IEEE 802 standards
continued .........
Switched 802.3 LANS
LAN switch is a device with multiple inputs and outputs
to interconnect multiple numbers of hosts and computing
terminals. LAN switch prevents data packet collision, and
maximizes transmission speed as well as bandwidth allocation.
The core job of a switch is to take packets that arrive
on an input and forward (or switch) them to the right output
so that they will reach their appropriate destination.
The heart of a LAN switch is a high-speed backplane and
room for typically 4-32 plug in line cards, each containing
one to eight connectors. Further each connector has a 10
Base-T twisted pair connection to a single host computer.
The transmitting station sends a standard frame to the switch.
The plug-in card receives the frame and check for the destination
address. If the address is of local stations the frame will
be sent to local station otherwise the frame is sent over
the high-speed back plane to the destination station's card.
The backplane runs at over 1Gbps using proprietary protocol.
Collision prevention: There are two types of collision
prevention; they are local on-card LAN and buffer-input
port card.
In case of local on-card LAN, collision are detected and
handled like any other CSMA/DC network- with retransmission
using the binary backoff algorithm. Where as in buffer-input
port card, the incoming frame-data are stored in RAM. Once
a frame is completely received the card can then check to
see if the frame is destines for another port on the same
card, or for a distant port.
In the figure above the port in upper right hand corner
is connected to a single station, but a 12-port hub. Even
at the hub, the frames are transmitted to destination station
in same way through collision detection and binary backoff.
Successful frames are switched to the correct output line
over the high speed backplane. If all the input ports are
connected to hubs, rather than to individual stations, the
switch just becomes an 802.3 to 802.3bridge.
IEEE802.4: Token bus
In this system, the nodes are physically connected as
a bus, but logically form a ring with tokens passed around
to determine the turns for sending. It has the robustness
of the 802.3 broadcast cable and the known worst case behavior
of a ring. The structure of a token bus network is as follows:
Token bus or 802.4 is more robust and reliable than 802.3
frames. The standard 802.4 is a linear or tree shaped cable
onto which the stations are attached.
Network is a physical bus but a logical ring. The stations
are numbered and are allowed to access the medium sequentially.
If there are n stations, and packet transmission time is
bounded to T, then the maximum waiting time is nT. Token
bus MAC is simple and robust. Token bus is mostly used in
automation systems.
Data is subdivided into four priority classes. Each station
has four packet queues, one for each priority. When a station
receives token, it is allowed to transmit certain fixed
time. During that time it transmits packets in the decreasing
order of priorities.
Frame Structure:
A 802.4 frame has the following fields:
Preamble: The Preamble is for synchronizing the receiver's
clock.
Starting Delimiter (SD) and End Delimiter (ED): The
SD and ED fields are used to mark frame boundaries. Both
contain analog encoding of symbols other than 1 or 0 so
that they cannot occur accidentally in the user data. Lengthy
field is no more needed.
Frame Control (FC): FC is used to distinguish data
frames from control frames. Data frames carries the frame's
priority as well as a bit which the destination can set
as an acknowledgement. For control frames, the Frame Control
field is used to specify the frame type. Token passing and
various ring maintenance frames are the allowed frame types.
Destination and Source Address: The Destination and
Source address fields may be of 2 bytes (for a local address)
or 6 bytes (for a global address).
Data: This is the actual data and it is of 8182 bytes
when 2 byte addresses are used and 8174 bytes for 6 byte
addresses.
Checksum: A 4-byte checksum of the data for error
detection.
Ring Maintenance:
Whenever the first node on the token bus comes up, it
sends a Claim_token packet to initialize the ring. If more
than one station send this packet at the same time, there
is a collision. Collision is resolved by a contention mechanism,
in which the contending nodes send random data for 1, 2,
3 and 4 units of time depending on the first two bits of
their address. The node sending data for the longest time
wins. If two nodes have the same first two bits in their
addresses, then contention is done again based on the next
two bits of their address and so on.
