Multiple access What if multiple machines are sharing the same link?
Random access • In random access protocols, any device can send data at any time, so long as the rules for sending are followed. Nobody is privileged. • The goal of the protocols is to maximize use of the link and minimize collisions - when two devices try to talk at the same time.
ALOHA • Pure ALOHA: Whenever the sender has a frame to send, it sends the frame. • If no ACK is received, assume a collision: • Wait a random amount of time. This avoids everybody resending the collided frames at the same time. (the back-off time, TB) • Re-send the frame and repeat. • After some maximum number of attempts, Kmax, give up and wait until later.
Pure ALOHA is pretty lousy at delivering frames, and has a very large vulnerable time. • Slotted ALOHA tries to fix this by having rules for when a station can send. • Time is divided into slots of Tfr length. • A station can send only at the beginning of a slot. • Thus, if you are the only station that sends a frame at the beginning of a slot, that frame will get through.
CSMA • Carrier Sense Multiple Access • Before sending, a station must check the link to make sure it is free. • Significantly reduces the probability of a collision, at the cost of extra overhead. • Vulnerable time of Tp. • What are the rules if the link is busy or idle? • 1-persistent: Send frame immediately when the link is idle. (Ethernet) • Nonpersistent: Check link; if idle, send; if busy, wait random time and check again. • p-persistent: • If the link is idle, send frame with a probability p. • With probability q = 1-p, wait for next slot and check link again. • If idle, do 1. • Else, wait random time and try again.
CSMA/CD • CSMA leaves it unclear what to do in case of a collision. • To detect a collision, the sender listens to the link after sending a frame: • If it detects nothing else in Tfr, it assumes no collision. • If, however, it detects other bits in less than the frame transmission time, there must have been a collision.
Frame size in CSMA/CD • If the frame size is too small, there will be lots of undetected collisions. • In particular, the problem is if Tfr < 2*Tp.
CSMA/CA • Ideally, we would like to avoid collissions all together (hence Collision Avoidance). • Collision detection is not always feasible, especially in high-attenuation media (e.g., wireless). • Collision avoidance involves not transmitting the instant the link is idle. • Wait the interframe space (IFS). This ensures that some distant device has not already started sending. • If the channel is still idle, wait through the contention window - essentially a random number of time slots. • If the channel is busy, stop the timer until the channel is idle, then restart it. • If the channel is idle after the contention window, send.
Controlled Access • Random access methods can reduce, but never eliminate, the chance of a collision. • They do so at the cost of higher overhead, but at the benefit of maximal use of the link. • Controlled access protocols eliminate collisions by only giving sending rights to one device at a time. • However, the link might be wasted if an authorized device has nothing to send or hogs the credentials.
Reservation • To use the link, a device needs to make a reservation. • Each time slot is preceded by set of reservation slots, one for each device on the network. • If a device wants to send a data frame, it makes a reservation. • After the reservation slots have passed, each frame that made a reservation sends its data in order.
Polling • Polling works in networks set up as primary/secondary architectures, where all communication must go through the primary device. • The primary can send data whenever the link is free. (the select function) • If the primary has nothing to send, it polls each of the secondary devices, one at a time, until it finds one with data to send.
Token passing • Used in token-ring networks, where each devidce has a predecessor and a successor. • A device can only send when it has the token - a special packet handed around the ring • A token can only be received the predecessor and sent to the successor.
Channelization • In channelization, sending devices share the bandwidth of a link and send simultaneously. • Primarily useful in cases where communication is relatively continuous (such as phones) • FDMA - link is divided into frequency bands and each sending device is assigned a band. • TDMA - link is divided into time slots which are allocated to sending devices.
Code Division Multiple Access • CDMA works a little differently. The link consists of only one channel, but all devices can transmit at the same time. • Each device is assigned a code, ci. • Codes have two important properties: • The result of multiplying any two codes together is 0. • The result of multiplying any code by itself is the number of devices on the network. • The data on the link are:
Codes are simply vectors, with one slot for each device on the network. • The rule for sending to utilize the codes is: 0 bit = -1; 1 bit = 1; idle = 0 • If a device wants to send a bit, it multiplies that bit by its code, which yields a vector. • Since the nature of signals is such that they are additive, each device sending to the link simultaneously has the effect of adding these vectors together. • For another device to extract a bit it multiplies the total signal on the link by the code of the device it wants to listen to.
