NETW 501 Communication
Networks
Tutorial 6
By: Mariham Wasfy
Outline
• Random Access Dynamic MAC
Outline
• Random Access Dynamic MAC


o
o
ALOHA
Slotted ALOHA
CSMA
CSMA/CD
Random Access MAC


o
o
ALOHA
Slotted ALOHA
CSMA
CSMA/CD
Random Access MAC


o
o
ALOHA
Slotted ALOHA
CSMA
CSMA/CD
CSMA
• Carrier Sense
SENSE Medium
Medium Access
Access
∵ Busy
∴ Don’t send the frame
∵ Idle (Free)
∴ Send the frame
CSMA
• Vulnerable Period = Tprop
A
Ttrans = 2
Tprop = 1
B
A 0
B
C
D
D
C
0
2
1
3
1
3
1
3
CSMA
• Types:
P
Success
11Persistent
Persistent
NonPersistent
Run
Exponential
Backoff
Algorithm
1-P
Failure
PPPersistent
Persistent
CSMA
• Collision
Run Exponential
Backoff Algorithm
11Persistent
Persistent
NonPersistent
PPPersistent
Persistent
CSMA
• Example
A
Ttrans = 2
Tprop A-B = 2
Tprop A-C = 1
B
A 0
B
C
C
2
0 1 2 3 4
1
3
Random Access MAC