Once the ring is set up, new nodes which are powered up
may wish to join the ring. For this a node sends Solicit_successor_1
packets from time to time, inviting bids from new nodes
to join the ring. This packet contains the address of the
current node and its current successor, and asks for nodes
in between these two addresses to reply. If more than one
nodes respond, there will be collision. The node then sends
a Resolve_contention packet, and the contention is resolved
using a similar mechanism as described previously. Thus
at a time only one node gets to enter the ring. The last
node in the ring will send a Solicit_successor_2 packet
containing the addresses of it and its successor. This packet
asks nodes not having addresses in between these two addresses
to respond.
A question arises that how frequently should a node send
a Solicit_successor packet? If it is sent too frequently,
then overhead will be too high. Again if it is sent too
rarely, nodes will have to wait for a long time before joining
the ring. If the channel is not busy, a node will send a
Solicit_successor packet after a fixed number of token rotations.
This number can be configured by the network administrator.
However if there is heavy traffic in the network, then a
node would defer the sending of bids for successors to join
in.
There may be problems in the logical ring due to sudden
failure of a node. What happens if a node goes down along
with the token? After passing the token, a node, say node
A, listens to the channel to see if its successor either
transmits the token or passes a frame. If neither happens,
it resends a token. Still if nothing happens, A sends a
Who_follows packet, containing the address of the down node.
The successor of the down node, say node C, will now respond
with a Set_successor packet, containing its own address.
This causes A to set its successor node to C, and the logical
ring is restored. However, if two successive nodes go down
suddenly, the ring will be dead and will have to be built
afresh, starting from a Claim_token packet.
When a node wants to shutdown normally, it sends a Set_successor
packet to its predecessor, naming its own successor. The
ring then continues unbroken, and the node goes out of the
ring.
The various control frames used for ring maintenance are
shown below:
| Frame Control Field |
Name |
Meaning |
| 00000000 |
Claim_token |
Claim token during ring maintenance |
| 00000001 |
Solicit_successor_1 |
Allow stations to enter the ring |
| 00000010 |
Solicit_successor_2 |
Allow stations to enter the ring |
| 00000011 |
Who_follows |
Recover from lost token |
| 00000100 |
Resolve_contention |
Used when multiple stations want to
enter |
| 00001000 |
Token |
Pass the token |
| 00001100 |
Set_successor |
Allow the stations leave the ring |
Priority Scheme:
Token bus supports four distinct priority levels: 0, 2,
4 and 6.
0 is the lowest priority level and 6 the highest. The following
times are defined by the token bus:
THT: Token Holding Time. A node holding the token can send
priority 6 data for a maximum of this amount of time.
TRT_4: Token Rotation Time for class 4 data. This is the
maximum time a token can take to circulate and still allow
transmission of class 4 data.
TRT_2 and TRT_0: Similar to TRT_4.
When a station receives data, it proceeds in the following
manner:
It transmits priority 6 data for at most THT time, or as
long as it has data.
Now if the time for the token to come back to it is less
than TRT_4, it will transmit priority 4 data, and for the
amount of time allowed by TRT_4. Therefore the maximum time
for which it can send priority 4 data is= Actual TRT - THT
- TRT_4
Similarly for priority 2 and priority 0 data.
This mechanism ensures that priority 6 data is always sent,
making the system suitable for real time data transmission.
In fact this was one of the primary aims in the design of
token bus.
IEEE 802.5: Token Ring Network
Token Ring is formed by the nodes connected in ring format
as shown in the diagram below.
The principle used in the token ring network is that a
token is circulating in the ring and whichever node grabs
that token will have right to transmit the data. Whenever
a station wants to transmit a frame it inverts a single
bit of the 3-byte token which instantaneously changes it
into a normal data packet. Because there is only one token,
there can atmost be one transmission at a time.
Since the token rotates in the ring it is guarenteed that
every node gets the token with in some specified time. So
there is an upper bound on the time of waiting to grab the
token so that starvation is avoided.
There is also an upper limit of 250 on the number of nodes
in the network. To distinguish the normal data packets from
token (control packet) a special sequence is assigned to
the token packet. When any node gets the token it first
sends the data it wants to send, then recirculates the token.
