CSC 335 Data Communications and Networking

1
CSC 335 Data Communications and Networking

• Lecture 7b Local Area Networking
• Dr. Cheer-Sun Yang
• Fall 2000

2
Topologies

• Bus A single communication line, typically a
  twisted pair, coaxial cable, or optical fiber,
  represents the primary medium.
• Ring packets can only be passed from one node to
  its neighbor.
• Star A hub or a computer is used to connect to
  all other computers.
• Tree no loop exists (logical connection).

3
Token Passing

• Token Ring (802.5) P. 183, Section 6.3
• Token Bus (802.4) P. 186, Section 6.4

4
Token Passing

• The difficulty with many networks is that no
  central control or authority makes decisions on
  who sends when.
• Token passing is designed to deal with this issue
  and hopefully the link utilization can be
  increased.

5
Token Passing

• In order to send, a station must obtain an
  admission pass, called a token.
• In a token ring, the token is passed from one
  station to another.
• When a station does not need it, it simply passes
  it on.
• Token ring network must pass the token orderly to
  its neighbor.
• Token bus network can pass a token to any other
  station directly.

6
Token Passing

• However, a token bus network cannot be added as
  simply as with the CSMA/CD bus.
• All stations must know who and where its neighbor
  is in a token bus.

7
6.3 Token Ring IEEE 802.5

• Each repeater connects to two others via
  unidirectional transmission links
• 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

8
Token Ring (802.5)

• MAC protocol
• Small frame (token) circulates when idle
• Station waits for token
• Changes one bit in token to make it SOF for data
  frame
• Append rest of data frame
• Frame makes round trip and is absorbed by
  transmitting station
• Station then inserts new token when transmission
  has finished and leading edge of returning frame
  arrives
• Under light loads, some inefficiency
• Under heavy loads, round robin

9
Dedicated Token Ring

• Central hub
• Acts as switch
• Full duplex point to point link
• Concentrator acts as frame level repeater
• No token passing

10
802.5 Physical Layer

• Data Rate 4 16 100
• Medium UTP,STP,Fiber
• Signaling Differential Manchester
• Max Frame 4550 18200 18200
• Access Control TP or DTR TP or DTR DTR
• Note 1Gbit in development

11
Ring Repeater States
12
Listen State Functions

• Scan passing bit stream for patterns
• Address of attached station
• Token permission to transmit
• Copy incoming bit and send to attached station
• Whilst forwarding each bit
• Modify bit as it passes
• e.g. to indicate a packet has been copied (ACK)

13
Transmit State Functions

• Station has data
• Repeater has permission
• May receive incoming bits
• 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

14
Bypass State

• Signals propagate past repeater with no delay
  (other than propagation delay)
• Partial solution to reliability problem (see
  later)
• Improved performance

15
Ring Media

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

16
Two observations

1. Ring contention is more orderly than with an
   Ethernet. No wasted bandwidth.

17
Two observations

• 2. The failure of one station can cause network
  failure. More discussion will be provided in next
  slide.

18
Potential Ring Problems

• Break in any link disables network
• Repeater failure disables 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 (the
  wire center approach).

19
Network Failure Problem

• The failure of one station can cause network
  failure This problem can be solved by using a
  wire center (Fig. 6.11). Instead of connecting
  neighboring stations directly, they all
  communicate through a wire center. The wire
  center contains a bypass relay. If a station
  fails, the bypass relay will allow a frame to
  bypass the station.
• This architecture is called a Star Ring
  Architecture.

20
Star Ring Architecture

• Feed all inter-repeater links to single site
• 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 concentrator
• Can lead to long cable runs
• Can connect multiple rings using bridges

21
Timing Jitter

• Clocking included with signal
• e.g. differential Manchester encoding
• Clock recovered by repeaters
• To know when to sample signal and recover bits
• Use clocking for retransmission
• Clock recovery deviates from midbit transmission
  randomly
• Noise
• Imperfections in circuitry
• Retransmission without distortion but with timing
  error
• Cumulative effect is that bit length varies
• Limits number of repeaters on ring

22
Solving Timing Jitter Limitations

• Repeater uses phase locked loop
• 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

23
Token Ring MAC Frame
24
Token and Frame Formats

• Start Delimiter (SD), End Delimiter (ED) 1 octet
• Access Control (AC) 1 octet, 3 priority bits, 1
  token bit, 1 monitor bit, 3 reserved bits.
• Frame Control (FC) used to distinguish control
  frame from data frame.
• Frame Status(FS) 1 octet (acxxacxx) A address
  recognized bit, C frame copied bit, X undefined
  bit.
• A 0, C0 dest not present or not power up
• A 1, C 0 dest present but frame is not
  accepted
• A 1, C 1 dest present and frame copied.

