CS-09 Data Communications and Networks
AGENDA (Session 10)
Quiz
802.3 Collision Control, Performance, Switching
Token Bus (802.4)
Token Ring (802.5)
Bridges
Transparent Bridges
Source Routing Bridges
Quiz
1.
FDM/TDM are Dynamic Channel Allocation Approaches. TRUE/FALSE
2.
The probability of traffic success in Pure ALOHA is
a)
e-2G.
b)
e-G.
c)
1/2e.
d)
1/e.
3.
p-persistent CSMA applies to slotted channels TRUE/FALSE.
4. In CSMA/CD after
collision detection.
a)
Frames shall be retried after a fixed time.
b)
Frames shall be totally discarded.
c)
Frame shall be delayed for twice the cable propagation delay.
d)
Frames shall be tried after a random time.
5. 10 Base T maximum allowed
distance is.
a)
200 m.
b)
100 m.
c)
1 Km.
d)
Ten Meter.
6.
Draw the
7.
In 802.3 setting MSB = 1 in Destination Address means.
a)
Group Address.
b)
Local Address.
c)
Remote Address.
d)
Control Frame.
802.3 Collision Control Performance - Switching
Binary Exponential Backoff Algorithm:
· After i Collision random number between 0 to (2i 1) is chosen and that many slots are skipped before resending the frame.
· After 16 collision failure is reported.
· Dynamically adapts number of stations.
· This allows lower collision probability successively but introduces more waiting delay.
Performance:
· Channel Efficiency = 1/(1+2BLe/cF
F = Frame Length, B=Bandwidth, L=Cable Length, c=signal propagation speed.
· Requirement is to increase BxL, Thus channel Efficiency goes down. So 802.3 not suitable for WAN.
Switching:
· A high-speed backplane with multiport cards that work as independent collision domains of LAN.
· Buffer based cards allow collision domain to be restricted to port level.
802.4 Token Bus
· Conceived by General Motors for real time assembly lines.
· Arbitrary wait on 802.3 had to be resolved.
· Important and unimportant priortisation required.
· Station organized as branches to a bus.
· Logical Ring, Each station has a logical left and right station.
· Highest n. station send the first frame & then sends Token (Permission of Sending) to its neighbor.
· No Contention design
·
Four Priority Classes 0,2,4,6. Each Maintaining
its own queue and timers. Token passes from
Frame:
|
Preamble (1) |
Start (1) |
Frame Control (1) |
Dest. Address (2 or 6) |
Source Address (2 or 6) |
Data (0-8182) |
CS (4) |
End |
Frame Control Distinguishes Data/Control, Frame Priority, Acknowledgement.
Source/Dest. 2/6 byte arrangement.
Data 8182 byte : 2byte address, 8174 byte: 6byte address.
Control Frames manage token passing and new station entry.
Token Ring 802.5
· Collection of point to point entities connected as ring and thus forming a broadcast medium.
· Popular on IBM platform.
· Ring Engineering is Digital.
·
Ring
Circumference has direct relation with DataRate and thus no. of bits together
on a ring could be a limitation.
· 1 bit delay at each interface due to buffering and checking of data at interface.
· 3 Byte Token.
· 1 Bit Inversion converts it to Data Frame.
·
Differential
· High-High and Low-Low are used as control bytes (start/end Delimiter) not appearing in data.
· To avoid ring breaking, WIRE-CENTRES are used at interfaces, where bypass relays are provided. Wire centers make it a star-shaped network similar to 10BaseT in Physical looks but different in protocol.
· In order to contain token on the ring artificial delay might be introduced, specially when some station go down at night.
Token Ring 802.5 Frame
· No Limit on Data as the whole channel may never contain a full frame.
Token:
|
SD |
AC |
FC |
Frame:
|
SD (1) |
Access Control (1) |
Frame Control (1) |
Dest. Address (2 or 6) |
Source Address (2 or 6) |
Data (No Limit) |
CS (4) |
ED 1 |
FS 1 |
· Sent Bits are removed when they come back.
· Station holds a token for Token Holding Timer=10ms.
· If Frame transmission is complete or next frame not possible to transmit during this time, token is reinserted into the channel.
· ACCESS CONTROL: Monitor Bit, Priority Bits, Reservation bits.
· Wait until station captures a token of priority less than n.
· Next token can also be reserved by a station.
· Station raising priority has to lower it back.
· FRAME STATUS: A&C bits checked by the sender to know the acknowledgement status.
· An Elected Monitoring Station.
Bridges
· Connection between Multiple LANs.
· Operate in Data-Link Layer (Unlike Routers that operate in Network Layer).
· Required to handle situation like:
o Separated LANs.
o Independently functional LANs.
o Secured LANs.
o Load Sharing on LANs.
o 802.x to 802.y conversion.
· Problems are associated with 802.x to 802.y conversion
o Frame sizes different
o Presence/absence/ different nature of priority schemes.
o Value of Acknowledgement timers.
o Acknowledgement methods.
o Different Data Rates.
Transparent Bridges
· Connects LANs transparently.
· Accepts all traffic on all sides and discards any one which doesnt need crossing over.
· Forwarding decisions are based on Hash tables, prepared inside that bridges that updates by using backward learning algorithm.
· Table entries are dynamically changes as stations go up/down. Last Arrival Time from every node is also maintained. Flooding algorithm is used initially to fill hash tables.
· Spanning Tree Bridges assume just 1 connection between 2 LANs.
· Root is elected between bridges by selecting lowest unique serial number.
· Spanning avoids traffic looping.
· Bridges can be used to connect LAN using WANs.
Source Routing Bridges
· Connects optimally.
· Sender Knows whether receiver is on same or different LAN and thus sets the addressing accordingly.
· Frame header contains path to follow.
· Each LAN: 12 bit ID, Bridge: 4 bit ID.
· Frame Examination and forwarding implemented by Hardware/Software or hybrid way.
· Discovery frame sent to locate distant LAN.
· Discovery frames can cause frame Explosion as they are copied on multiple parallel paths enroute.
· Connection Oriented.
Bridges can be
remotely connected by point-to-point links.