network topologies, csma/cd, csma/ca

network topologies, csma/cd, csma/ca
Network Topologies, CSMA/CD, CSMA/CA
NETWORK TOPOLOGIES,
CSMA/CD, CSMA/CA
Mohammad S. Hasan
1
Bus Topology
2



The original linear topology
Based on 10base2 coaxial cable
Limited to 185m
Terminating resistor 50 ohm
Network Topologies, CSMA/CD, CSMA/CA
5-4-3 rule
3

5
 At

4
 At

most 5 segments
most 4 repeaters
3
 At
most 3 segments
with active senders
Network Topologies, CSMA/CD, CSMA/CA
Connectors & Cable
4

Coaxial cable and BNC
connector
 Bayonet

Tee connector

Terminating resistor
Neill-Concelman
Network Topologies, CSMA/CD, CSMA/CA
Connection issues
5
The dropper cable
produces
circulating currents
Network Topologies, CSMA/CD, CSMA/CA
Star Network
6
Network Topologies, CSMA/CD, CSMA/CA
Star Network based on Hub
7


All nodes are connected to a central point
Original was via a hub
Network Topologies, CSMA/CD, CSMA/CA
Star Network based on Switch
8

A better more efficient and faster solution is via a
switch
Network Topologies, CSMA/CD, CSMA/CA
UTP/STP
9
Network Topologies, CSMA/CD, CSMA/CA
Straight through Ethernet cable
10
Network Topologies, CSMA/CD, CSMA/CA
Crossover cable
11
Network Topologies, CSMA/CD, CSMA/CA
Token Ring Network (IEEE 802.5)
12
Data flow
Network Topologies, CSMA/CD, CSMA/CA
Ring Network
13
Network Topologies, CSMA/CD, CSMA/CA
Ring Network
14



Data flows theoretically in one direction
Each station can only transmit when it has the
TOKEN
A further enhancement is FDDI (Fibre Distributed
Data Interface)
 IEEE802.4
token bus based
 Up to 124 miles (200 km)
 Normally optical fibre but can be copper
 2 counter rotating rings
Network Topologies, CSMA/CD, CSMA/CA
Carrier Sense Multiple Access (CSMA)
Protocol
15

A station wishing to transmit has to first listen to the
channel for a predetermined amount of time to
check for any activity.
 If
the channel is sensed "idle" then the station is
permitted to transmit.
 If the channel is sensed as "busy" the station has to
defer its transmission.

This is the essence of both CSMA/CA and
CSMA/CD.
Network Topologies, CSMA/CD, CSMA/CA
Carrier Sense Multiple Access /
Collision Detection (CSMA/CD)
16

Typically Ethernet LANs
A

carrier sensing scheme is used
If a transmitting station detects another signal while
transmitting a frame
 stops
transmitting that frame,
 transmits a jam signal,
 then waits for a random time interval “backoff delay”
before trying again
 The
backoff delay is determined using the truncated binary
exponential backoff algorithm
Network Topologies, CSMA/CD, CSMA/CA
CSMA/CD and Wired Networks
17
Start
There is data from a user to send
Assemble a frame
Physical addresses are used (MAC)
Attempt 1
Yes
Is another station
transmitting?
No
Collision recovery
subalgorithm
Transmit 1st bit of frame
Yes
Collision detected?
Yes
Transmit next bit
of the frame
Recovered?
No
No
Transmission
finished?
No
End
Yes
frame successfully transmitted
Network Topologies, CSMA/CD, CSMA/CA
Truncated binary exponential backoff
18

After i collisions, a random number of slot times
between 0 and 2i − 1 is chosen.
 For
the first collision, each sender might wait 0 or 1 slot
times.
 After the second collision, the senders might wait 0, 1,
2, or 3 slot times, and so forth.

As the number of retransmission attempts increases,
the number of possibilities for delay increases.
Network Topologies, CSMA/CD, CSMA/CA
Truncated binary exponential backoff
(cont.)
19

The 'truncated'

after a certain number of increases, the exponentiation
stops i.e. the retransmission timeout reaches a ceiling, and
thereafter does not increase any further.


For example, if the ceiling is set at i=10, then the maximum delay
is 1023 slot times.
Because these delays cause other stations who are
sending to collide as well, there is a possibility that, on
a busy network, hundreds of people may be caught in
a single collision set. Because of this possibility, after 16
attempts at transmission, the process is aborted.
Network Topologies, CSMA/CD, CSMA/CA
Wireless Network - Hidden Node
Problem
20


A and B can communicate
with the hub
A and B can NOT
communicate with each other
directly
 hidden

from each other
Typical wireless problem
 Simultaneous
transmission
from A and B
Network Topologies, CSMA/CD, CSMA/CA
Carrier Sense Multiple Access /
Collision Avoidance (CSMA/CA)
21

Typical application is on Wireless networks
 Collision

detection is NOT possible
Collision Avoidance (CA) protocol
 The
sender sends Request To Send (RTS) to the receiver
 The Receiver replies back with a Clear To Send (CTS) to
the Sender
 The Sender starts data transmission
 The RTS or the CTS tell other nodes NOT to send data
for a given amount of time.
Network Topologies, CSMA/CD, CSMA/CA
CSMA/CA
22
Network Topologies, CSMA/CD, CSMA/CA
LAN limitations
23

The choice of connection equipment is extremely
important
 For

example, do you connect via a hub or switch
In a previous lecture
 hubs
share bandwidth
 so available bandwidth
Total _ bandwidth
Number _ of _ users
Network Topologies, CSMA/CD, CSMA/CA
LAN limitations
24

A switch however
uses a process
called
microsegmentation
to create
dedicated full
bandwidth links
between the ports
Network Topologies, CSMA/CD, CSMA/CA
Proof of Hub limitations – Ping
command
25

Type in command prompt (where x is the address of
another machine
 C:\>ping
192.168.1.x –l 65500 –n 65500
Dlink Hub
192.168.1.1
.2
.3
.4
Network Topologies, CSMA/CD, CSMA/CA
.5
192.168.1.6
Ping with hub - 3 PC’s, 1 cmd prompt
only
26
Note round trip time
Network Topologies, CSMA/CD, CSMA/CA
Ping with hub - 3 PC’s, 4 cmd prompts
per PC
27
Note increased round trip time
Network stalled due to too many collisions
Network Topologies, CSMA/CD, CSMA/CA
Proof of Switch Performance – Ping
Command
28

Type in the command prompt (where x is the
address of another machine:
 C:\>ping
192.168.1.1
192.168.1.x –l 65500 –n 65500
.2
.3
.4
Network Topologies, CSMA/CD, CSMA/CA
.5
192.168.1.6
Ping with switch - 3 PC’s, 4 cmd
prompts per PC
29
Noticed the reduced round trip time
Network Topologies, CSMA/CD, CSMA/CA
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