Ring Local Area Network

1
Ring Local Area Network
2
Ring LANs

• The ring is a series of bit repeaters, each
  connected by a unidirectional transmission link
• All arriving data is copied into a 1-bit buffer
  and then copied out again (1-bit delay)
• Data in the buffer can be modified before
  transmission
• Ring interface can be in one of three states
• Listen State
• Transmit State
• Bypass State

3
States of the Ring Interface

• Listen State Incoming bits are copied to output
  with 1-bit delay
• Transmit State Write data to the ring
• Bypass State Idle station does not incur
  bit-delay

4
Ring LANs

• If a frame has traveled once around the ring it
  is removed by the sender
• Ring LANs have a simple acknowledgment scheme
• Each frame has one bit for acknowledgment.
• If the destination receives the frame it sets the
  bit to 1.
• Since the sender will see the returning frame, it
  can tell if the frame was received correctly.

5
What is the "Length" of a Ring?

• The length of a ring LAN, measured in bits, gives
  the total number of bits which are can be in
  transmission on the ring at a time
• Note Frame size is not limited to the length of
  the ring since entire frame may not appear on the
  ring at one time. Why?
• Bit length propagation speed length of ring
  data rate No. of stations bit delay at
  repeater

6
Example

• Calculate the length of the following ring LAN
• 3 km ring
• 1 Mbps data rate
• 5 us/km propagation speed
• 20 stations _at_ 1 bit delay
• Bit length 15 20 35 bits

7
Ring LAN

• Advantages
• Can achieve 100 utilization
• No collisions
• Can achieve deterministic delay bounds
• Can be made efficient at high speeds
• Disadvantages
• Long delays due to bit-delays
• Solution Bypass state eliminates bit-delay at
  idle station
• Reliability Problems
• Solution 1 Use a wire center
• Solution 2 Use a second ring (opposite flow)

8
Ring LANS Wire Center (802.5)
9
Ring LANS Use a second ring
10
Token Ring LANS

• Token is a small packet that rotates around the
  ring
• When all stations are idle, the token is free and
  circulates around the ring
• Possible Problem All stations are idle and in
  the Bypass state. What is the problem?

11
802.5(Token Ring) MAC Protocol

• In order to transmit a station must catch a free
  token
• The station changes the token from free to busy
• The station transmits its frame immediately
  following the busy token
• IF station has completed transmission of the
  frame AND the busy token has returned to the
  station THEN station inserts a new free token
  into the ring

12
Token Ring LANs

• Note
• If the bit length of the ring is less than the
  packet length, then the completion of a packet
  transmission implies return of busy token
• Only one station can transmit at a time. If a
  station releases a free token, the next station
  downstream can capture the token

13
Transmission in a Token Ring
Sender looks for free token
14
Transmission in a Token Ring
Sender changes free token to busy token and
appends data to the token
15
Transmission in a Token Ring

• Receiver recognizes that it is the destination of
  the frame
• Receiver copies frame to station
• Note Frame also returns to sender

16
Transmission in a Token Ring

• Receiver recognizes that it is the destination of
  the frame
• Receiver copies frame to station
• Note Frame also returns to sender

17
Transmission in a Token Ring
Sender generates free token when it is done
transmitting (Note The busy token has returned)
18
Properties of the 802.5 Token Ring

• No collisions of frames
• Full utilization of bandwidth is feasible
• Transmission can be regulated by controlling
  access to token
• Recovery protocols is needed if token is not
  handled properly, e.g., token is corrupted,
  station does not change to "free" etc

19
Token Format / Data Frame Format
20
IEEE 802.5 Frame Format

• One 3-byte token circulates if all stations are
  idle.
• AC "PPPTRRR" where
• "PPP" Priority fields
• "RRR" Reservation fields
• "T" Indicates "Token" or "Data frame"
• SD, ED Start/End delimiter of a frame
• FC Identifies type of a control frame
• FS Contains address recognized bit (A bit) and
  frame copied bit (C bit)
• Receiver sets A1 when frame arrives
• Receiver sets C1 when frame has been copied

