Network Technology CSE3020 Week 7

1
Network Technology CSE3020 Week 7
2
Local Area Network (LAN)

• Stations are near to each other (short
  propagation delay). ?
• Number of stations per LAN generally small (easy
  to manage).
• Stations access the network occasionally
  (bandwidth sharing).
• Size increases from time to time (flexible
  scalable).
• Connection Broadcasting.
• Bandwidth Allocation Dynamic.
• Protocol Service Connectionless.

3
LAN Applications

• Personal computer LANs
• Around small businesses and homes (share
  printers, disk, internet access).
• Built with Hubs and/or Switches
• Data rate 10/100 Mbps
• Back end and storage area LANs
• Interconnecting large systems - mainframes and
  large storage devices.
• Data rate, 100 Mbps, and high speed parallel I/O
  interface or Fibre Channel
• Limited distance and limited number of devices
  Computer room

4
LAN Applications

• High speed office LANs
• Use hubs and/or switches
• Desktop image processing transfer of high
  quality images.
• High capacity local storage.
• 100Mbps
• Backbone LANs
• Use switches
• Interconnect low speed local LANs.
• Improves reliability, capacity and cost of a
  single LAN.
• 100Mbps

5
LAN Architecture

• Protocol architecture.
• Topologies.
• Logical Link Control.
• Media Access Control.

6
Protocol Architecture

• Higher layer protocols (above layer 3) are
  independent of network architecture.
• LAN protocols concern with lower layers of the
  OSI model.
• IEEE 802 reference model
• Logical link control (LLC).
• Media access control (MAC).
• Physical.

7
IEEE 802 Layers

• Logical Link Control
• Interface to higher levels.
• Flow and error control.
• Media Access Control
• Assembly of data into frame with address and
  error detection fields.
• Disassembly of frame, Address recognition and
  Error detection.
• Govern access to transmission medium.
• For the same LLC, several MAC options may be
  available.
• Physical
• Encoding/decoding of signal.
• Preamble generation for synchronization.
• Bit transmission/reception.
• Specification of Transmission medium and topology

8
LAN Protocols - Encapsulation
9
LAN Topologies
Star and Extended Star are ubiquitous
10
Bus and Tree Topologies

• Bus is a special case of the tree only one trunk
  and no branches.
• Multipoint medium.
• Transmission propagates throughout the medium.
• Broadcast - heard by all stations.
• Need to identify target station - each station
  has unique address.
• Data transmitted in small blocks frames.
• Full duplex connection between station and its
  tap, channel for
• Transmission of outgoing frame
• Collision detection or reception of incoming
  frame
• Need to regulate transmission collisions,
  hogging.
• Terminator absorbs frames at end of medium
  avoid signal reflection.

11
Bus LAN Frame Transmission
12
Ring Topology

• Repeaters joined by point-to-point links in
  closed loop.
• Simple device.
• Receive data on one link and retransmit on
  another.
• Links unidirectional.
• Stations attach to the repeaters.
• Data is transmitted in frames.
• Circulate frames past all stations.
• Destination recognizes address and copies frame.
• Frame circulates back to source where it is
  removed.
• Media access control determines when station can
  insert frame.

13
Ring LAN Frame Transmission
14
Star Topology

• Each station is connected directly to a common
  central node.
• Usually via two point-to-point links.
• One for transmission and one for reception.
• Central node can broadcast to all stations.
• Physically a star, but logically bus.
• Only one station at a time may successfully
  transmit.
• Central node can act as frame switch.
• Incoming frames are buffered at the central node.
• Then, retransmitted on an outgoing link to the
  destination.

15
Media Access Control

• Controlling access to the transmission medium for
  an orderly and efficient use of the networks
  transmission capacity.
• Where control is exercised
• Central a controller grants access to the
  network.
• Greater control (e.g., priority, overrides and
  guaranteed capacity).
• Simple access logic at station.
• Avoids problems of co-ordination among peer
  stations.
• Creates a single point of network failure.
• It may act as a potential bottleneck, reducing
  performance.
• Distributed stations collectively control access.

16
LAN - Media Access Control

• How constrained by topology, cost, performance
  and complexity.
• Synchronous MAC
• Specific capacity dedicated to connection (like
  FDM and TDM).
• Not optimal in LANs as the needs of the stations
  are unpredictable.
• Asynchronous MAC
• Allocate capacity in response to demand
  (dynamic).
• Three categories
• round robin
• reservation
• contention.