25
Reserving and Claiming Tokens
B
A
token
C
D
26
Reserving and Claiming Tokens
B
A
Station A requests the token and sends its data
to D
C
D
27
Reserving and Claiming Tokens
B
A
C
D
Station C can reserve the next open token By
entering its priority code in the AC field.
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Reserving and Claiming Tokens
B
A
Station D copies the frame and sends the data
back to the ring.
C
D
29
Reserving and Claiming Tokens
B
A
Station A receives the frame and releases the
token
C
D
30
Reserving and Claiming Tokens
B
A
C
D
Station C can send its data now.
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Token RingOperation
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Disadvantage of Token Ring

• Token maintenance requires extra work.
• Loss of token prevents further utilization of the
  ring.
• Duplication token can disrupt the operation.
• A monitor station is required. It becomes a
  crucial point for a single point failure.

33
Advantage of Token Ring

• The flexible control over access that it
  provides.
• The access is fair.
• It is easy to provide priority and guaranteed
  bandwidth services.

34
Priority Scheme

• A station having a higher priority frame to
  transmit than the current frame can reserve the
  next token for its priority level as the frame
  passes by.
• When the next token is issued at a station A, it
  will be at the reserved priority level. The
  station reserving the token can use this token to
  transmit data frame.
• The station A is responsible to down-grade the
  priority of the token later.

35
Priority Scheme
36
Priority Scheme

• A sends a frame to B at priority 0.
• When the frame passes by D, D makes a reservation
  at priority 3.
• When the token is sent back to A, A changes the
  priority to 3 and issues a new token.
• D can use this token to send a frame to any
  station.
• After the data is seized by the destination and
  the token is passed back to A, A is responsible
  for changing the priority back to 0. (Why A?)

37
Time Limits

• Token holding time the time duration a station
  is allowed to hold the token
• Token rotation time the total time a token is
  allowed to rotate around the ring.
• TRT gt N THT

38
Ring Maintenance

• Things can go wrong. For example
• A station sends a short frame over a long ring
  and subsequently crashes. It is not able to drain
  the token. This frame is called an orphan frame.
• A station receives a frame or token crashes
  before it can send it. Now there is no token
  circulating.
• Line noise damages a frame.

39
Ring Maintenance

• Some problems can be handled by giving one of the
  stations a few different responsibilities and
  designating it a monitor station.
• When a monitor station receives a frame, it sets
  the monitor bit to 1. If the frame is received
  the second time and the monitor bit is still set
  to 1, the monitor station deletes the frame.

40
Ring Maintenance

• 2. The monitor station also detect a lost token
  using a built-in timer which is determined based
  on the length of the ring, number of stations,
  and maximum frame size. Whenever the monitor
  sends a frame or token, it starts the timer. If
  the monitor does not receive another frame or
  token before the timer expires, it assumes that
  the token is lost. It then creates another one.

41
Ring Maintenance

• Some problems cannot be solved even with a
  monitor station. For example, what if the
  malfunction station is the monitor station? What
  if a break in the ring causes a lack of tokens?
  Sending new ones does nothing to correct the
  problem. These problems are handled using control
  frames.

42
Ring Maintenance

• Some example control frames
• Claim token frame for submitting bids to elect
  a monitor station.
• Active monitor present (AMP) frame to notify
  others that a monitor station has been produced.
• Standby monitor present (SMP) frame.
• Beacon frame to inform stations that a problem
  has occurred and the token-passing protocol has
  stopped.

43
Ring Efficiency

• T1 time to send a frame
• T2 time to send a token

44
Other Ring Networks FDDI

• 100Mbps
• LAN and MAN applications
• Token Ring

45
FDDI MAC Frame Format
46
FDDI MAC Protocol

• As for 802.5 except
• Station seizes token by aborting token
  transmission
• Once token captured, one or more data frames
  transmitted
• New token released as soon as transmission
  finished (early token release in 802.5)

47
FDDI Operation
48
FDDI Physical Layer

• Medium Optical Fiber Twisted Pair
• Data rate 100 100
• Signaling 4B/5B/NRZI MLT-3
• Max repeaters 100 100
• Between repeaters 2km 100m

49
LAN Generations

• First
• CSMA/CD and token ring
• Terminal to host and client server
• Moderate data rates
• Second
• FDDI
• Backbone
• High performance workstations
• Third
• ATM
• Aggregate throughput and real time support for
  multimedia applications

50
Third Generation LANs

• Support for multiple guaranteed classes of
  service
• Live video may need 2Mbps
• File transfer can use background class
• Scalable throughput
• Both aggregate and per host
• Facilitate LAN/WAN internetworking

51
ATM LANs

• Asynchronous Transfer Mode
• Virtual paths and virtual channels
• Preconfigured or switched
• Gateway to ATM WAN
• Backbone ATM switch
• Single ATM switch or local network of ATM
  switches
• Workgroup ATM
• End systems connected directly to ATM switch
• Mixed system

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Example ATM LAN
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ATM LAN HUB
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Compatibility

• Interaction between end system on ATM and end
  system on legacy LAN
• Interaction between stations on legacy LANs of
  same type
• Interaction between stations on legacy LANs of
  different types

