EDUSAT SESSION FOR COMPUTER NETWORKSI CS64

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EDUSAT SESSION FOR COMPUTER NETWORKS-I
(CS64) Date 14.03.2006 Session VI TopicToken
Ring and FDDI Faculty Anita Kanavalli MSRIT
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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

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Token Ring Operation
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Example Ring Latency and Token reinsertion
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Ring Latency

• Let there be M stations
• b bits delay in stations
• The delay in interface is Mb bits
• typically b2.5
• d total ring length
• additional delay is d/v or dR/v v-delay in
  medium
• v2108 m/sec
• therefore it is 5microsec to travel 1 kms

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Ring Latency

• ring latency is defined as the time that it
  takes for a bit to travel around ring is given by
• Td/vMb/R and TR dR/vMb bits
• Example
• Let R4Mbps M20 stations separated by 100m
  b2.5
• Latency 201004106 /2108 202.590 bits

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Token reinsertion

• IEEE 802.5-After the last bit arrives
• IBM token ring-after the header bit arrives
• IEEE 802.5 and IBM token ring 26Mbps- after last
  bit transmitted
• Conclusion-improves efficiency

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Token Ring MAC Frame IEEE802.5 Token Frame
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Access Control (1 Byte)
Start Frame Delimiter (1 Byte)
End Frame Delimiter (1 Byte)
S
AC
E
Contents of 1-Byte Access Control Field
Reservation bits 3-bit field used for reserving a
token at a prescribed priority level
Token bit 0 token frame 1 nontoken frame
Priority bits 000 lowest 111 highest
Monitor bit 0 clean 1 orphaned frame
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Token Ring MAC Frame IEEE802.5 General Frame
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End Frame Delimiter (1 Byte)
Start Frame Delimiter (1 Byte)
Access Control(1 Byte)
Frame Control(1 Byte)
S
AC
E
FC
Destination Address 2 or 6 bytes
Source Address 2 or 6 bytes
User Data 0 or more bytes
Checksum 4 bytes
FS
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Token Ring MAC Frame Start and end delimiter
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S
AC
E
FC
Destination Address 2 or 6 bytes
Source Address 2 or 6 bytes
User Data 0 or more bytes
Checksum 4 bytes
FS
Start and End delimiter (SD/ED) J symbol has
same polarity as the preceding symbol K symbol
has opposite polarity to preceding symbol I and E
both 0 in token, in frame can be 0 or 1 E used
for error detection
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Token Ring MAC Frame Access control
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S
AC
E
FC
Destination Address 2 or 6 bytes
Source Address 2 or 6 bytes
User Data 0 or more bytes
Checksum 4 bytes
FS
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Token Ring MAC Frame Frame control
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S
AC
E
FC
Destination Address 2 or 6 bytes
Source Address 2 or 6 bytes
User Data 0 or more bytes
Checksum 4 bytes
FS
Sample Frame Control 00000000 duplicate address
test test if two lobes have the same
address 00000010 beacon used to locate breaks
in the ring 00000011 claim token attempt to
become monitor 00000100 purge used to
reinitialize the ring
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S
AC
E
FC
Destination Address 2 or 6 bytes
Source Address 2 or 6 bytes
User Data 0 or more bytes
Checksum 4 bytes
FS
Sample Frame Control 00000101 active monitor
present active monitor informs all lobes that it
is alive 00000110 standby monitor present
announces the present of potential monitors
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Token Ring MAC Frame User data, source and
destination address, checksum
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S
AC
E
FC
Destination Address 2 or 6 bytes
Source Address 2 or 6 bytes
User Data 0 or more bytes
Checksum 4 bytes
FS

• Source and Destination address
• 2 or 6 bytes, same as 802.3
• Data
• contains LLC data unit
• maybe as long as necessary, provided that frame
  still be transmitted within the token-holding
  time
• Frame check sequence
• 4 bytes, same as 802.3

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Token Ring MAC Frame Frame status
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S
AC
E
FC
Destination Address 2 or 6 bytes
Source Address 2 or 6 bytes
User Data 0 or more bytes
Checksum 4 bytes
FS
Frame Status A address C Copy x
reserved 00xx00xx address not recognized
nothing copied. 10xx10xx address recognized
nothing copied 01xx01xx address not
recognized data copied 11xx11xx address
recognized data copied
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Token Ring Priority scheme
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Token Ring Active ring monitor

• Overall Ensures ring operates correctly
• Ensures token is not lost
• Remove garbled and orphan frames
• Purge ring and issues new token
• Inserts bits to ensure minimum latency of 24 bits
• Supply master clock data encoding/decoding
• Every station is capable of being monitor, but
  only one at a time good for reliability

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Token Ring Token release

• Normal release
• Token released only when response bits have been
  received - 4 Mbps
• Early release
• Token released as soon as last bit of frame
  transmitted - 16 Mbps

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Fiber Distributed Data Interface

• Token Ring
• 100 Mbps
• Distances up to 100 km
• Up to 1000 stations
• LAN and MAN applications

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FDDI Token Ring network
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Primary Ring
Primary Ring
Secondary Ring
Secondary Ring
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Timed token ring protocol

• if we assume 500 stations each introducing a
  latency of 10 bits and max length of 200 kms
  ring.
• Then the latency is 50010100Mbps(2001000m)/2
  108m/sec
• 105000bits
• Thus FDDI LAN can have high latency
• Only one token circulates at a time

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FDDI Physical Layer
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FDDI MAC Token and Frame Format
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FDDI token format
FDDI frame format
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FDDI MAC Token and Frame Format (1)
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FDDI MAC Token and Frame Format (1)

• Preamble synchronizes the frame with each
  station's clock. The originator of the frame uses
  a field of 16 idle symbols
• starting delimiter (SD) indicates start of the
  frame
• Frame control has the bit format CLFFZZZZ.
• C indicates whether this is a synchronous or
  asynchronous frame
• L indicates the use of 16- or 48 bit addresses,
• FF indicates whether this is an LLC, MAC control
  or reserved frame. For a control frame the
  remaining 4 bits indicate the type of control
  frame.

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FDDI MAC Token and Frame Format
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FDDI MAC Token and Frame Format (1)

• Destination address
• Source address
• Information contains an LLC data unit or
  information related to a control operation
• Frame Check Sequence ? 32 bit CRC
• Ending delimiter
• Frame status contains
• the error detected (E), address recognized and
  frame copied indicators

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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)

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FDDI MAC Protocol

• Handle two type of traffic
• synchronous
• asynchronous
• TTRT-target token rotation time
• TRT-token rotation timer
• THT-token holding time

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FDDI Operation
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