone) and some wired LANs• Use multiple access schemes to determine who transmits and when

– Fixed Assignment : e.g. FDMA(Frequency Division Multiple Access), TDMA(Time Division Multiple Access)• The sub channel assignments are fixed and the same.• FDMA used in radio systems to share the radio spectrum. • TDMA used in digital transmission technology• This techniques is suitable for broadcast satellite communication

– Demand Assignment: Eg. Reservation, polling• The sub channel allotment depends on user’s demand(use time to divide).• Ignore idle nodes and consider only nodes that are ready to transmit• This technique is suitable for point to point communication.

– Random Access : eg. CSMA• unpredictable performance

– Demand Assignment and Random access called Medium Access Control (MAC)


Medium Access Control• In a shared medium, if two (or more) stations transmit at the same time,

there is a chance the two transmissions will interfere with each other• Collision of frames: receiver receives two or more frames partially

overlapping in time; assume all frames are corrupted/lost• Medium Access Control: allow one station to use the shared medium at a

time (avoiding collisions)• MAC techniques must give stations opportunities to transmit: fair and

efficientDemand Assignment reservation, polling, round-robin

– Stations are coordinated by a scheduleRandom Access Aloha, CSMA

– Stations wait a random time and transmit if no-one else is currently transmitting


WANs and LANs

• Wide Area Networks(WAN)– Connect devices/networks over large geographical area– Between campuses, offices buildings, cities, countries– Owned and operated by organizations on behalf of– users, e.g. TOT, CAT, TT&T– Leased to users, e.g. unis, companies, smaller ISPs

• Local Area Networks(LAN)– Connect end-user devices over small area– Within campuses, buildings, homes– Owned and operated by organization using the network– Typically support higher data rates than WANs (internal– communications, multiplexing)


Selecting the Link Configuration

• Point-to-point– Dedicated link for users ! high, predictable– Performance– Need many links, planning of end-points (Topology)– Wired links, wireless links with directional antennas

• Point-to-multipoint– Cover multiple users with single link– Requires sharing of medium: multiple access, Medium– Access Control protocol– Wireless links with omnidirectional antennas, shared– wired links


Identifying a Network Type• Communications medium– Wire cable, fiber-optic cable, radio waves, microwaves

• Protocol– How networked data is formatted into discrete units– How each unit is transmitted and interpreted

• Topology– Physical layout of cable and logical path

• Network type– Private versus public


LAN/WAN Integration

• Becoming more advanced through networking devices– Bridges– Routers– Gateways– Switches


Bridges

• Connect different LANs or LAN segments using the same access method


Routers

• Connect networks having the same or different access methods and media

• Forward packets and frames to networks by using a decision-making process based on:– Routing table data– Discovery of most efficient routes– Preprogrammed information from network

administrator


Routers


Gateways

• Enable communications between two different types of networked systems


Switches• Link network

segments• Forward and filter

frames between segments


Integrating Data Between LANs and WANs

• Translation(Gateway)• Encapsulation(Data PDU)• LAN emulation (LANE): Local Area Network

Emulation (LANE) enables local area network (LAN) data bridging and routing over an Asynchronous Transfer Mode (ATM) network and facilitates the exchange of Ethernet and token ring network data.


Media Access

• Perhaps the most important part of data link protocols. This part determines the way a particular device gains the right to transmit data on the medium

• There are two main standards that are adhered to—contention and token passing.


Contention

• Each station has equal access to the medium.– Each node monitors the medium to see if a

message is being transmitted.– If no message is detected then any node can

transmit.


Collision

• One of the challenges with contention is the possibility that two stations will transmit at the same time.


CSMA/CD

• An alternative format to straight contention• Known as a fair protocol– All nodes listen to the medium to see whether a message

is being transmitted– If medium is quiet, transmit message. If medium is busy,

wait for the signal to clear then transmit.– If a collision occurs, wait for the signal to clear, then wait

for a random interval, and then retransmit


Token Passing Media Access Protocol

– Nodes wait to transmit token– If transmit token is received and there is no message to

send, send the token to the next node.– If token is received and there is a message to send, then:

• Transmit message• Wait for acknowledgement• When acknowledgement is received, pass the token to the next

mode.


Token Ring versus Ethernet (CSMA/CD)

• Token Ring networks are deterministic in nature -nodes may only transmit at certain well defined times. Result is high bandwidth efficiency. Up to 90% in Token Ring, 40% in Ethernet.

• Guaranteed sequential access to network eliminates fluctuating response times experienced on other network topologies.

• Token Ring performance does not deteriorate to the same extent as Ethernet when network traffic increases. This means that at high loads, the presence of collisions of data frames on Ethernet networks becomes a major problem and can seriously affect the throughput.

• By its nature Token Ring has a higher reliability, the ring can continue normal operation in most cases despite any single fault.

• Ethernet has an advantage over Token Ring in that the cost of network equipment is lower for Ethernet. Token Ring networks tend to be more expensive to set up and maintain than Ethernet, although hardware costs for Token Ring are decreasing.

• Advances in Ethernet technology have tended to be much more rapid than Token Ring. Gigabit Ethernet being an example of this. Token Ring technologies are being developed however to allow data transfer rates of 100Mbps using technologies such as Token Ring Switching.

Technology

Computer Networking: LAN and WAN Technologies