If a node transmits the token and nobody wants to send
the data the token comes back to the sender. If the first
bit of the token reaches the sender before the transmission
of the last bit, then error situation araises. So to avoid
this we should have:
propogation delay + transmission of n-bits (1-bit delay
in each node ) > transmission of the token time
A station may hold the token for the token-holding time.
which is 10 ms unless the installation sets a different
value. If there is enough time left after the first frame
has been transmitted to send more frames, then these frames
may be sent as well. After all pending frames have been
transmitted or the transmission frame would exceed the token-holding
time, the station regenerates the 3-byte token frame and
puts it back on the ring.
If a node transmits the token and nobody wants to send
the data the token comes back to the sender. If the first
bit of the token reaches the sender before the transmission
of the last bit, then error situation araises. So to avoid
this we should have:
propogation delay + transmission of n-bits (1-bit delay
in each node ) > transmission of the token time
A station may hold the token for the token-holding time.
which is 10 ms unless the installation sets a different
value. If there is enough time left after the first frame
has been transmitted to send more frames, then these frames
may be sent as well. After all pending frames have been
transmitted or the transmission frame would exceed the token-holding
time, the station regenerates the 3-byte token frame and
puts it back on the ring.
Modes of Operation
Listen Mode: In this mode the node listens to the
data and transmits the data to the next node. In this mode
there is a one-bit delay associated with the transmission.
Transmit Mode: In this mode the node just discards
the any data and puts the data onto the network.
I
By-pass Mode: In this mode reached when the node
is down. Any data is just bypassed. There is no one-bit
delay in this mode.
Token Ring Using Ring Concentrator
One problem with a ring network is that if the cable breaks
somewhere, the ring dies. This problem is elegantly addressed
by using a ring concentrator. A Token Ring concentrator
simply changes the topology from a physical ring to a star
wired ring. But the network still remains a ring logically.
Physically, each station is connected to the ring concentrator
(wire center) by a cable containing at least two twisted
pairs, one for data to the station and one for data from
the station. The Token still circulates around the network
and is still controlled in the same manner, however, using
a hub or a switch greatly improves reliability because the
hub can automatically bypass any ports that are disconnected
or have a cabling fault. This is done by having bypass relays
inside the concentrator that are energized by current from
the stations. If the ring breaks or station goes down, loss
of the drive current will release the relay and bypass the
station. The ring can then continue operation with the bad
segment bypassed.
Removing Packet from the Ring
There are 3 possibilities-
1. The source itself removes the packet after one full
round in the ring.
2. The destination removes it after accepting it: This has
two potential problems. Firstly, the solution won't
work for broadcast or multicast, and secondly, there would
be no way to acknowledge the sender
about the receipt of the packet.
3. Have a specialized node only to discard packets: This
is a bad solution as the specialized node would
know that the packet has been received by the destination
only when it receives the packet the second
time and by that time the packet may have actually made
about one and half (or almost two in the
worst case) rounds in the ring.
Thus the first solution is adopted with the source itself
removing the packet from the ring after a full one round.
With this scheme, broadcasting and multicasting can be handled
as well as the destination can acknowledge the source about
the receipt of the packet (or can tell the source about
some error).
Token Format
The token is the shortest frame transmitted (24 bit)
MSB (Most Significant Bit) is always transmitted first -
as opposed to Ethernet
SD = Starting Delimiter (1 Octet)
AC = Access Control (1 Octet)
ED = Ending Delimiter (1 Octet)
Starting Delimiter Format:
J = Code Violation
K = Code Violation
Access Control Format:
T=Token
T = 0 for Token
T = 1 for Frame
When a station with a Frame to transmit detects a token
which has a priority equal to or less than the Frame to
be transmitted, it may change the token to a start-of-frame
sequence and transmit the Frame
P = Priority
Priority Bits indicate tokens priority, and therefore,
which stations are allowed to use it. Station can transmit
if its priority as at least as high as that of the token.
M = Monitor
The monitor bit is used to prevent a token whose priority
is greater than 0 or any frame from continuously circulating
on the ring. If an active monitor detects a frame or a high
priority token with the monitor bit equal to 1, the frame
or token is aborted. This bit shall be transmitted as 0
in all frame and tokens. The active monitor inspects and
modifies this bit. All other stations shall repeat this
bit as received.
