INDEX

1. Intoduction.

a. Broadcast networks

b. Need of protocols in Broadcast channel

2.Types of Multiple –access protocol

a. Random Access Protocols

i. ALOHA ii. CSMA

iii. CSMA/CDiv. CSMA/CA

b.Controlled AccessI. Reservation

II. Polling III. Token -based

c.Channelization I. FDMA

II. TDMAIII. CDMA

1.INTRODACTION

The media access control (MAC) data communication protocol sub-layer, also known as the medium access control, is a sublayer of the data link layer specified in the seven-layer OSI model (layer 2). It provides addressing and channel access control mechanisms that make it possible for several terminals or network nodes to communicate within a multiple access network that incorporates a shared medium, e.g. Ethernet. The hardware that implements the MAC is referred to as a medium access controller.

The MAC sub-layer acts as an interface between the logical link control (LLC) sublayer and the network's physical layer. The MAC layer emulates a full-duplex logical communication channel in a multi-point network. This channel may provide unicast, multicast or broadcast communication service.

The channel access control mechanisms provided by the MAC layer Is known as a multiple access protocol. This makes it possible for several stations connected to the same physical medium to share it. Examples of shared physical media are bus networks, ring networks, hub networks, wireless networks and half-duplex point-to-point links. The multiple access protocol may detect or avoid data packet collisions if a packet mode contention based channel access method is used, or reserve resources to establish a logical channel if a circuit switched or channelization based channel access method is used. The channel access control mechanism relies on a physical layer multiplex scheme.

a. Broadcast networksBroadcast networks have a single communication channel that is shared by all the machines on the network. A packet sent by one computer is received by all the other computers on the network. The packets that are sent contain the address of the receiving computer; each computer checks this field to see if it matches its own address. If it does not then it is usually ignored; if it does then it is read. Broadcast channels are sometimes known as multi-access channel.

b. Need of protocols in Broadcast channelIssues in multi-access channel :

• WHO is going to use the channel ?

• WHEN the channel is going to be used ?

• For HOW much time the channel is used ?

Due to shared channel and unregulated traffic over the network collisions and data loss occur. Some protocol must be followed for regulated and safe transmission over the network.

2.Types of Multiple –access protocol

Over the years, dozens of multiple access protocol have been implemented in a variety of link-layer technologies. Nevertheless ,we can classify just about any

shared wire or medium

multiple access protocol as belonging to one of three categories .Random access protocols, Controlled access protocols and channelization protocols.

a. Random Access Protocols

In a random access protocol, a transmitting node always transmits at the full rate of the channel, namely ,R bps .When there is a collision ,each node involved in the collision repeatedly retransmits its frame( that is ,packet) until the frame gets through without a collision. But when a node experiences a collision ,it doesn’t necessarily retransmitting the frame right away. Instead it waits a random delay before retransmitting the frame .Each node involved in a collision chooses independent random delays .Because the random delays are independently chosen , it is possible that one of the nodes will pick a delay that is sufficiently less than the delays of the other colliding nodes and will therefore be able to sneak its frame into the channel without a collision.

There are dozens of random access protocols in this we’ll describe a few of the most commonly used random access protocols – the ALOHA protocol , CSMA (carrier sense multiple access ) protocol , CSMA/CD (carrier sense multiple access /collision detection) protocol and CSMA/CA (carrier sense multiple access winth collision avoidance )protocol

I. ALOHA

The original version of ALOHA used two distinct frequencies in a hub/star configuration, with the hub machine broadcasting packets to everyone on the "outbound" channel, and the various client machines sending data packets to the hub on the "inbound" channel. If data was received correctly at the hub, a short acknowledgment packet was sent to the client; if an acknowledgment was not received by a client machine after a short wait time, it would automatically retransmit the data packet after waiting a randomly selected time interval. This acknowledgment mechanism was used to detect and correct for "collisions" created when two client machines both attempted to send a packet at the same time.

ALOHAnet's primary importance was its use of a shared medium for client transmissions. Unlike the ARPANET where each node could only talk directly to a node at the other end of a wire or satellite circuit, in ALOHAnet all client nodes communicated with the hub on the same frequency. This meant that some sort of mechanism was needed to control who could talk at what time. The ALOHAnet solution was to allow each client to send its data without controlling when it was sent, with an acknowledgment/retransmission scheme used to deal with collisions. This became known as a pure ALOHA or random-accessed channel, and was the basis for subsequent Ethernet development and later Wi-Fi networks. Various versions of the ALOHA protocol (such as Pure ALOHA, Slotted ALOHA) also appeared later in satellite communications, and were used in wireless data networks such as ARDIS, GSM.

PURE ALOHA

The first version of the was quite simple:

If you have data to send, send the data If the message collides with another transmission, try resending "later"

Note that the first step implies that Pure ALOHA does not check whether the channel is busy before transmitting. The critical aspect is the "later" concept: the quality of the backoff scheme chosen significantly influences the efficiency of the protocol, the ultimate channel capacity, and the predictability of its behavior.

