Module 8 LAN Part II

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Module 8LAN Part II
2

• Textbook sections
• LG Section 6.6.2 Token-Ring and IEEE 802.5 LAN
  Standard
• LG Section 6.7.1 Transparent Bridges
• LG Section 6.7.2 Source Routing Bridges
• Topics
• Token-Ring (IEEE802.5)
• Overview
• Frame Format
• Priority Access Mechanism
• Ring Maintenance
• Ring Latency and Efficiency
• Transparent Bridges
• Overview
• Bridge Learning Packet filtering/Forwarding
• Spanning Tree
• Source Routing Bridges

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1. Token Ring (IEEE 802.5) - Overview

• Introduction
• Token ring resolves collision problem by
  requiring that stations take turns sending data.
  
• Each station may transmit only during its turn
  and may send only one frame during each turn.
  The mechanism that coordinates this rotation is
  called token passing.
• A token is a simple placeholder frame that is
  passed from station to station around the ring.
  A station may send data only when it has
  possession of the token.
• Access method
• Token passing (more details later)
• Addressing
• Six-byte physical address, which is imprinted on
  the NIC card similar to Ethernet addresses
• Electrical specification
• Signaling Differential Manchester encoding
• Data Rate Up to 16 Mbps
• Frame Format (more detail later)
• Considerations (more detail later)

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1. Token Ring (IEEE 802.5) - Overview

• Token Passing Mechanism
• Free Token
• Whenever the network is unoccupied, it circulates
  a simple three-byte token
• This token is passed from Network Interface Card
  (NIC) to NIC in sequence until it encounters a
  station with data to send
• Station with data to send
• It waits for the token to enter its network
  board. If the token is free, the station may
  then send a data frame. It keeps the token and
  sets a bit inside its NIC as a reminder that it
  has done so.

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1. Token Ring (IEEE 802.5) - Overview

• Receiving station
• The data frame proceeds around the ring, being
  regenerated by each station. Each intermediate
  station examines the destination address.
• If it finds that the frame is addressed to
  another station then it relays the data frame to
  its neighbor.
• The intended recipient recognized its own
  address, copies the message, check for errors,
  and change four bits in the last byte of the
  frame to indicate address recognized and frame
  copied
• The full packet then continues around the ring
  until it returns to the sending station.
• Token release (by the sending station)
• The sending station receives the frame and
  recognizes itself in the source address field.
  It then examines the address-recognized bits. If
  they are set, it knows the frame was received.
• The sending station then discard the used data
  frame and release the token back to the ring.

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Token Passing Mechanism
7
Token Passing Mechanism
8
Token Passing Mechanism
9
Token Passing Mechanism
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1. Token Ring (IEEE 802.5) - Overview

• Advantages and disadvantages of token rings
• Advantages Fairness
• Disadvantages Consequences of a fault
• The entire network will fail if there is a break
  in any transmission link or a failure in the
  mechanism that relays a signal from one
  point-to-point link to the next.
• Remedy
• Star topology (LG Figure 6.58)

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LG Figure 6.58 Token-ring network implemented
using a star topology
Reliability is provided by relays that can
bypass the wires of stations that are deemed to
have failed (for example station E).
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LG Figure 6.61 IEEE 802.5 Token and data frame
structure
ED
SD
AC
Token Frame Format
Data Frame Format
1
1
4
2 or 6
1
2 or 6
1
1
Destination Address
Source Address
Information
AC
FS
SD
FCS
FC
ED
Starting delimiter
J K 0 J K 0
0
0
J, K non-data symbols (line code)
Access control
PPP Priority T Token bit M Monitor bit RRR
Reservation
P P P
T
M
R R R
FF frame type ZZZZZZ control bit
Frame control
Z Z Z Z Z Z
F F
I intermediate-frame bit E
error-detection bit
Ending delimiter
J K 1 J K 1
I
E
Frame status
A address-recognized bit xx undefined C
frame-copied bit
A
C
x x
A
C
x x
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• Starting delimiter (SD) field
• Alerts each station of the arrival of a token (or
  data frame). This field includes signals that
  distinguish the byte from the rest of the frame
  by violating the differential Manchester encoding
  scheme used elsewhere in the frame.
• J violation has the same polarity as the
  trailing element of the preceding symbol, but has
  no transition in the middle
• K violation has the opposite polarity as the
  trailing element of the preceding symbol, but has
  no transition in the middle.
• To avoid an accumulating dc component, non-data
  component are normally transmitted as a pair of J
  and K symbols.
• Access control (AC) field Includes four
  subfields.
• PPP subfield priority The IEEE 802.5 standard
  allows the token ring to operated with a priority
  access mechanism. To transmit a frame of a given
  priority, a station must wait to capture a token
  of equal or lower priority.
• RRR subfield reservation The station can
  reserve a token of the desired level by setting
  the RRR field in passing frames to the level of
  priority of its frame if the RRR level is lower
  than the priority the station is seeking.
• The priority bits (PPP) and the reservation bits
  (RRR) work together to provide a priority access
  mechanism