R = Reserved bits
The reserved bits allow station with high priority Frames
to request that the next token be issued at the requested
priority.
Ending Delimiter Format:
J = Code Violation
K = Code Violation
I = Intermediate Frame Bit
E = Error Detected Bit
Frame Format:
MSB (Most Significant Bit) is always transmitted first
- as opposed to Ethernet
| SD |
AC |
FC |
DA |
SA |
DATA |
CRC |
ED |
FS |
SD=Starting Delimiter(1 octet)
AC=Access Control(1 octet)
FC = Frame Control (1 Octet)
DA = Destination Address (2 or 6 Octets)
SA = Source Address (2 or 6 Octets)
DATA = Information 0 or more octets up to 4027
CRC = Checksum(4 Octets)
ED = Ending Delimiter (1 Octet)
FS=Frame Status
Starting Delimiter Format:
J = Code Violation
K = Code Violation
Access Control Format
T=Token
When a station with a Frame to transmit detects a token
which has a priority equal to or less than the Frame to
be transmitted, it may change the token to a start-of-frame
sequence and transmit the Frame.
P = Priority
Bits Priority Bits indicate tokens priority, and therefore,
which stations are allowed to use it. Station can transmit
if its priority as at least as high as that of the token.
M = Monitor
The monitor bit is used to prevent a token whose priority
is greater than 0 or any frame from continuously circulating
on the ring. if an active monitor detects a frame or a high
priority token with the monitor bit equal to 1, the frame
or token is aborted. This bit shall be transmitted as 0
in all frame and tokens. The active monitor inspects and
modifies this bit. All other stations shall repeat this
bit as received.
R = Reserved bits the reserved bits allow station with
high priority Frames to request that the next token be issued
at the requested priority
Frame Control Format:
| F |
F |
CONTROL BITS (6 BITS) |
FF= Type of Packet-Regular data packet or MAC layer packet
Control Bits= Used if the packet is for MAC layer protocol
itself
Source and Destination Address Format:
The addresses can be of 2 bytes (local address) or 6 bytes
(global address).
local address format:
| I/G (1 BIT) |
NODE ADDRESS (15 BITS) |
alternatively
| I/G (1 BIT) |
RING ADDRESS (7 BITS) |
NODE ADDRESS (8 BITS) |
The first bit specifies individual or group address.
universal (global) address format:
| I/G (1 BIT) |
L/U (1 BIT) |
RING ADDRESS (14 BITS) |
NODE ADDRESS (32 BITS) |
The first bit specifies individual or group address.
The second bit specifies local or global (universal) address.
local group addresses (16 bits):
| I/G (1 BIT) |
T/B(1 BIT) |
GROUP ADDRESS (14 BITS) |
The first bit specifies an individual or group address.
The second bit specifies traditional or bit signature group
address.
Traditional Group Address: 2Exp14 groups can be defined.
Bit Signature Group Address: 14 grtoups are defined. A host
can be a member of none or any number of them. For multicasting,
those group bits are set to which the packet should go.
For broadcasting, all 14 bits are set. A host receives a
packet only if it is a member of a group whose corresponding
bit is set to 1.
universal group addresses (16 bits):
| I/G (1 BIT) |
RING NUMBER |
T/B (1 BIT) |
GROUP ADDRESS (14 BITS) |
The description is similar to as above.
Data Format:
No upper limit on amount of data as such, but it is limited
by the token holding time.
Checksum:
The source computes and sets this value. Destination too
calculates this value. If the two are different, it indicates
an error, otherwise the data may be correct.
Frame Status:
It contains the A and C bits.
A bit set to 1: destination recognized the packet.
C bit set to 1: destination accepted the packet.
This arrangement provides an automatic acknowledgement
for each frame. The A and C bits are present twice in the
Frame Status to increase reliability in as much as they
are not covered by the checksum.
Ending Delimiter Format:
J = Code Violation
K = Code Violation
I = Intermediate Frame Bit
If this bit is set to 1, it indicates that this packet is
an intermediate part of a bigger packet, the last packet
would have this bit set to 0.