21
Priority of Transmission in 802.5

• Eight levels of priorities
• Priorities handled by 3-bit priority field and
  3-bit reservation field
• Define
• Pm priority of the message to be transmitted
• Pr token priority of received token
• Rr reservation priority of received token

22
IEEE 802.5 (Token Ring)

• 1. A station wishing to transmit a frame with
  priority Pm must wait for a free token with Pr lt
  Pm
• 2. The station can reserve a future priority Pm
  token as follows
• If busy token comes by, then set Rr Pm (if Rr lt
  Pm)
• If free token comes by, then set Rr Pm (if Rr lt
  Pm Pm lt Pr)

23
IEEE 802.5 ( Token Ring)

• 3. If a station gets a free token, it sets the
  reservation field to 0 and leaves the priority
  field unchanged and transmits
• 4. After transmission send a free token with
  Priority max(Pr, Rr, Pm), Reservation max(Rr,
  Pm)
• 5. Station which upgraded the priority level of a
  token must also downgrade the priority (if no one
  used the token)

24
Ring Maintenance

• Token ring selects one station as the monitor
  station
• Duties of the monitor
• Check that there is a token
• Recover ring if it is broken
• Detect garbled frames
• Make sure the token (24-bit) is shorter than the
  ring length

25
Terms in IEEE 802.5

• IEEE 802.5 requires to maintain a large number of
  counters
• THT Token Holding Timer (one per station)
• Limits the time that a station can transmit
  (Default 10 ms)
• TRR Time-to-Repeat Timer (one per station)
• Limits the time that a station waits for return
  of own message (Default 2.5 ms)
• TVX Valid Transmission Timer (in monitor
  station)
• Verifies that station which accessed the token
  actually used it (Default THT TRR 12.5 ms)
• TNT No-Token Timer (one per station)
• If expire, a new token is generated (Default N
  (THT TRR))

26
Performance of Token Rings

• Parameters and Assumption
• End-to-end propagation delay a
• Packet transmission time 1
• Number of stations N
• Assume that each station always has a packet
  waiting for transmission
• Note The ring is used either for data
  transmission or for passing the token

27
Performance of Token Rings

• Define
• T1 Average time to transmit a frame. Per
  assumption, T1 1
• T2 Average time to pass the token
• Maximum Throughput
• Frame Time T1
• ------------------------------ ----------
• Time Frame Overhead T1 T2

28
Effect of propagation delay

• Effect of propagation delay on throughput
• Case 1 a lt 1 (Packet longer than ring)
• T2 time to pass token to the next station a/N
• Case 2 a gt 1 (Packet shorter than ring)
• Note Sender finishes transmission after T1 1,
  but cannot release the token until the token
  returns
• T1T2 max(1, a) a/N

29
Performance of Token Rings

• Illustration of Analysis (agt1)

30
Performance of Token Rings

• Illustration of Analysis (alt1)

31
Ethernet vs. Token Ring

• Maximum throughput as a function of "a"

32
Ethernet vs. Token Ring

• Maximum throughput as a function of N

33
FDDI

• Some Facts
• FDDI Fiber Distributed Data Interface
• FDDI is a high-speed token ring
• Fiber-optic (dual redundant counter rotating)
  ring LAN
• Multimode fiber
• Standardized by ANSI and ISO X3T9.5 committee
• 100 Mbps data rate
• Maximum frame size is 4500 bytes
• Allows up to 1000 connected stations
• Maximum ring circumference 200 km

34
FDDI

• FDDI distinguishes 4 Service Classes
• Asynchronous
• Synchronous
• Immediate (for monitor and control)
• Isochronous (only in FDDI-II)

35
FDDI - Protocol Architecture
36
Dual Redundant Counter Rotating Ring

• Second ring adds a certain level of fault
  tolerance

37
Station Types - Class A Station

• Two PHY (and one or two MAC) entities
• Connects to another Class A station or to a
  concentrator

38
Station Types - Class B Station

• Class B station has one PHY (and one MAC) entity
• Connects to a concentrator

39
Station Types - Concentrator

• Connects Class A and Class B stations into one of
  the counter rotating rings.
• Concentrator can bypass failing stations.