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1. Token Ring (IEEE 802.5) Frame Format

• T subfield T 0 indicates a token frame, and T
  1 indicates a data frame
• M subfield The monitor bit is used by a
  designated monitor station to identify and remove
  orphan frames that are not removed from the
  ring by their sending station. (details later)
• Frame control field
• This field indicates whether a frame contains
  data or control information. FF01 indicates a
  data frame and the Z bits are ignored. FF00
  indicates a control frame and the Z bits indicate
  the type of control frame
• Ending delimiter (ED) field
• I bit indicates the last frame in a sequence of
  frames exchanges between two stations
• E bit indicates that a station interface has
  detected an error such as a line code violation
  or a frame check sequence error.

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1. Token Ring (IEEE 802.5) Frame Format

• Frame status (FS) field
• This field allows the receiving station to convey
  transfer status information to the sending
  station.
• A 1 indicates that the destination address was
  recognized by the receiving station
• C 1 indicates that the frame was copied onto
  the receiving stations buffer
• Receiving station sets A and C bits. Sending
  station examines A and C bits.
• Setting A C bits
• When a frame arrives at the interface of the
  receiving station, the interface turns on the A
  bit as it passes through
• If the interface copies the frame to the
  receiving station, it turns on the C bit.

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1. Token Ring (IEEE 802.5) Frame Format

• Frame status (FS) field
• Examining A C bits
• When the sending station drain the frame from the
  ring, it examine the A and C bits
• A 0 , and C 0 receiving station not
  present or not powered up
• A 1, and C 0 receiving station present
  but frame not accepted
• A 1, and C 1, receiving station present
  and frame copied.
• The A, C bits come at the beginning of the field
  and are repeated in the fifth and sixth bits.
  This repetition is for the purpose of preventing
  errors and is necessary because the field
  contains information inserted after the frame
  leaves the sending station. It therefore cannot
  be included in the CRC and so has no error
  checking performed on it.

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1. Token Ring (IEEE 802.5) Priority Access
Mechanism

• Priority access mechanism
• To transmit a frame of a given priority, a
  station must wait to capture a token of equal or
  lower priority
• The station can reserve a token of the desired
  level by setting the RRR field in passing frames
  to the level of priority of its frame if the RRR
  level is lower than the priority the station is
  seeking
• When the token arrives at a station that has a
  frame of higher or equal priority, the token is
  removed and a data frame is inserted into the
  ring. The RRR field in the data frame is set to
  0, and the priority field is kept at the same
  value as the token frame.
• When the station is done transmitting its frames,
  it issues a token at the reserved priority level.

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1. Token Ring (IEEE 802.5) Priority Access
Mechanism

• The priority access mechanism is a little
  confusing and non-intuitive at first. The basic
  rules are
• To capture the token
• Any station wishing to capture the token can only
  do so if the current priority of the token is
  lower than its priority.
• To reserve the token
• If the priority of the token is higher than the
  priority of the station, the station may set the
  priority reservation to request a lower priority,
  but only if another station has not already set
  the priority reservation to a higher value than
  this stations priority
• To reset the token priority
• Any station that raises the priority of the token
  must lower the priority back down to its original
  value the next time it sees a free token. This
  makes sure that everybody gets a chance to talk
  eventually.

