Download Token Ring management

LAN and OSI relationship
Layers in
OSI
7
Application
Layers in
a LAN
Higher layers
(depend on the
used system of
protocols)
3
Network
LLC
2
Data link
MAC
1
Physical
Physical
LLCMACNIC-
Logical Link Control
Media Access Control
Network Interface Card
To/from higher layer
MAC Header
NIC
This a datagram
(for TCP/IP)
frame data
frame data
frame
MAC Frame
MAC Trailer (FCS)
1
MAC frame in Ethernet
(IEEE 802.3 standard and DIX standard)
Physical address (MAC addresses)
in the LAN
If 802.3
preamable Dest. Source Length
addr. addr. or Type
8
octets
6
6
2
Frame check
sequence(FCS)
Data
46 – 1 500
4
If DIX
72 – 1526 octets
101010...10
64 bits
Physical (MAC) address
1st Bit --> 47
transmitted
0
Address in LAN
Type
Type =
0 – a unique (individual) address
1 – a group (multicast) address
If all 48 bits are 1, this is the broadcast address.
MAC addresses are controlled by IEEE Registration Authority.
2
Example of a MAC (phys.) address
(a) As written in hexadecimal form
F0 12 01 A3 52 10
0000 1111 0100 1000 … 0000 1000
The 1st bit to be transmitted
(b) As transmitted (in binary): each octet is transmitted
starting from the least significant bit (LSB).
In this example:
The 1st transmitted bit is 0, so that this is an individual
address( not multicast address).
3
Multicasting in TCP/IP
Application
This is done
by
setsockopt
sys. call
Join a multicast group
with IP=230.4.5.6
UDP
Individual
IP = 193.15.20.30
IP layer:
Checks both
193.15.20.30 and
230.4.5.6
Datalink layer:
Receives all frames
with destination MAC
Fixed NIC
address
Muticast
IP = 230.4.5.6
Mapping to
temporary
MAC address
02:60:2f:6c:e4:12
and
01:00:5e:04:05:06
LAN
How to map a multicast IP address into a MAC address:
See Stewens, UNIX Netw. Progr, p.489.
4
Hub
(a multiport repeater)
Hub
Coax. cable
To other hub(s)
Hub
Ports
Up to 100 m
Two unshielded
twisted pairs (UTP)
………
Host 1
Host N
A logical view
of a repeating
hub
One collision domain for all ports!
(like a “compressed” Ethernet LAN)
5
A switched Ethernet LAN
The purpose is to have large number of stations, with the
possibility more than one station transmitting at the same
time.
A line card
A switch
A high-speed backplane
With buffer memory
A high-speed bus (1 Gbps)
Like usual
Ethernet
Card 1
Stations of
card 1
Switch
Card M
Stations of
card M
6
CSMA/CD (IEEE 802.3 Standard)
Major Advantages:
1. Algorithm is completely distributed.
2. The protocol is simple (the standard 802.3 is presented as
Pascal procedures).
3. No special procedures to include or remove a station from
the network.
4. No control frames.
Major Disadvantages:
1. A station might wait indefinitely long to start transmitting
(real type application is not suitable).
2. Data packets have no priorities.
3. Bad performance under heavy load.
What is the minimal size of a frame in IEEE 802.3?
l = 8 + 6 + 6 + 2 + 46 + 4 = 72 bytes = 576 bits
8:preamble, 2:length of data, 46:data, 4:frame check sequence
This requires (576 bits/10 Mbps) = 576/107 sec = 57.6 s
The window slot = 512bits/10 Mbps = 51.2 s (5 segments
of cable with 500 m long each plus 4 repeaters between the
segments).
7
Performance of a CSMA/CD network
Let there be
k – number of stations,
p – the probability that each
station wants to transmit during a
collision window.
CSMA/CD
network
1
2
k
Then
A = kp(1-p)k-1 , A = Amax when k=1/p
is the probability that some station acquires the medium
during a collision window.
And channel efficiency be
T/(T+2/A)
Where
T – mean time to send a frame,
2 - duration of the collision window ( 51.2 s for 2.5 km of
cable and 4 repeaters).
Channel 1.0
efficiency .9
1024
.7
256
.5
64 byte
51.2 s to
send, with
10 Mbps
.3
.1
0 1 2
4
8 16 32 64 128 256
Number of stations
8
Token ring access method (IEEE 802.5)
A wants to send to C.