17
Asynchronous MAC - LAN

• Round robin
• Each station in turn is given the opportunity to
  transmit.
• Control of sequence may be centralized or
  distributed.
• Very efficient, if many stations have data to
  transmit over an extended period.
• Considerable overhead if only a few stations have
  data to transmit.
• Reservation
• A station dynamically reserves future time slots
  for transmission.
• Reservations may be made in a centralized or
  distributed fashion.
• Good for stream traffic voice, video, telemetry
• Contention (the most common)
• No control to determine the turn - all stations
  contend for time.
• Good for bursty traffic - short, sporadic
  transmission terminal traffic.
• Simple to implement and efficient under moderate
  load.
• May collapse under heavy load.

18
MAC Frame Format

• MAC layer receives data from LLC layer.
• Responsible for medium access and transmitting
  data.
• MAC layer detects errors and discards frames.
• LLC optionally retransmits unsuccessful frames.

19
Logical Link Control

• Transmission of link level protocol data units
  (PDUs) between two stations
• Must support multi-access, shared medium.
• Relieved of some link access details by MAC
  layer.
• Addressing involves specifying source and
  destination LLC users.
• Referred to as service access points (SAP).
• User typically higher level protocol or network
  management function.

20
LLC Services and Protocol

• Operation and format is based on HDLC.
• Unacknowledged connectionless service (type 1
  operation)
• Best Effort Service - delivery of frame not
  guaranteed, higher layer deals with reliability
  issues
• No prior logical connection is set up
• No flow and error control.
• Unnumbered information (UI) frame used to
  transfer data
• Error detection and discard at the MAC level

21
LLC Services and Protocol

• Connection-mode service (type 2 operation)
• Reliable Delivery Service
• Logical connection is set up
• Flow and error control.
• Uses HDLC in asynchronous balanced mode (ABM)
• Acknowledged connectionless service (type 3
  operation)
• Frames acknowledged
• No prior logical connection is set up

22
Extended Star LANs

• Use unshielded twisted pair wire UTP Cat5
• Stations are attached to a central active hub
  (act as a repeater).
• Two links
• Transmit and receive.
• Hub repeats incoming signal on all outgoing
  lines.
• Link lengths limited to about 100m.
• Fiber optic - up to 500m.
• Logically bus - with collisions.

23
Extended Star LANs Two Level Topology

• One header hub (HHUB) and one or more
  intermediate hubs (IHUB).

24
Extended Star LANs Hubs and Switches

• Hub Shared medium
• Central hub and a star wiring arrangement.
• Hub retransmits incoming signal to all outgoing
  lines.
• To avoid collision, only one station can transmit
  at a time.
• Switch
• Incoming frame switched to appropriate outgoing
  line.
• Unused lines can also be used to switch other
  traffic.

25
Extended Star LANs Hubs and Switches
26
Extended Star LANs Switches

• No change to software or hardware of devices to
  change from shared to switched.
• Each device has dedicated capacity.
• Scales well.
• Two types of switching hubs
• Store and forward switch
• Accept input, buffer it check CRC, then output.
• Cut through switch
• Take advantage of the destination address being
  at the start of the frame.
• Begin repeating incoming frame onto output line
  as soon as address recognized.
• Highest possible throughput.
• May propagate some bad frames since no CRC check
  performed.

27
Bridges

• Ability to expand beyond single LAN.
• Provide interconnection to other LANs/WANs using
• Bridge.
• Router.
• Bridge is simpler
• Connects similar LANs.
• Identical protocols for physical and link layers.
• Minimal processing.
• More sophisticated bridges are capable of mapping
  from one MAC format to another.
• To connect an Ethernet and a Token ring LAN.
• Router more general purpose
• Interconnect various LANs and WANs.
• Covered in higher layers of the protocol stack.

28
Bridges

• Why use a bridge rather than a single LAN?
• Reliability A fault on the network may disable
  all devices.
• Performance Reduce the number of devices on a
  single LAN.
• Security Keep different levels of secure
  information on separate physical media.
• Geography Multiple LANs separated by
  geographical distances.

