Multiple access Flashcards
What is the data link layers purpose?
> Enables the exchange of information between end hosts
> Controls errors
> Determines the start and end of bits and frames
> Delivers information reliably
What is TDMA? What does it stand for?
TDMA = Time division multiple access Each user gets a fixed time slot to transmit packets at full link speed. Unused slots are wasted
What is FDMA? What does it stand for?
FDMA = Frequency division multiple access Each user gets a frequency band to send data over Unused frequencies are wasted.
What are the advantages of FDMA?
Users are guaranteed to send data bits continuously
What are the disadvantages of FDMA?
> Unused frequency bands reduce utilisation.
> An additional channel is required to help allocate bands
> Gaps between bands reduce channel utilisation
> Dividing up into channels reduces transmission rate Rigid allocation of resources requires advanced coordination
What is OFDMA? What does it stand for?
OFDMA = Orthogonal frequency-division multiple access
This is the combination of FDMA and TDMA
What is CDMA?
CDMA = Code Division Multiple Access
All users transmit over the same frequencies and at the same time with no interference, in theory. In practice it preforms well
How is CDMA implemented?
Using special coding that reduces interference of multiple transmitters
What are the disadvantages of TDMA?
> Dividing up into slots reduces transmission rate
> Rigid allocation of resources requires advanced coordination
What is contention based random access?
This is when a user transmits without any prior communication among nodes and the user hopes that no collision occurs. When collisions occur, both information sets (frames) are lost
What is ALOHA un-slotted?
This is simplest type of medium access control.
Anyone can transmit a packet at any time without delay.
Each packet has a specific duration time.
Collision can occur
What is the vulnerable period for a node transmitting a single frame?
This is the time in which a collision may occur. It is ideal that a packet is not sent by another node during this time.
The vulnerable period starts 1 frame before the packet is sent and during the time the packet is sent.
The length of time is 2 frames. [IMAGE 12]
What is the equation to calculate the probability a packet begins in a time interval?
Probability = λ × ∆t
λ = Rate of transmission from N nodes
What is does the graph of utilisation of a link look like for un-slotted ALOHA? What is the max utilisation?
Max utilisation = 0.5e ≈ 18% [IMAGE 13]
What is carrier sense multiple access?
A transmitting node will sense if there is data currently being transmitted before transmitting its data
What is slotted ALOHA?
This is when each node is given a specific time slot for them to transmit during.
How are bits represented as binary levels for communication?
0 = +5V
1 = -5V
What is a CDMA code, how does it work?
Each user is given a specific code.
The message that the user wants to send is multiplied by their code.
e.g. if the users code is 01 and the message is 110 then the transmission would be 01 01 10 . For the 0 bit in the message the users code is inverted.
[IMAGE 19]
How are CDMA user codes represented on a diagram?
They are represented as vectors on a diagram. The vectors need to be perpendicular to each so that if the vectors are dot producted the result will equal 0.
[IMAGE 21]
How long is a CDMA message that M bits long and the user has C code bits?
Message length = M × C
When two users with unique codes transmit at the same time, what happens to the signal received by the receiver?
The signals combine and the received signal is neither bob or cathys message.
When they combine, the amplitudes of the signals sum so there may be different voltage levels of bits
When two users with unique codes transmit at the same time, how is the message from a single user isolated? what is the maths behind it?
If you multiply the recieved message with the code from one of the users then that users message can be extracted as the answer.
M[Trans] = M[a] × C[a] + M[b] × C[b]
M[trans] ∙ C[a] = C[a]∙M[a]×C[a] + C[a]∙M[b]×C[b]
C[a]∙M[b]×C[b] = 0
C[a]∙M[a]×C[a] = M[a] × 1
M[trans] ∙ C[a] = M[a]
What is the order of things to do to transmit a CDMA message?
- Give each user a unique ID
- For each users message multiply their message by their unique ID. If the message has a 0 bit then the inverse of their unique ID is used
- The expanded and unique message is transmitted
How is a CDMA mesage decoded?
If the reciever knows the ID code of a user that sent a message then the reciever find the magnitude of each section of the data sent and the magnitude of the user ID code and multiplies the together.
The angle between the two vectors is calculated and plugged into the equation:
|MessageVector| * |IDCode| * Sin(θ) = Message bit
How are user ID codes chosen (when it comes to 2 users)?
The codes need to orthogonal to each other.
When using 2D vectors, they need to be 90° to each other.
How are user ID codes chosen when having more users?
A square matrix is formed with the matrix format being:
M = M M
M -M
This can then be formed into a bigger matrix:
4x4 matrix:
M M M M
M -M M -M
M M -M -M
M -M -M M
Each square of 4 in the matrix follows the M matrix format. Each elemtent in the larger matrix is in the form of the M matrix format:
8x8
M M M M M M M M
M -M M -M M -M M -M
M M -M -M M M -M -M
M -M -M M M -M -M M
M M M M -M -M -M -M
M -M M -M -M M -M M
M M -M -M -M -M M M
M -M -M M -M M M -M
