5. Multiple Access Links and
Protocols
Three types of “links”:
Point-to-point (single wire, e.g. PPP,
SLIP)
Broadcast (shared wire or medium; e.g,
Ethernet, WiFi/WaveLAN, etc.)
Switched (e.g., switched Ethernet, ATM etc)
6. Multiple access problem
Example:
Classroom– many people gather together in a
large room
Broadcast medium – air
Human protocols:
“Give everyone a chance to speak”
“Don’t speak until you are spoken to”
“Don’t monopolize the conversation”
“Raise your hand if you have a question”
“Don’t interrupt when someone is speaking”
“Don’t fall asleep when someone else is talking”
7. Multiple access protocols
In LANs, WiFi, satellite networks
If more than 2 users send @ the same time collision
All collided packets are lost -> waste of bandwidth
Ideally, the MAC protocol for a broadcast channel
with the bit-rate R bps should satisfy:
if only 1 node is sending than the throughput is R
when M nodes have data to send than the throughput is
R/M
decentralized protocol – no master
simple & inexpensive to implement
8. MAC Protocols: Taxonomy
Three broad classes:
Channel Partitioning
Random Access
divide channel into smaller “pieces” (time slots,
frequency)
allocate piece to node for exclusive use
allow collisions
“recover” from collisions
“Taking turns”
tightly coordinate shared access to avoid
collisions
Goal: efficient, fair, simple, decentralized
12. Random Access Protocols
In random access or contention methods,
no station is superior to another station
and none is assigned the control over
another. No station permits, or does not
permit, another station to send. At each
instance, a station that has data to send
uses a procedure defined by the protocol
to make a decision on whether or not to
send.
13. Random Access
1.
2.
3.
4.
If more than one station wants to send,
there is an access conflict -- Collision—
To avoid access conflict each station has
to follow procedure that will answers the
following questions:
When can the station access the medium?
What can the station do if the medium is busy?
How can the station determine the success or failure of the
transmission
What can the station do if there is an access conflict?
14. ALOHA Network
Developed by Norm Abramson at the Univ. of Hawaii
the guy had interest in surfing and packet switching
mountainous islands → land-based network difficult to install
fully decentralized protocol
ACK
ACK
ACK
ACK
17. Example
Calculate possible values of TB, when
stations on an ALOHA network are a
maximum of 600 km apart
Tp = (600 × 103) / (3 × 108) = 2 ms
When K=1, TB ∈ {0ms,2ms}
When K=2, TB ∈ {0ms,2ms,4ms,6ms}
:
17
19. ALOHA: Throughput
Assume number of stations trying to
transmit follow Poisson Distribution
The throughput for pure ALOHA is
S = G × e−2G
where G is the average number of frames
requested per frame-time
The maximum throughput
Smax = 0.184 when G= 1/2
19
20. Example
A pure ALOHA network transmits 200-bit
frames on a shared channel of 200 kbps.
What is the throughput if the system (all
stations together) produces
1000 frames per second
500 frames per second
250 frames per second
20
23. Slotted ALOHA:
Throughput
The throughput for Slotted ALOHA is
S = G × e−G
where G is the average number of frames
requested per frame-time
The maximum throughput
Smax = 0.368 when G= 1
23
24. Example
A Slotted ALOHA network transmits 200bit frames on a shared channel of 200
kbps. What is the throughput if the system
(all stations together) produces
1000 frames per second
500 frames per second
250 frames per second
24
31. CSMA/CD
C arrier S ense M ultiple A ccess with
C ollision D etection
Station monitors channel when sending a
frame
31
32. Carrier Sensing Multiple Access with
Collision detection (CSMA-CD)
Jika station dp mengetahui apakah collision terjadi,
maka bandwith yang terbuang dpt dikurangi dengan
menghentikan transmisi
34. Carrier Sensing Multiple Access with
Collision detection (CSMA-CD)
Station yang mempunyai paket mendeteksi kanal dan
transmit jika kanal idle
Jika kanal sibuk, gunakan strategi dari CSMA (persist,
backoff segera atau persist dengan prob. p)
Jika collision terdeteksi saat transmisi, sinyal short
jamming ditransmisikan untuk meyakinkan semua
station mengetahui terjadi collision sebelum
menghentikan transmisi, selanjutnya algoritma backoff
digunakan untuk rescheduling waktu resensing
35. Carrier Sensing Multiple Access with
Collision detection (CSMA-CD)
Kanal mempunyai 3 kondisi (state):
sibuk mentransmisikan frame
idle
perioda contention (dimana station berusaha menduduki kanal)
Throughput 1-Persistent CSMA-CD dapat dianalisa dg
asumsi waktu dibagi dalam minislot sebesar 2tprop det
(untuk menjamin station selalu dapat mendeteksi
collision)
Setiap kanal menjadi idle, station memperebutkan
(contend) kanal dengan transmit dan mendengar untuk
mengetahui apakah sukses menduduki kanal
36. CSMA/CD: Minimum Frame Size
Each frame must be large enough for a sender
to detect a collision
Worst case scenario:
"A" is transmitting
"D" starts transmitting just before A's signal arrives
A
B
C
D
Long enough to
hear colliding signal
from D
36
37. Example
A CSMA/CD network has a bandwidth of
10 Mbps. If the maximum propagation
time is 25.6 μs, what is the minimum size
of the frame?
37
40. CSMA/CA
C arrier S ense M ultiple A ccess with
C ollision A voidance
Used in a network where collision cannot
be detected
E.g., wireless LAN
IFS – Interframe Space
40
41. CSMA/ CA
In wireless networks collision is avoided.
Collisions are avoided through following
strategies:
1. Interframe Space
2. Contention Window
3. Acknowledgment
44. Control Access
A station must be authorized by someone
(e.g., other stations) before transmitting
Three common methods:
Reservation
Polling
Token passing
44
45. Reservation
A station must make a reservation before sending
data
Time is divided into intervals
A reservation frame proceeds each time interval
Number of stations and number of time slots in the
reservation frame are equal
Each time slot belongs to a particular station
46. Polling
Devises are categorized into:
All data exchange must go through the primary station
Primary station controls the link and initiates the session
Secondary station obey the instructions of PS.
PS polls stations
Primary station (PS)
Secondary station (SS)
Asking SS if they have something to send
PS select a SS
Telling it to get ready to receive data
49. Token passing
the stations in a network are organized in a logical ring
for each station, there is a predecessor and a successor
for a station to access the channel, it must posses a token
(special packet) that gives the station the right to access the
channel and send its data
once the station has finished its task, the token will then be
passed to the successor (next station)
the station cannot send data until it receives the token again in
the next round
token management is necessary
Every station is limited in the time of token possession
Token must be monitored to ensure no lose or destroyed
Assign priorities to the stations and to the types of data transmitted
To make low-priority stations release the token to high priority stations
51. Token passing
Logical Ring
in a token passing network, stations do not have to be physically connected
in a ring; the ring can be a logical one.