• Functions of a Bridge
• Read all frames transmitted on one LAN and accept
  those addresses to any station on the other LAN.
• Using MAC protocol for second LAN, retransmit
  each frame.
• Do the same the other way round.

29
Bridges Operation
30
Bridges Design Aspects

• No modification to content or format of frame.
• No encapsulation.
• Exact bitwise copy of frame.
• Enough buffering to meet peak demand.
• Contains routing and address intelligence
• Must be able to tell which frames to pass.
• May be more than one bridge to cross.
• May connect more than two LANs.
• Bridging is transparent to stations
• Appears to all stations on multiple LANs as if
  they are on one single LAN.

31
Bridges Protocol Architecture

• Function at MAC level.
• IEEE 802.1D defines the protocol architecture.
• Station address is at this level.
• Bridge does not need LLC layer.
• It is relaying MAC frames.
• Can pass frame over external communication
  systems
• e.g. WAN link.
• Capture the MAC frame and encapsulate it in the
  appropriate packaging.
• Forward it across link to the target bridge.
• Target bridge removes encapsulation and forward
  over LAN link.

32
Bridges Connection of Two LANs
33
Bridges Fixed Routing

• Complex large LANs need alternative routes
• Load balancing.
• Fault tolerance.
• Bridge must decide whether to forward frame.
• Bridge must decide which LAN to forward frame on.
• Routing selected for each source-destination pair
  of LANs
• Done in configuration.
• Usually least hop route is selected.
• Only changed when the topology changes.
• A central routing matrix to be created.
• Each bridge needs one routing table (derived from
  routing matrix).
• Simple and minimal processing requirements.
• Too limited for complex network.

34
Bridges Spanning Tree

• Bridge automatically develops routing table.
• Automatically update in response to changes.
• Frame forwarding.
• Address learning.
• Loop resolution.

35
Bridges Frame forwarding

• Bridge maintains a forwarding database for each
  port.
• List station addresses reached through each port.
• For a frame arriving on port X
• Search forwarding database to see if MAC address
  is listed for any port except X.
• If address not found, forward to all ports except
  X.
• If address listed for port Y, check port Y for
  blocking or forwarding state.
• Blocking prevents port from receiving or
  transmitting.
• If not blocked, transmit frame through port Y.

36
Bridges Address Learning

• Can preload forwarding database into each bridge
  (as in fixed routing).
• Bridges can learn forwarding database.
• When frame arrives at port X, it has come form
  the LAN attached to port X.
• Use the source address to update forwarding
  database for port X to include that address.
• Timer on each entry in database entry removed
  when timer expires.
• Each time frame arrives, source address checked
  against forwarding database.
• If the element is already in database, update
  entry and reset timer.
• Otherwise, a new entry is created, with its own
  timer.

37
Bridges Spanning Tree Algorithm

• Address learning works for tree layout (no closed
  loops).

38
Bridges Spanning Tree Algorithm

• For any connected graph, there is a spanning tree
  that maintains connectivity but contains no
  closed loops.
• Spanning tree algorithm developed by IEEE 802.1.
• Each bridge assigned unique identifier and costs
  is assigned to each port.
• Brief exchange of messages among bridges to
  establish minimum-cost spanning tree.

39
Required Reading

• W. Stallings, Data and Computer Communications
  Prentice-Hall.
• gtgt Ch13 6E, Ch15 7E
• The End

40

• Following Slides for Interest ONLY

41
Bus LANs

• Multipoint Configuration more than two devices.
• Signal balancing
• Signal must be strong enough to meet receivers
  minimum signal strength requirements.
• Strong enough to maintain adequate signal to
  noise ratio.
• Not so strong that it overloads transmitter.
• Must satisfy these for all combinations of
  sending and receiving station on bus.
• Divide network into small segments.
• Link segments with amplifies or repeaters.

42
Bus LAN Transmission Media

• Twisted pair
• Not practical in shared bus at higher data rates.
• Baseband coaxial cable
• Used by the original Ethernet and IEEE 802.3
  systems.
• Broadband coaxial cable
• More expensive and difficult to install and
  maintain.
• Included in 802.3 specification but no longer
  made.
• Optical fiber
• Expensive, difficulty with availability, not
  commonly used.
• Few new installations
• Replaced by star based twisted pair and optical
  fiber.