40
Topology Example
41
FDDI Media Access Control

• FDDI uses a Token Ring Protocol, similar to 802.5
• Differences of FDDI and 802.5
• To release a token, a station does not need to
  wait until the token comes back after a
  transmission. The token is released right after
  the end of transmission
• In FDDI, multiple frames can be attached to the
  token
• FDDI has a different priority scheme

42
FDDI Token Ring Protocol

• 1. A awaits token
• 2. A seizes token, begins transmitting frame F1
  addressed to C

43
FDDI Token Ring Protocol

• 3. A appends token to end of transmission
• 4. C copies frames F1 as it goes by

44
FDDI Token Ring Protocol

• 5. C continues to copy F1 B seizes token and
  transmits frame F2 addressed to D
• 6. B emits token D copies F2 A absorbs F1

45
FDDI Token Ring Protocol

• 7. A lets F2 and token pass B absorbs F2
• 8. B lets token pass

46
Frame and Token Format

• SD Starting Delimiter
• FC Frame Control (type of frame)
• DA Destination Address
• SA Source Address
• FCS Frame Check Sequence (CRC)
• ED End Delimiter
• FS Frame Status
• Total Frame length lt 4500bytes

47
Timed Token Protocol

• FDDI has a timed token protocol which determines
  how long a station can transmit
• Each station has timers to measure the time
  elapsed since a token was last received
• TTRT Target Token Rotation Time
• Value of TTRT is negotiated during initialization
  (default is 8 ms)
• Set to the maximum desired rotation time

48
Parameters of Timed Token Protocol

• Station Parameters
• TRT Token Rotation Time
• Time of the last rotation of the token.
• If TRT lt TTRT, then token is early,
  asynchronous traffic can be transmitted
• If TRT gt TTRT then token is late, asynchronous
  traffic cannot be transmitted.
• THT Token Holding Time
• Controls the time that a station may transmit
  asynchronous traffic.
• fi Percentage of the TTRT that is allocated for
  synchronous traffic at station i.

49
Timed Token Protocol

• If a station receives the token it sets
• THT TRT
• TRT TTRT
• Enable TRT (i.e., start the timer)
• If the station has synchronous frames are waiting
  the transmit synchronous traffic for up to time
  TTRTfi (with sum(fi) lt1)
• If the station has asynchronous traffic
• enable THT
• while THT gt 0 transmit asynchronous traffic.

50
Timed Token Protocol

• Note
• Transmission is not interrupted if THT expires
  during a transmission.
• If a station does not utilize its maximum
  transmission time (i.e., THT), the next station
  can use it.

51
FDDI Synchronous Traffic
Assume not all transmit buffers are full. TTRT
5 msec, Sync 4 msec, Asy 1 ms
Station 1 Station 2 Station 3 Station 4
Frames
Frames
Frames
Frames
1.0ms
1.0ms
1.0ms
1.0ms 1.0msec
No Tx Nothing to Tx
Frames
Frames
Frames
1.0ms
1.0ms 1.0ms
1.0ms 1.0ms
No Tx Nothing to Tx
No Tx Nothing to Tx
Frames
Frames
1.0ms 1.0ms
1.0ms 1.0ms
52
FDDI Asynchronous Traffic
Assume 4 stations, all have full buffers to
send. TTRT 3ms
Station 1 Station 2 Station 3 Station 4
Frames
No Tx TRT expired
No Tx TRT expired
No Tx TRT expired
3ms
gt 3ms
No Tx TRT expired
No Tx TRT expired
No Tx TRT expired
Frames
No Tx TRT expired
No Tx TRT expired
No Tx TRT expired
Frames
53
FDDI Asynchronous Traffic
Assume not all transmit buffers are full.
Station 1 Station 2 Station 3 Station 4
Frames
No Tx Nothing to Tx
Frames
Frames
0.8ms
0.5ms
1.0ms
lt 3ms
No Tx TRT expired
Frames
Frames
Frames
1.0ms
0.7ms
0.5ms
No Tx Nothing to Tx
No Tx Nothing to Tx
Frames
Frames
1.5ms
0.5ms
54
Analysis of FDDI