E = Error Detected Bit
This bit is set if any interface detects an error.
Ring Maintenance
Each token ring has a monitor that oversees the ring. Among
the monitor's responsibilities are seeing that the token
is not lost, taking action when the ring breaks, cleaning
the ring when garbled frames appear and watching out for
orphan frames. An orphan frame occurs when a station transmits
a short frame in it's entirety onto a long ring and then
crashes or is powered down before the frame can be removed.
If nothing is done, the frame circulates indefinitely.
Detection of orphan frames: The monitor detects orphan
frames by setting the monitor bit in the Access Control
byte whenever it passes through. If an incoming frame has
this bit set, something is wrong since the same frame has
passed the monitor twice. Evidently it was not removed by
the source, so the monitor drains it.
Lost Tokens: The monitor has a timer that is set to the
longest possible tokenless interval : when each node transmits
for the full token holding time. If this timer goes off,
the monitor drains the ring and issues a fresh token.
Garbled frames: The monitor can detect such frames by their
invalid format or checksum, drain the ring and issue a fresh
token.
The token ring control frames for maintenance are:
| Control field |
Name |
Meaning |
| 00000000 |
Duplicate address test |
Test if two stations have the same
address |
| 00000010 |
Beacon |
Used to locate breaks in the ring |
| 00000011 |
Claim token |
Attempt to become monitor |
| 00000100 |
Purge |
Reinitialize the ring |
| 00000101 |
Active monitor present |
Issued periodically by the monitor |
| 00000110 |
Standby monitor present |
Announces the presence of potential
monitors |
The monitor periodically issues a message "Active Monitor
Present" informing all nodes of its presence. When
this message is not received for a specific time interval,
the nodes detect a monitor failure. Each node that believes
it can function as a monitor broadcasts a "Standby
Monitor Present" message at regular intervals, indicating
that it is ready to take on the monitor's job. Any node
that detects failure of a monitor issues a "Claim"
token. There are 3 possible outcomes :
If the issuing node gets back its own claim token, then
it becomes the monitor.
If a packet different from a claim token is received, apparently
a wrong guess of monitor failure was made. In this case
on receipt of our own claim token, we discard it. Note that
our claim token may have been removed by some other node
which has detected this error.
If some other node has also issued a claim token, then the
node with the larger address becomes the monitor.
In order to resolve errors of duplicate addresses, whenever
a node comes up it sends a "Duplicate Address Detection"
message (with the destination = source) across the network.
If the address recognize bit has been set on receipt of
the message, the issuing node realizes a duplicate address
and goes to standby mode. A node informs other nodes of
removal of a packet from the ring through a "Purge"
message. One maintenance function that the monitor cannot
handle is locating breaks in the ring. If there is no activity
detected in the ring (e.g. Failure of monitor to issue the
Active Monitor Present token...) , the usual procedures
of sending a claim token are followed. If the claim token
itself is not received besides packets of any other kind,
the node then sends "Beacons" at regular intervals
until a message is received indicating that the broken ring
has been repaired.
Phase Jitter Compensation :
In a token ring the source starts discarding all it's previously
transmitted bits as soon as they circumnavigate the ring
and reach the source. Hence, it's not desirable that while
a token is being sent some bits of the token which have
already been sent become available at the incoming end of
the source. This behavior though is desirable in case of
data packets which ought to be drained from the ring once
they have gone around the ring. To achieve the aforesaid
behavior with respect to tokens, we would like the ring
to hold at least 24 bits at a time. How do we ensure this?
Each node in a ring introduces a 1 bit delay. So, one approach
might be to set the minimum limit on the number of nodes
in a ring as 24. But, this is not a viable option. The actual
solution is as follows. We have one node in the ring designated
as "monitor". The monitor maintains a 24 bits
buffer with help of which it introduces a 24 bit delay.
The catch here is what if the clocks of nodes following
the source are faster than the source? In this case the
24 bit delay of the monitor would be less than the 24 bit
delay desired by the host. To avoid this situation the monitor
maintains 3 extra bits to compensate for the faster bits.
The 3 extra bits suffice even if bits are 10 % faster. This
compensation is called Phase Jitter Compensation.
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