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1. Token Ring (IEEE 802.5) Priority Access
Mechanism

• An example of the priority access mechanism

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1. Token Ring (IEEE 802.5) Priority Access
Mechanism

• An example of the priority access mechanism
  (continue)

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1. Token Ring (IEEE 802.5) Priority Access
Mechanism

• An example of the priority access mechanism
  (continue)

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1. Token Ring (IEEE 802.5) Priority Access
Mechanism

• An example of the priority access mechanism
  (continue)

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1. Token Ring (IEEE 802.5) Ring Maintenance

• Monitor station
• Token rings have a designated monitor station to
  ensure the health of the ring.
• Any station on the ring can become the monitor
  station
• There are defined procedures by which the monitor
  is elected when the ring is first connected or on
  the failure of the current monitor station
• Functions performed by the monitor station
• Insert additional delay into the ring if
  necessary
• Ensure there is a token in the ring
• Use a timer to detect a missing token. (timer
  equal to the maximum possible token rotation
  time)
• For example, a token may vanish because a failure
  occurred in the station that was holding the
  token.

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1. Token Ring (IEEE 802.5) Ring Maintenance

• Check for corrupted or orphaned frames (without
  monitor stations intervention, these frames
  could circulate forever on the ring)
• Corrupted frames are the ones having checksum
  errors or invalid formats.
• An orphaned frame is one that was transmitted
  correctly onto the ring, but whose parent died
  for example, the sending station went down before
  it could remove the frame from the ring.
• Approach Use the monitor bit (M) in the access
  control field. This is 0 on transmission and set
  to 1 the first time the frame passes the monitor
  station. If the monitor sees a frame with this
  bit sets, it knows the frame is going by for the
  second time and the monitor station drains the
  frame off the ring.

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1. Token Ring (IEEE 802.5) Ring Latency and
Efficiency

• Ring latency and its impact on ring efficiency
• The ring latency is defined as the time that it
  takes for a bit to travel around the ring and is
  given by
• ring latency propagation delay total station
  interface delay
• t d/v (Mb)/R in seconds
• where
• d total length of the links around the ring
• v propagation speed in the medium (typical
  2108 meters/second)
• M number of stations of the ring
• b interface delay incurred at each station
  between when the interface receives a frame and
  forwards it along the outgoing link (typical
  value is 2.5)
• R bit rate (typically between 4Mbps to 16
  Mbps)
• Ring latency can also be expressed in bits
• tR dR/v (Mb)

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1. Token Ring (IEEE 802.5) Ring Latency and
Efficiency

• Token reinsertion approaches
• Single-token operation
• Single-packet operation
• Multi-token operation

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LG Figure 6.59 Ring latency and token reinsertion
strategies
(a) Low Latency Ring
A
A
A
A
t90, return of first bit
t400, transmit last bit
t490, reinsert token
t0, A begins frame
(b) High Latency Ring
A
A
A
A
t0, A begins frame
t840, return of first bit
t1240, reinsert token
t400, last bit of frame enters ring
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LG Figure 6.60 Reinsert token after header of
frame returns
(a) Low Latency Ring
A
A
A
A
t400, last bit enters ring, reinsert token
t90, return of first bit
t210, return of header
t0, A begins frame
(b) High Latency Ring
A
A
A
A
t400, transmit last bit
t840, arrival first frame bit
t960, reinsert token
t0, A begins frame
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2. Transparent Bridges

• Bridges
• Three functions
• Address learning
• Based on the sender address
• Packet filtering/forwarding/flooding
• Based on the destination address
• Loop avoidance

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2. Transparent Bridges

• Packet forwarding/filtering
• Filtering
• If the destination device is on the same segment
  as the frame, the bridge blocks the frame from
  going on to other segments.
• Forwarding
• If the destination device is on a different
  segment, the bridge forwards the frame to the
  appropriate segments.
• Flooding
• If the destination address is unknown to the
  bridge, the bridge forwards the frame to all
  segments except the one on which it was received.

