Multiple access means to access a plurality of transmitting and receiving nodes to a shared broadcast channel.
It refers to a multiple access protocol nodes to govern the behavior of a shared broadcast channel transmitted by these protocols.
When all nodes simultaneously to a plurality of frames, the transmission frames in the received collision has arisen, when a collision occurs, a node can not receive any valid frame transmission, multiple access protocol is required to coordinate.
Multiple access protocol has the following three types:
- Channel is divided Protocol
- Random access protocol
- Turns agreement
1. The channel is divided Protocol
1.1 Time Division Multiplexing TDM:
Time is divided into time frames and each time frame is further divided into N time slots. Each slot is then allocated to one of the N nodes when the node need to be divided when there is a packet, which is assigned to the transmission of packets within its slot in the TDM frame bit cycle.
Advantages: to give each node a dedicated transmission rate R / N bps in each frame time.
Disadvantages: the node is restricted to R / N bps of average rate.
Node must wait for its rounds in the transmission series.
1.2 Frequency Division Multiplexing FDM:
The R bps channel is divided into different frequency bands each having a R / N bandwidth and frequency assigned to each node N a.
Advantages: avoiding the collision between the N nodes in a fair division of bandwidth.
Disadvantages: a node can only limits the use of R / N bandwidth, even when it is the only node in a packet needs to occur.
1.3 CDMA Code Division Multiple Access
CDMA Each node is assigned a different code, each node is encoded data transmitted with it its unique code.
Advantages: different nodes can be transmitted simultaneously, a receiver can correct reception.
2. Random Access Protocol
In a random access protocol, a node always transmitted in its entirety R bps data transmission channel, when there is a collision, each node design repeated collisions its frame retransmission until the frame without a collision by far .
2.1 Slotted ALOHA
Slotted ALOHA is operable to:
- When the new node has a frame to be transmitted, it until the next time slot begins and the entire frame in the time slot transmission.
- If there is no collision, the node successfully transmitted its frame, without retransmission.
- If there is a collision, the node before the end of the slot collision is detected, the node with probability p for each time slot in a subsequent retransmission of the frame until the frame is transmitted out of the collision.
The probability of successful transmission to a given node is
Agreement for maximum efficiency
2.2 ALOHA
Slotted ALOHA protocol requires that all nodes synchronize their transmissions to at the beginning of each time slot for data transmission.
Pure ALOHA, when a first arrival , the node immediately the full transmission frame into a broadcast channel. If a transmission frame with other transmission experienced a collision, the node immediately transmits the frame with a probability p. Otherwise, the node waits for a frame transmission time, after waiting that the frame transmission with probability p, with probability 1-p or another frame waiting time.
The probability of successful transmission to a given node is
The maximum efficiency of the protocol is 1 / (2e), the half slotted ALOHA.
2.3 carrier sense multiple access CSMA
And pure ALOHA time slots, the node determines a transmission independent of other nodes connected to the active broadcast channel. That is, a node does not care if there are other nodes when it starts to happen in transit transport, and even if there is another node begin to interfere with its transmission does not stop transmission.
Carrier Sense : a first node before transmission channel to listen, if there is no frame from another node is transmitted on a channel, it waits until the node is not transmitting a short time, and then starts transmitting detected.
Since all nodes are listening, why there will be a collision?
Because the broadcast channel having a channel transmission delay end.
2.4 carrier with collision detection Sense Multiple Access CSMA / CD
Collision Detection: When a node when the transmission has been listening on the transmission channel, if the detected another node is transmitting frames of interference, it stops transmitting, repeating "listening - when the idle transmission" wait for a while before the loop at any time time.
CSMA / CD operation:
- Adapter takes packets from the network layer, link layer frame prepared, and placed in the adapter frame buffer.
- If the adapter listens to the channel is idle, start a transmission frame; if listening to the channel is busy, it waits until the transmission start signal when there is no other transmission frame.
- During transmission, the adapter monitors the presence of signals from other energy using the broadcast channel adapter.
- If the adapter to transmit an entire frame without detecting signal energy from other adapters, the adapter frame to complete the transfer. If the detected signal energy from other adapters in the transmission, the transmission is aborted.
- After discontinuing the transmission, the adapter waits a random amount of time, return to step 2.
3. turns agreement
Refers to a protocol turns the node designated as the master node, the master node polls a cyclic manner poll each node.
Advantages: random access protocols to eliminate the problems of collisions and empty slots.
Disadvantages: introducing polling delay.
If the master node is faulty, the entire channel is inoperable.
Token passing protocol: The protocol uses a token called a small specific frame exchanged between the node in some fixed order. When a node receives a token, only if it has some frame to be transmitted, it only holds the token, or immediately forwarded to the next node of the token.
Advantages: dispersion, high efficiency.
Disadvantages: a node failure may cause the collapse of the entire channel.