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Fiber Channel - Background

• I/O channel
• Direct point to point or multipoint comms link
• Hardware based
• High Speed
• Very short distance
• User data moved from source buffer to destiation
  buffer
• Network connection
• Interconnected access points
• Software based protocol
• Flow control, error detection recovery
• End systems connections

56
Fiber Channel

• Best of both technologies
• Channel oriented
• Data type qualifiers for routing frame payload
• Link level constructs associated with I/O ops
• Protocol interface specifications to support
  existing I/O architectures
• e.g. SCSI
• Network oriented
• Full multiplexing between multiple destinations
• Peer to peer connectivity
• Internetworking to other connection technologies

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Fiber Channel Elements

• End systems - Nodes
• Switched elements - the network or fabric
• Communication across point to point links

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Fiber Channel Network
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Fiber Channel Protocol Architecture (1)

• FC-0 Physical Media
• Optical fiber for long distance
• coaxial cable for high speed short distance
• STP for lower speed short distance
• FC-1 Transmission Protocol
• 8B/10B signal encoding
• FC-2 Framing Protocol
• Topologies
• Framing formats
• Flow and error control
• Sequences and exchanges (logical grouping of
  frames)

60
Fiber Channel Protocol Architecture (2)

• FC-3 Common Services
• Including multicasting
• FC-4 Mapping
• Mapping of channel and network services onto
  fiber channel
• e.g. IEEE 802, ATM, IP, SCSI

61
Wireless LANs

• IEEE 802.11
• Basic service set (cell)
• Set of stations using same MAC protocol
• Competing to access shared medium
• May be isolated
• May connect to backbone via access point (bridge)
• Extended service set
• Two or more BSS connected by distributed system
• Appears as single logic LAN to LLC level

62
Types of station

• No transition
• Stationary or moves within direct communication
  range of single BSS
• BSS transition
• Moves between BSS within single ESS
• ESS transition
• From a BSS in one ESS to a BSS in another ESS
• Disruption of service likely

63
Wireless LAN - Physical

• Infrared
• 1Mbps and 2Mbps
• Wavelength 850-950nm
• Direct sequence spread spectrum
• 2.4GHz ISM band
• Up to 7 channels
• Each 1Mbps or 2Mbps
• Frequency hopping spread spectrum
• 2.4GHz ISM band
• 1Mbps or 2Mbps
• Others under development

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Media Access Control

• Distributed wireless foundation MAC (DWFMAC)
• Distributed coordination function (DCF)
• CSMA
• No collision detection
• Point coordination function (PCF)
• Polling of central master

65
802.11 MAC Timing
66
Interconnecting LANs

• Layer 1 connection repeaters
• Layer 2 connection - bridges

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Repeaters

• Layer 1 connections
• Used to expand physical length of a cable when it
  exceeds the distance limit and attenuation can
  occur.

68
Bridges

• Ability to expand beyond single LAN
• Provide interconnection to other LANs/WANs
• Use Bridge or router
• Bridge is simpler
• Connects similar but different types of LANs
• Identical protocols for physical and link layers
• Minimal processing
• Router more general purpose
• Interconnect various LANs and WANs
• see later

69
Why Bridge?

• Reliability
• Performance
• Security
• Geography

70
Functions of a Bridge

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

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Bridge Operation
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Bridge Design Aspects

• No modification to content or format of frame
• No encapsulation
• Exact bitwise copy of frame
• Minimal 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

73
Bridge Protocol Architecture

• IEEE 802.1D
• MAC level
• Station address is at this level
• Bridge does not need LLC layer
• It is relaying MAC frames
• Can pass frame over external comms system
• e.g. WAN link
• Capture frame
• Encapsulate it
• Forward it across link
• Remove encapsulation and forward over LAN link

74
Connection of Two LANs
75
Types of Bridges

• Transparent bridges
• Spanning tree bridges
• Source routing bridges

76
Transparent Bridges

• Complex large LANs need alternative routes
• Load balancing
• Fault tolerance
• Bridge(NOT the source) 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
• Only changed when topology changes

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Multiple LANs
78
Spanning Tree

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

79
Frame forwarding

• Maintain 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

80
Address Learning

• Can preload forwarding database
• Can be learned
• 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
• Each time frame arrives, source address checked
  against forwarding database

81
Spanning Tree Algorithm

• Address learning works for tree layout
• i.e. no closed loops
• For any connected graph there is a spanning tree
  that maintains connectivity but contains no
  closed loops
• Each bridge assigned unique identifier
• Exchange between bridges to establish spanning
  tree

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Loop of Bridges
83
Source Routing Bridges

• Although source routing bridges can be used with
  any type of LAN segment, they are used primarily
  for the interconnection of token ring LAN
  segments.
• The spanning tree bridges perform the routing in
  a way that is transparent to the end stations.
  Conversely, with source routing, the end stations
  perform the routing function.
• The necessary information must be included in a
  frame.

84
Comparison of LAN Bridges

• See Table 6.8

85
Reading

• Chapter 6 6.1-6.5