2-MAC sublayer
1. FDM
is a multiplexing scheme which divides the bandwidth into equal-sized portions, with
each user being assigned to one portion.
2. ALOHA is a protocol that let users transmit whenever they have a frame to be sent, And if collision
happens, it retransmits the frame again after waiting a random amount of time.
3. CSMA are protocols in which stations listen to the channel and send their data if the channel is idle.
4. CSMA/CD is a protocol in which stations listen to the channel for collision while it is transmitting.
5. Collision-Free Protocols are protocols that ensure totally no collision could happen.
6. A Bit Map Protocol is a protocol in which there are contention slots to decide the order of stations
that are going to make transmission.
7. Token Passing is a protocol in which the station that has a token (permission to send), it has the
right to start transmission, otherwise it must pass this token to another station.
8. Binary Countdown is a protocol in which higher- numbered stations have a higher priority.
9. Limited-Contention Protocols
are protocols combine the best properties of the contention
protocols and collision-free protocols.
10. Wireless LAN Protocols (MACA) is a protocol in which the sender must send a (Request to Send)
and wait for (Clear to send) before start transmission.
11. The hidden terminal problem
is a wireless Lan problem of a station not being able to detect
a potential competitor for the medium because the competitor is too far away.
12. The exposed terminal problem is a wireless Lan problem that happened when a station falsely
concludes that it may not start transmission while it is possible.
13. Match between each MAC protocol and its function
Protocol name
function
1. FDM
(1)divides the bandwidth into equal-sized portions, with each user being assigned to
2. ALOHA
one portion.
3. CSMA
(2)lets stations transmit whenever they have a frame to be sent.
4. CSMA/CD
(3)in which stations listen to the channel and send their data if the channel is idle.
5. Bit-Map
(4)in which stations listen to the channel for collision while it is transmitting.
6. Token Passing
(5)in which there are contention slots to decide the order of stations that are going to
7. Binary
make transmission.
countdown
(6)in which the station that has a token (permission to send), it has the right to start
8. MACA
transmission, otherwise it must pass this token to another station.
9. Collision-Free
(7)in which higher- numbered stations have a higher priority.
10. Limited(8)in which the sender must send a (Request to Send) and wait for (Clear to send)
Contention
before start transmission.
(9) ensure totally no collision could happen.
(10)combines the best properties of the contention protocols and collision-free
protocols.
14. Describe the static channel allocation (e.g. FDM ) problem.
1. Part of the bandwidth will be wasted when some users are quiescent or if the number of spectrum
portions > the number of users
2. Some of users will be denied permission If the number of spectrum portions < number of users
15. Compare between ALOHA and SLOTTED ALLOHA by (a)how they work, (b) channel utilization, (c) delay?
1-persistent CSMA
nonpersistent CSMA
how they work If the stations transmit whenever
A station (ready to send) waits for the
channel is busy,
they have data to be sent
beginning of the next slot.
channel utilization
Better channel utilization (less collisions)
delay
lower delay with low load
lower delay with high load
16. Compare between 1-persistent CSMA , nonpersistent CSMA , by (a)how they work if the channel busy, (b)
channel utilization, (c) delay?
1-persistent CSMA
nonpersistent CSMA
how they work If the
stations (ready to send)
stations (ready to send) waits a
channel is busy,
continuously sense it for
random period of time before sense it
collision.
again.
channel utilization
Better channel utilization (less
collisions)
delay
lower delay with low load
lower delay with high load
17. Compare between 1-persistent CSMA , CSMA/CD , by (a)how they work, (b) channel utilization (c) delay?
1-persistent CSMA
CSMA/CD
how they work
stations don’t sense the
stations stop transmitting when it detect
channel for collision while
a collision
transmitting
channel utilization
Better channel utilization (less collisions)
delay
lower delay
18. Compare between ALOHA and CSMA protocols by (a)how they work, (b) channel utilization, (c) delay?
ALOHA
CSMA
how they work
stations don’t sense the
stations sense the channel for
channel for collisions
collisions
channel utilization
Better channel utilization (less
collisions)
delay
lower delay
19. A group of N stations share a 56-kbps pure ALOHA channel. Each station outputs a 1000-bit frame on an
average of once every 100 sec, even if the previous one has not yet been sent (e.g., the stations can buffer
outgoing frames). What is the maximum value of N? (maximum throughput = 0.184)
With pure ALOHA the usable bandwidth is 0.184 x 56 kbps = 10.3 kbps.
Each station requires 10 bps, so N = 10300/10 = 1030 stations.
20. Sixteen stations, numbered 1 through 16, are contending for the use of a shared channel by using the
adaptive tree walk protocol. If all the stations whose addresses are prime numbers suddenly become
ready at once, how many bit slots are needed to resolve the contention?
Answer:
 13 bit slots
21. Consider five wireless stations, A, B, C, D, and E. Station A can communicate with all other stations. B can
communicate with A, C and E. C can communicate with A, B and D. D can communicate with A, C and E. E
can communicate A, D and B.
(a) When A is sending to B, what other communications are possible?
(b) When B is sending to A, what other communications are possible?
(c) When B is sending to C, what other communications are possible?
Answer:
a) D C , D E
b) No possible communication
c) E  D, E  A
22. An ALOHA network transmits 200-bit frames on a shared channel of 200 kbps. What is the throughput if
the system (all stations together) produces
a. 1000 frames per second
b. 500 frames per second
c. 250 frames per second.
Answer
a. data transfer rate = 1000* 200 = 200kbps
G (the load) = data transfer rate / channel bandwidth
= 200k / 200k = 1
The throughput percentage for pure ALOHA is: S = G × e −2G = 0.135
The throughput = 1000 × 0.135 = 135 frames will be successfully transmitted
b. data transfer rate = 500* 200 = 100kbps
G (the load) = data transfer rate / channel bandwidth
= 100k / 200k = 1/2
The throughput percentage for pure ALOHA is: S = G × e −2G = 0.184
The throughput = 500 × 0.184 = 92 frames will be successfully transmitted
c. data transfer rate = 250* 200 = 50kbps
G (the load) = data transfer rate / channel bandwidth
= 50k / 200k = 1/4
The throughput percentage for pure ALOHA is: S = G × e −2G = 0.152
The throughput = 250 × 0.152 = 38 frames will be successfully transmitted