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2. Transparent Bridges - Bridge Learning Packet
Filtering/Forwarding

• Algorithm of build forwarding tables and
  forwarding frames
• When the bridge receives a frame, the source
  address is compared to the forwarding table. If
  the source address in not there, the source
  address and port number of the frame are added to
  the forwarding table.
• The bridge then compares the destination address
  with the forwarding table
• If the destination address is in the table and is
  on the same segment as the source address, the
  frame is discarded. This filtering helps to
  reduce network traffic and isolate segments of
  the network.
• If the destination address is in the table and
  not in the same segment as the source address,
  the bridge forwards the frame out of the
  appropriate port to reach the destination
  address.
• If the destination address is not in the table,
  the bridge forwards the packet to all of its
  ports, except the one on which the frame
  originated

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LG Figure 6.81Initial configuration
S5
S1
S2
S3
S4
LAN1
LAN2
LAN3
B1
B2
port 1
port 2
port 1
port 2
Address Port
Address Port
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LG Figure 6.82 S1 sends a frame to S5
S1
S2
S5
S3
S4
S1 S5
LAN1
LAN2
LAN3
B1
B2
port 1
port 2
port 1
port 2
Address Port
Address Port
S1
1
S1
1
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LG Figure 6.83 S3 sends a frame to S2
S1
S2
S5
S3
S4
S3 S2
LAN1
LAN2
LAN3
B1
B2
port 1
port 2
port 1
port 2
Address Port
Address Port
S1
1
S1
1
S3
2
1
S3
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LG Figure 6.84 S4 sends a frame to S3
S5
S1
S2
S3
S4
S4 S3
LAN1
LAN2
LAN3
B1
B2
port 1
port 2
port 1
port 2
Address Port
Address Port
S1
1
S1
1
S3
2
S3
1
2
S4
2
S4
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LG Figure 6.85 S2 sends a frame to S1
S1
S2
S5
S3
S4
S2 S1
LAN1
LAN2
LAN3
Bridge1
Bridge 2
port 1
port 2
port 1
port 2
Address Port
Address Port
S1
1
S1
1
S3
2
S3
1
2
S4
2
S4
S2
1
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2. Transparent Bridges - Bridge Learning Packet
Filtering/Forwarding

• Collision/broadcast domains
• Collision domain A group of devices connected to
  the same physical media such that if two devices
  access the media at the same time, the result is
  a collision of the two signals
• Broadcast domain A group of devices in the
  network that receive one anothers broadcast
  messages
• Bridges reduce collisions
• Bridges reduce collisions by giving each segment
  its own collision domain
• Bridges do not stop broadcast messages
• Because a bridge learns all the station
  destinations by listening to source addresses, it
  will never learn the broadcast address.
  Therefore, all broadcasts will always be flooded
  to all the segments on the bridge. All segments
  in a bridged environment are therefore considered
  to be in the same broadcast domain

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2. Transparent Bridges Spanning Tree

• Multiply redundant paths exist between LAN
  segments
• To improve fault tolerance
• Side effect frames can cycle and multiply
  within the interconnected LAN
• Bridges dynamically selected a subset of the LAN
  interconnections that provides a loop-free path
  from any LAN to any other LAN using the spanning
  tree algorithm
• Even after the spanning tree has been
  established, the algorithm continues to run in
  order to automatically detect topology changes
  and update the tree. The distributed algorithm
  used for constructing the spanning tree was
  invented by Perlman.

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2. Transparent Bridges Spanning Tree
Two LAN segments are connected by two bridges
Segment 1
Segment 2
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2. Transparent Bridges Spanning Tree

• Example Two LANs connected by two bridges
• Initially, host B has not sent out any packet,
  so neither bridge knows to which segment host B
  is connected.
• Host A sends a packet to host B
• One of the bridge, say Br1, receives the packet
  first and, not knowing where host B is, forwards
  the packet to segment 2.
• The packet goes to its destination (host B), but,
  at the same time, Br2 receives the packet via
  segment 2.
• The packet source address is host A, and its
  destination address is host B. Br2 erroneously
  assumes that host A is connected to segment 2 and
  updates its table accordingly. Because it does
  not have any information about host B. Br2
  forwards the packet to segment 1.

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2. Transparent Bridges Spanning Tree

• Example Two LANs connected by two bridges
  (continues)
• 5. The packet is then received for the second
  time by Br1. Br1 thinks it is a new packet from
  host A and, because it has no information about
  host B. Br1 forwards the packet to segment2.
• 6. Now Br2 receives the packet once again, and
  the cycle will repeat endlessly.