43
Bus LAN Baseband Coaxial Cable

• Uses digital signaling Manchester or
  Differential Manchester encoding.
• Entire frequency spectrum of cable used (single
  channel on cable).
• Bi-directional transmission over few kilometer
  range.
• Original use for Ethernet (basis for 802.3) at
  10Mbps.
• 50-ohm cable (less reflection and noise than the
  standard CATV 75-ohm cable).
• 10Base5 (10 Mbps, baseband, 500 m cable)
• Ethernet and 802.3 originally used 0.4 inch
  diameter cable.
• 10Base2 (10 Mbps, baseband, 200 m cable (185m))
• Cheapernet using 0.25 inch cable.
• More flexible, easier to bring to workstation,
  cheaper electronics.
• Greater attenuation and lower noise resistance.

44
Bus LAN Repeaters

• Transmits in both directions.
• Joins two segments of cable.
• No buffering.
• No logical isolation of segments.
• If two stations on different segments send at the
  same time, packets will collide.
• Only one path of segments and repeaters between
  any two stations.

45
Ring LANs

• Each repeater connects to two others via
  unidirectional transmission links to create a
  single closed path.
• Data transferred bit by bit from one repeater to
  the next.
• Repeater regenerates and retransmits each bit.
• Repeater performs data insertion, data reception,
  data removal.
• Repeater acts as attachment point.
• Packet removed by transmitter after one trip
  round ring.
• Permits automatic acknowledgement.
• Permits multicast addressing.
• MAC protocol for determining how and when packets
  are inserted.
• Ring repeater - Pass all data that comes its
  way. - Provide access point to the
  attach stations to send and
  receive data.

46
Ring LANs Repeater States

• Listen State Functions
• Scan passing bit stream for patterns.
• Address of attached station.
• Token for permission to transmit.
• Copy incoming bit and send it to attached
  station, while forwarding each bit.
• Modify a bit as it passes by.
• To indicate a packet has been copied
  (acknowledgement).

47
Ring LANs Repeater States

• Transmit State Functions
• Station has data and the repeater has permission
  to send.
• Repeater receives bits from the station and
  retransmit them on its outgoing link.
• May receive incoming bits during transmission
• If ring bit length shorter than packet.
• Pass back to station for checking (ACK).
• May be more than one packet on ring.
• Buffer for retransmission later.
• Bypass State
• Signals propagate past repeater with no delay
  (other than propagation delay).
• Partial solution to reliability problem.
• Improved performance by eliminating repeater
  delay for inactive station.

48
Ring LANs Transmission Media

• Twisted pair.
• Baseband coaxial.
• Fiber optic.
• Not broadband coaxial.
• Would have to receive and transmit on multiple
  channels, asynchronously.

49
Ring LANs Timing Jitter

• Clocking included with signal
• e.g. differential Manchester encoding.
• Clock recovered by each repeaters.
• To know when to sample signal and recover bits.
• Use the clocking for retransmitting the signal to
  the next repeater.
• Clock recovery deviates from mid-bit transitions
  randomly
• Due to noise during transmission.
• Imperfections in receiver circuitry.
• Known as timing jitter.
• Retransmission without distortion but with timing
  error.
• Cumulative effect is that bit length varies.
• Limits number of repeaters in a ring.

50
Ring LANs Solving Timing Jitter

• Cannot be entirely overcome.
• Two approaches are used in combination.
• Repeater uses phase locked loop.
• Uses feedback to minimize deviation from one bit
  to the next.
• Use buffer at one or more repeaters.
• Hold a certain number of bits.
• Expand and contract to keep bit length of ring
  constant.
• Significant increase in maximum ring size.

51
Ring LANs Potential Problems

• Break in any link disables the network.
• Repeater failure disables the network.
• Installation of new repeater to attach new
  station requires identification of two
  topologically adjacent repeaters.
• Timing jitter.
• Method of removing circulating packets required.
• With backup in case of errors.
• Mostly solved with star-ring architecture.

52
Ring LANs Star Ring Architecture

• Feed all inter-repeater links to single site
  (wiring concentrator).
• Provides central access to signal on every link.
• Easier to find faults.
• Can launch message into ring and see how far it
  gets.
• Faulty segment can be disconnected and repaired
  later.
• New repeater can be added easily.
• Bypass relay can be moved to the wiring
  concentrator.
• Can lead to long cable runs.
• Can connect multiple rings using bridges