o
o
ALOHA
Slotted ALOHA
CSMA
CSMA/CD
CSMA/CD
• Carrier Sense Medium Access / Collision
COLLISION
Detection
DETECTION
A
Ttrans = 2
Tprop = 1
B
A 0
B
C
2
0 1 2
1
3 4
3
Abort transmission
when collision detected
C
CSMA/CD
• Vulnerable Period = Tprop
A
Ttrans = 2
Tprop = 1
B
A 0
B
C
D
D
C
0
2
1
3
1
3
1
3
NB: Number of trials
CW: Clear window
BE: Backoff window
CSMA and CSMA/CD
• Exponential Backoff Algorithm
R value from 0 to
𝟐𝑩𝑬
−𝟏
CCA: Clear Channel Assessment
ABORT
Yes
NB >
max_NB
?
No
NB = 1
CW = 2
BE = min(BE+1, max_BE)
SUCCESS
Yes
CW
= 0?
NB = 0
CW = 2
BE = min_BE
Choose and wait
R (time)
Perform 2 CCA
No
CW = CW -1
No
Yes
CCA
Success
?
Tutorial
1. Consider an Ethernet of diameter 300 m, frame size 1500 bytes, Speed of 2.1 × 108
m/sec and transmission rate of 1 Mbps.
a. Calculate the vulnerable period in CSMA.
b. If CSMA/CD is used. Stations A & B are located at the edges of the network.
Given that station A starts transmission at t and station B starts transmission at t +
0.5 µsec.
i. At what time will B abort transmission?
ii. At what time will A abort transmission?
Given:
• D = 300 m
• Lframe = 1500 B
• C = 2.1 × 108 m/sec
• R = 1 Mbps
𝑫
𝟑𝟎𝟎
Tprop = 𝑪 = 𝟐.𝟏 ×𝟏𝟎𝟖 = 1.428 µsec
𝑺𝒊𝒛𝒆
Ttrans = 𝑹𝒂𝒕𝒆 =
𝟏𝟓𝟎𝟎 × 𝟖
𝟏 × 𝟏𝟎𝟔
A
= 12 msec
a. Vulnerable period in CSMA = Tprop = 1.428 µsec
B
Given:
• A starts transmission at t
• B starts transmission at t + 0.5 µsec
𝑫
A
B
𝟑𝟎𝟎
Tprop = 𝑪 = 𝟐.𝟏 ×𝟏𝟎𝟖 = 1.428 µsec
𝑺𝒊𝒛𝒆
Ttrans = 𝑹𝒂𝒕𝒆 =
𝟏𝟓𝟎𝟎 × 𝟖
𝟏 × 𝟏𝟎𝟔
= 12 msec
t + 1.928 µs
t + 1.928 µs
b. i. B aborts transmission @: t + Tprop = t + 1.428 µs
ii. A aborts transmission @: t + 0.5 µs + Tprop = t + 1.928 µs
t + 2.856 µs
t
t + 12 ms
t + 0.5 µs
A
B
t + 1.428 µs
t + 12 ms
2. A channel using random access protocols has three stations on a bus with end-to-end
propagation delay t. Station A is located at one end of the bus and stations B & C are
located together at the other end of the bus. Frames arrive at the three stations and
are to be transmitted at stations A, B & C at the respective time tA = 0, tB = 0.5 t and tC
= 1.5 t. Frames require transmission time of 4t. in appropriate figures, with time at
the horizontal axis, show the transmission activity of each of the three stations for:
a. Non-persistent CSMA
b. Non-persistent CSMA/CD
tB = 0.5 t
ttrans = 4 t
tprop A-B = tprop A-C = t
tprop B-C = 0 t
a. Non-persistent CSMA
B
A
A
B
C
0
1.5 t
0.5 tt
4t
5.5 t
tA = 0 t
C
tC = 1.5 t
4.5 t5 t
Busy doesn’t send and will back off
0.5 tt
1.5 t
4.5 t5 t
tB = 0.5 t
ttrans = 4 t
tprop A-B = tprop A-C = t
tprop B-C = 0 t
a. Non-persistent CSMA
B
A
A
B
C
0
0.5 t
4t
4.5 t
tA = 0 t
C
tC = 1.5 t
tB = 0.5 t
ttrans = 4 t
tprop A-B = tprop A-C = t
tprop B-C = 0 t
b. Non-persistent CSMA/CD
B
A
A
0 1.5 t 2 t
B
0.5 t t
C
0.5 t tt
1.5 t
tA = 0 t
4t
C
tC = 1.5 t
5t
Busy doesn’t send and will back off
5t
tB = 0.5 t
ttrans = 4 t
tprop A-B = tprop A-C = t
tprop B-C = 0 t
b. Non-persistent CSMA/CD
B
A
A
0
B
0.5 t t
C
1.5 t
tA = 0 t
C
tC = 1.5 t
3. Consider the network shown below where terminals A, B, C and D are
communicating over a bus with a central station. The transmission time for packets
from terminal B and C is t units while the transmission time for packets from
terminals A and D is 0.5 units.
a. What is the vulnerable period in the following scenarios: (Calculate based on
worst case scenario)
i. 1-persistent CSMA
ii. Non-Persistent CSMA/CD
b. Provided that terminals A,B,C and D start their transmission at tA=1.5t, tB=0.5t, tC=2t
and tD=2t respectively. Assuming that A & C implement 1-persistant CSMA/CD while
B & D implement Non-persistent CSMA/CD. Show the transmission activity for the
four stations in the plot below until 6t. (Assume that back-off interval is equal to 1.5t
for all terminals)
c. Indicate the time interval during which node D senses the channel to be idle.
d. Indicate which of the four packets will be received correctly at central station and
which will suffer from collision on timing diagram.
a. The vulnerable period:
i.
ii.
1-persistent CSMA
Non-Persistent CSMA/CD
Vulnerable period 1-persistent CSMA = Vulnerable period Non-Persistent CSMA/CD = Tprop_MAX
TpropA-B = t
TpropA-Central Station = 1.5 t
TpropA-C = 2t
TpropA-D = 3t
TpropD-C = t
TpropD-Central Station = 1.5 t
TpropD-B = 2t
TpropD-A = 3t
∴3t
b.
back-off interval is equal to 1.5 t for all terminals
A
B
C
D
Send
at
2.5 t
1.5
0.5 t
2.5
2tt
2t
Tt
0.5 t
t
t
0.5 t
Tp
A-Bt
A-C2t
A-D3t
B-At
B-Ct
B-D2t
C-A2t
C-Bt
C-Dt
D-A3t
D-B2t
D-Ct
Proto
col
1persistant
CSMA/CD
Nonpersistent
CSMA/CD
1persistant
CSMA/CD
Nonpersistent
CSMA/CD
c. Node D will sense the channel idle at:
From 0 to 2 t
From 4 t to 5.5 t
From 6 t to further
d. All 4 packets received correctly at
central station
A
B
C
D
Send
at
1.5 t
2.5
0.5 t
2.5
25tt tt
4.5
2t
Tt
0.5 t
t
t
0.5 t
Tp
1.5 t
0.5 t
0.5 t
1.5 t
Proto
col
1persistant
CSMA/CD
Nonpersistent
CSMA/CD
1persistant
CSMA/CD
Nonpersistent
CSMA/CD
Backoff
4. Four stations are located on a bus network as shown below. The distance between station
A and B is 2d meters. The distance between A and H is d meters. The distance between
station H and C is 3d meters. The propagation delay between A and C is t sec.
Frames arrive at the four stations A, B, C and H at times tA=0, tB= 0.75 t, tC= 2.5 t and
tH=2.75 t respectively.
All frames require transmission time of t sec.
If we assume that the stations A, C, H apply 1-persistent CSMA/CD while station B
applies Non-Persistent CSMA/CD. Show the transmission activity for the four stations in
the plot below until 6t. (Assume that if collision is detected or occurs for terminals A & C
the back-off window size is 2 slots while if collision is detected or occurs for terminal B
& H the back-off window size is 1 slot)
A
B
H
C
D
A-B2d
A-Hd
A-C4d
B-A2d
B-Hd
B-C2d
H-Ad
H-Bd
H-C3d
C-A4d
C-B2d
C-H3d
Tprop
A-C t
Cross multiplication
Distance
Tprop
4d
t
3dd
2
x
x=
𝑡×3𝑑
𝑡×𝑑
𝑡×2𝑑 𝑡 3𝑡
2𝑡
=
=
4𝑑
4𝑑 4 4
𝑡
2
=
A
B
H
C
Send
at
0
1.75 t
0.75
3t t
2.75
3.25
5.75
2.5 ttt
Tt
t
t
t
t
Tp
A - B 0.5t
A - H 0.25t
A-Ct
B - A 0.5t
B – H 0.25t
B - C 0.5t
H-A2t
H - B 0.25t
H - C 0.75t
C-At
C - B 0.5 t
C - H 0.75t
Prot
ocol
1persistant
CSMA/CD
Nonpersistent
CSMA/CD
1persistant
CSMA/CD
1persistant
CSMA/CD
Back
off
2 slots
1 slot
1 slot
2 slots
Backoff
Backoff