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2. Transparent Bridges Spanning Tree

• Concept
• The spanning tree algorithm is used in data
  structure to create a tree out of a graph
• A tree should include all nodes with minimum
  number of lines connected to nodes
• A node has to be selected as the root
• Trees are not unique. However, we are interested
  in one spanning tree where each node has the
  shortest path to the roots

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2. Transparent Bridges Spanning Tree

• Spanning tree algorithm
• Root bridge
• Select a root bridge among all the bridges in the
  bridged LAN. The root bridge is the bridge with
  the lowest bridge ID
• Root port
• Determine the root port for each bridge except
  the root bridge in the bridged LAN
• The root port is the port with the least-cost
  path to the root bridge.
• In case of ties the root port is the one with
  lowest port ID
• Cost is assigned to each LAN according to some
  criteria. (One criteria could be to assign higher
  costs to lowest speed LANs)
• A path cost is the sum of the costs along the
  path from one bridge to another

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2. Transparent Bridges Spanning Tree

• Spanning tree algorithm
• Designated bridge
• Select a designated bridge for each LAN. The
  designated bridge is the bridge that offers the
  least-cost path from the LAN to the root bridge.
• In case of ties the designated bridge is the one
  with the lowest bridge ID
• The port that connects the LAN and the designated
  bridge is called a designated port

45
LG Figure 6.86 Sample topology
(1)
(2)
(3)
(2)
(1)
(2)
(1)
(2)
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3. Source Routing Bridges

• Source route bridging is a methods whereby one
  end host locates another end host by discovering
  available paths. Once all paths to the
  destination are known, the source host choose the
  route to use. Paths are determined by sending
  explore frames
• Tradeoff
• Transparent bridges have the advantage of being
  easy to install. On the other hand, they do not
  make optimal use of the bandwidth, since they
  only use a subset of the topology (the spanning
  tree)
• Background
• A split within the 802 committee
• The CSMA/CD and token bus people chose the
  transparent bridge
• The ring people (with encouragement from IBM)
  preferred a scheme called source routing.

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3. Source Routing Bridges

• Description
• Source routing assumes that the sender of each
  frame knows whether or not the destination is on
  its own LAN
• When sending a frame to a different LAN, the
  source sets the high-order bit of the source
  address to 1, to mark it
• Source includes in the frame header the exact
  path that the frame will follow

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3. Source Routing Bridges

• Frame header
• Source address field When sending a frame to a
  different LAN, the source sets the high-order bit
  of the source address to 1
• Routing control field (2 bytes)
• Types of frame
• Length of the routing information field
• Direction of the route given by the route
  designator fields
• Largest frame supported over the path
• Route designator field (2 bytes)
• 12-bit LAN number
• 4-bit bridge number

49
LG Figure 6.88 Frame format for source routing
Routing
Route-1
Route-2
Route-m
Control
Designator
Designator
Designator
2 bytes
2 bytes
2 bytes
2 bytes
Destination
Routing
Source
Data
FCS
Address
Address
Information
50
3. Source Routing Bridges

• Route discovery procedure
• Single-route broadcast frame first the station
  broadcasts a special frame. The frame visits
  every LAN in the bridged LAN exactly once,
  eventually reaching the destination station.
• Upon receipt of this frame, the destination
  station responds with another special frame,the
  all-route broadcast frame, which generates all
  possible routes back to the source station.
• To prevent all-route broadcast frames from
  circulating in the network, a bridge first checks
  whether the outgoing LAN number is already
  recorded in the route designator field. The
  bridge will not forward the frame if the outgoing
  LAN number is already recorded
• After collecting all routes, the source station
  chooses the best route and saves it.

51
LG Figure 6.89 LAN interconnection with source
routing bridges
52
LG Figure 6.90 Route followed by single-route
broadcast frames
LAN3
B6
LAN5
B3
LAN1
B1
B4
LAN4
53
LG Figure 6.91 Routes followed by all-routes
broadcast frames
B3
B2
LAN1
B1
LAN2
B5
LAN4
B4
B7
LAN1
B1
B2
LAN3
B6
B3
LAN2
B5
B4
LAN4
B7
B1
LAN1
B2
B4
LAN2
LAN4
B5
B3
LAN5
B7
B3
LAN1
B5
B1
B2
LAN3
B6
B4
LAN2
LAN1
B1
B2
B3
LAN3
B5
B7
LAN4
B6
B3
B2
LAN1
B1
LAN2
B4
LAN1
LAN3
B5
B1
B2
B3
LAN2
B4
B6