• Analysis of
• Synchronous traffic
• Asynchronous traffic
• Synchronous Traffic
• Recall that each station can transmit synchronous
  traffic for up to time TTRTfi (with sum(fi)lt1)
• If sum(fi)1, the maximum throughput of
  synchronous traffic is 100.
• One can show that the maximum delay until a frame
  is completely transmitted is
• Maximum Access Delay lt 2TTRT

55
Analysis of FDDI

• Asynchronous Traffic
• Parameters
• D Ring latency
• n Number of active sessions (all heavily loaded)
• T Value of TTRT
• Assumption
• No synchronous traffic

56
Analysis of FDDI

• From the Example we see
• Cycle in a system has a length of nT D
• Time in a cycle used for transmission n(T - D)
• We obtain for the maximum throughput for
  asynchronous traffic is

• ... and the maximum access delay for asynchronous
  traffic
• Max. Access DelayT(n-1)2D

57
Analysis of FDDI

• Numerical Example
• Number of stations 16
• Length of fiber 200 km
• Speed of Signal 5 ms/km
• Delay per station 1 ms/station
• TTRT 5 msec
• Ring Latency D (20 km) x (5 msec/km) (16
  stations) x 1 msec/station 0.12 ms.

58
Analysis of FDDI
59
Numerical Results

• We show plots for 3 different FDDI networks.
• "Typical" FDDI
• 20 stations (single MAC)
• 4 km ring
• "Big" FDDI
• 100 stations (single MAC)
• 200 km ring
• "Largest" FDDI
• 500 stations (dual MAC)
• 200 km ring

60
Throughput vs. TTRT
61
Maximum Access Delay vs. TTRT
62
Other LANs

• Token Bus

63
IEEE 802.4 (Token Bus)

• Problems with 802.3
• Collisions of frames can lead to unpredictable
  delays
• In some real-time scenarios, collisions and
  unpredictable delays can be catastrophic
• Solution via Token Bus
• A control packet (Token) regulates access to the
  bus
• A station must have the token in order to
  transmit
• A station can hold the token only for a limited
  time
• The token is passed among the stations in a
  cyclic order
• This structures the bus as a logical ring

64
IEEE 802.4 (Token Bus)

• Stations form a logical ring
• Each station knows its successor and predecessor
  in the ring

65
Feature of Token Bus

• Bandwidth is 1, 5, or 10 Mbps
• The token bus MAC protocol is very complex
• Typically, token bus is free of collisions
• Defines priority transmissions and can offer
  bounded transmission delays

66
IEEE 802.4 (Token Bus)

• 802.4 requires each station to implement the
  following management functions
• Ring Initialization
• Addition to ring
• Deletion from ring
• Fault management

67
Adding a Station to the Token Bus

• Each node periodically sends a solicit successor
  packet which invites nodes with an address
  between itself and the next node to join the ring
• Sending node waits for response for one round
  trip
• One of the following three cases apply
• (1) No Response
• Pass token
• (2) Response from one node
• Reset successor node
• Pass token to new successor node
• (3) Response from more than one node
• Collision has occurred
• Node tries to resolve contention

68
Add a station to the Token Bus

• Assume Response from more than one node has
  resulted in a collision.
• Station sends a resolve contention packet and
  waits for four windows
• (window 1 round trip time) for a response
• In window 1, stations with address prefix 00 can
  reply
• In window 2, stations with address prefix 01 can
  reply
• In window 3, stations with address prefix 10 can
  reply
• In window 4, stations with address prefix 11 can
  reply
• If there is a another collision, procedure is
  repeated for the second pair of bits. Only the
  nodes which replied earlier can join the next
  round
• First successful reply joins the ring

69
IEEE 802.4 (Token Bus)

• Four priority levels
• Levels 6, 4, 2, 0
• Priority 6 is the highest level
• Token Holding Time (THT)
• Maximum time a node can hold a token
• Token Rotation Time for class i (TRTi)
• Maximum time of a full token circulation at which
  priority i transmissions are still permitted

70
Token Bus Transmission Rules

• Each station can transmit class 6 data for a time
  THT
• For i 4, 2, 0
• Transmit class i traffic if all traffic from
  class i2 or higher is transmitted
• and the time of the last token circulation
  (including the transmission time of higher
  priority packets during the current holding of
  the token) is less than TRT i .

71
Token Bus Priority Scheme