Ring
B
interface
Sender A
A token
(NIC)
 waits for free token.
 Then changes free token to
C
A
busy token and
 appends Data.
waits
Station
D
Now receiver C
 copies data addressed to it.
 Data continues traveling
along the ring.
B
A
C
D
Sender:
 receives its own packets,
 removes (absorbs) it from
the ring and then
 generates a free token.
B
A
C
D
Station A will generate a new free
token when both of the following conditions take place:
1. The station has completed transmission of its packet.
2. The busy token has returned to the station.
With the speed = 10 Mbps, a data packet of 512 bytes can
occupy  200 000 000 m/sec.(512*8)bits/107 bit/sec  80 km,
if there are no delays in stations’ interfaces
9
Operating modes of ring interfaces:
a) Listen.
1 bit delay
Station
Ring interface
Media
To station From station
Ring interface (Repeater)
Media
b) Transmit
Station
Media
Ring
interface
To station From station
c) bypass
Media
To station From station
10
Algorithm of the ring interface
Start
Continue
listening
No
A free token arrived
Is there a frame
to transmit?
Yes
Change the free token to
busy token (and let it go)
Switch to “transmit” mode.
Transmit the frame.
Frame has been
transmitted AND Busy
token returned?
Token holding
time=10 ms
for IEEE 802.5
No
Yes
Generate new free token.
Switch to “listen” mode.
11
Token Ring Protocol (802.5)
Token format:
1
1
Bytes
1
SD AC ED
1
1
1 2 or 6 2 or 6
No limit
Data frame: SD AC FC Dest. Source Data
Start
delimiter
4
1
CRC ED FS
Frame control
(data or control frame)
Access
control
1
Frame
status
End
delimiter
Token holding time = 10 ms
With the rate = 4 Mbps, up to 10*10-3*4*106 = 40 000 bits
may be transmitted.
Frame status bits A and C in FS byte:
1) A=0, C=0 : destination is not present or not powered.
2) A=1, C=0 : destination is present but frame not accepted.
3) A=1, C=1 : destination is present, frame copied.
12
AC byte:
Contains:
 the token bit (0 – token, 1 – frame)
 the Monitor bit
 Priority bit
 Reservation bits
A station wants to
transmit a priority n frame
A token
arrived with
priority m
n>=m ?
Yes
No
Transmit the
frame
A station wants to receive
next token with priority s
Wait for a data
frame
The token is already No
with priority t>s
Yes
Set priority s in
Reservation
bits
13
Features of the standard IEEE 802.5
1) Transmission Media: shielded twisted pair, coaxial Cable,
fiber
2) Transmission Rate: 4 Mbps, 16 Mbps
3) Physical signals: Manchester code
4) There is a possibility using a wire center.
Station
Wire center
Station
Station
Bypass relay
Physical structure: star
Logical structure: ring
Station
Wire center 1
2
3
Extending the ring
by using many wire
centers.
14
Using priorities
Pm – station wants to transmit a data
Station
packet with priority Pm.
Pr – priority value in the received
token (free or busy).
Pm
Rr – reservation priority value in the
received token (free or busy).
A token arrived
Busy
No
Yes
Rr  Pm
Pr
Free
Free or busy ?
Pm > Rr ?
Rr
Pr is the highest ?
Yes
Yes
No
Pm >= Pr ?
Seize a
token
Reserve
future token
at Pm
Store Pr, Rr,
Rr0
Transmit with Pr
No
Pm>Rr
Yes
Decrease Pr
to stored
value
No
RrPm
Generate new free token
with Pr=max(oldRr, Pr, Pm)
Rr=max(oldRr, Pm)
15
Token Ring management
Active
Monitor
Monitor
station
token
Start time-out
Time-out expired
Other operations
Purge the ring
Generate new
free token
Active
Monitor
passive
Monitor
Busy token
Is monitor bit
set ?
No
Set it
Yes
(Circulating busy token,
packet was not purged by
transmission station)
Change busy token
to free token
Active monitor
failed ?
No
Yes
Start an election
algorithm
Advantages:
1) Traffic can be easily regulated.
2) Advantages of the token bus.
Disadvantages:
1) Complexity of ring maintenance.
16