Multiple Access

Multiple Access

Random Access
Controlled Access
Channelization

Data Link Layer: Two sublayers

Data link layer divided into two functionality-oriented sublayers
IEEE made this division for LANs

Medium Access Protocols



Random Access

Each station has the right to the medium without being controlled by any other station
Collision, a access conflict, if more than one station tries to send

ALOHA

The earliest random access method developed at the Univ. of Hawaii in the early 1970s
Designed for a radio (wireless) LAN
Pure ALOHA and Slotted ALOHA

Pure ALOHA Protocol: Procedure

The throughput for pure ALOHA is S = G × e −2G .
The maximum throughput Smax = 0.184 when G= (1/2).

Slotted ALOHA

Pure ALOHA vulnerable time = 2 x Tfr because there is no rule that defines when the station can send
Slotted ALOHA was invented to improve the efficiency of pure ALOHA

throughput for slotted ALOHA is S = G × e−G .
The maximum throughput Smax = 0.368 when G = 1
Slotted ALOHA vulnerable time = Tfr


Carrier Sense Multiple Access (CSMA)

CSMA
“Sense before transmit”
“Listen before talk”
CSMA can reduce the possibility of collision, but it can not eliminate it

CSMA: Vulnerable Time

Vulnerable time for CSMA is the propagation time Tp needed for a signal to propagate from one end of the medium to the other

Persistence Strategy

Nonpersistent strategy
Reduces the chance of collision
Reduces the efficiency of the network
1-persistent
Increases the chance of collision
p-persistent
Reduces the chance of collision and improves the efficiency by combining the other two strategies.

CSMA/CD: Min. Frame Size

Example: A network using CSMA/CD has a bandwidth of 10 Mbps. If the maximum propagation time (including the delays in the devices and ignoring the time needed to send a jamming signal, as we see later) is 25.6 μs, what is the minimum size of the frame?


Solution
The frame transmission time is Tfr = 2 × Tp = 51.2 μs. This means, in the worst case, a station needs to transmit for a period of 51.2 μs to detect the collision. The minimum size of the frame is 10 Mbps × 51.2 μs = 512 bits or 64 bytes. This is actually the minimum size of the frame for Standard Ethernet.


CSMA/CD: Energy Level & Throughput

Throughput of CSMA/CD is greater than that of ALOHA
The max. throughput occurs at a different value of G and is based on the persistent method and the value of p in the p-persistent approach
The max throughput is around 50% when G=1 for 1-persistent, up to 90% when G is between 3 and 8 for non-persistent

CSMA/CA (Collision Avoidance)

Invented for wireless network where we cannot detect collisions
Collision are avoided through the use of CSMA/CA’s three strategies: the interframe space, the contention windows, and acknowledgement
IFS can also be used to define the priority of a station or a frame
If the station finds the channel busy, it does not restart the timer of the contention window; it stops the timer and restarts it when the channel becomes idle


Controlled Access

The stations consult one another to find which station has the right to send
Reservation/Polling/ Token passing
Reservation access method

Polling: Select and Poll Functions


Channelization: FDMA

FDMA
Available bandwidth of the common channel is divided into bands that are separated by guard bands
FDMA is an access method in data link layer protocol. But, FDM is a physical layer technique

Channelization: TDMA

TDMA
The bandwidth is just one channel that is timeshared between different stations
TDMA is an access method. But, TDM is a physical layer technique

One channel carries all transmissions simultaneously
Two properties: If we multiply each code by another, we get 0. If we multiply each code by itself, we get 4
Data = (d1.c1 + d2.c2 + d3.c3 + d4.c4) .c1
= d1.c1.c1 + d2.c2.c1 + d3.c3.c1 + d4.c4.c1 = 4.d1

CDMA: Chips

Orthogonal sequences have the following properties:
Each sequence is made of N elements, where N is the number of stations
If we multiply a sequence by a number, every element in the sequence is multiplied by that element (scalar multiplication)
If we multiply two equal sequence, element by element, and add the results, we get N (inner product)
If we multiply two different sequence, element by element, and add the results, we get 0
Adding two sequence means adding the corresponding elements. The result is another sequence
Data representation in CDMA