To assess Pure ALOHA, we need to predict its throughput, the rate of (successful) transmission of frames. First, let's make a few simplifying assumptions:

All frames have the same length. Stations cannot generate a frame while transmitting or trying to transmit. The population of stations attempts to transmit (both new frames and old

frames that collided) according to a Poisson distribution.

Let "T" refer to the time needed to transmit one frame on the channel, and let's define "frame-time" as a unit of time equal to T. Let "G" refer to the mean used in the Poisson distribution over transmission-attempt amounts: that is, on average, there are G transmission-attempts per frame-time.

Overlapping frames in the pure ALOHA protocol. Frame-time is equal to 1 for all frames.

Pure Aloha efficiencyP(success by given node) = P(node transmits) .

P(no other node transmits in [t0-1,t0] .

P(no other node transmits in [t0,t0+1]

= p . (1-p)N-1 . (1-p)N-1

= p . (1-p)2(N-1)

… choosing optimum p and then letting n -> ¥ ...

Efficiency = 1/(2e) = .18

Slotted ALOHA

An improvement to the original ALOHA protocol was "Slotted ALOHA", which introduced discrete timeslots and increased the maximum throughput. A station can send only at the beginning of a timeslot, and thus collisions are reduced. In this case, we only need to worry about the transmission-attempts within 1 frame-time and not 2 consecutive frame-times, since collisions can only occur during each timeslot. Thus, the probability of there being zero transmission-attempts in a single timeslot is:

Slotted ALOHA is used in low-data-rate tactical satellite communications networks

by military forces, in subscriber-based satellite communications networks, mobile telephony call setup, and in the contactless RFID technologies.

Pros

• single active node can continuously transmit at full rate of channel

• highly decentralized: only slots in nodes need to be in sync

• simple

Cons

• collisions, wasting slots

• idle slots

• nodes may be able to detect collision in less than time to transmit packet

• clock synchronization

Efficiency is the long-run fraction of successful slots when there are many nodes, each with many frames to send

• Suppose N nodes with many frames to send, each transmits in slot with probability p

• prob that node 1 has success in a slot = p(1-p)N-1

• prob that any node has a success = Np(1-p)N-1

• For max efficiency with N nodes, find p* that maximizes Np(1-p)N-1

• For many nodes, take limit of Np*(1-p*)N-1 as N goes to infinity, gives 1/e = .37

• Efficiency is 37%, even with optimal p

II. Carrier Sense Multiple Access

Carrier Sense Multiple Access (CSMA) is a probabilistic Media Access Control (MAC) protocol in which a node verifies the absence of other traffic before transmitting on a shared transmission medium, such as an electrical bus, or a band of the electromagnetic spectrum.

"Carrier Sense" describes the fact that a transmitter uses feedback from a receiver that detects a carrier wave before trying to send. That is, it tries to detect the presence of an encoded signal from another station before attempting to transmit. If a carrier is sensed, the station waits for the transmission in progress to finish before initiating its own transmission.

"Multiple Access" describes the fact that multiple stations send and receive on the medium. Transmissions by one node are generally received by all other stations using the medium.

Advantages

Fairly simple to implement

Functional scheme that works

Disadvantages

Can not recover from a collision

(inefficient waste of medium time)

III. CSMA/CD (Collision Detection)

Carrier sense multiple access with collision detection (CSMA/CD) is a Media Access Control method in which.

a carrier sensing scheme is used. a transmitting data station that detects another signal while transmitting a

frame, stops transmitting that frame, transmits a jam signal, and then waits for a random time interval before trying to resend the frame.

CSMA/CD is a modification of pure carrier sense multiple access (CSMA). CSMA/CD is used to improve CSMA performance by terminating transmission as soon as a collision is detected, thus shortening the time required before a retry can be attempted.

Algorithm

When a station wants to send some information, it uses the following algorithm.

Main procedure

1. Frame ready for transmission.2. Is medium idle? If not, wait until it becomes ready

3. Start transmitting.4. Did a collision occur? If so, go to collision detected procedure.5. Reset retransmission counters and end frame transmission.

Collision detected procedure

1. Continue transmission until minimum packet time is reached to ensure that all receivers detect the collision.

2. Increment retransmission counter.3. Was the maximum number of transmission attempts reached? If so, abort

transmission.4. Calculate and wait random backoff period based on number of collisions.5. Re-enter main procedure at stage 1.

Methods for collision detection are media dependent, but on an electrical bus such as 10BASE-5 or 10BASE-2, collisions can be detected by comparing transmitted data with received data or by recognizing a higher than normal signal amplitude on the bus.

Jam signal

The jam signal is a signal that carries a 32-bit binary pattern sent by a data station to inform the other stations that they must not transmit.

Advantages

More efficient than basic CSMA

Disadvantages

Requires ability to detect collisions

IV. Carrier Sense Multiple Access with Collision Avoidance (CSMA-CA)

Carrier sense multiple access with collision avoidance (CSMA/CA), in computer networking, is a wireless network multiple access method in which:

a carrier sensing scheme is used. a node wishing to transmit data has to first listen to the channel for a

predetermined amount of time to determine whether or not another node is transmitting on the channel within the wireless range. If the channel is sensed "idle," then the node is permitted to begin the transmission process. If the channel is sensed as "busy," the node defers its transmission for a random period of time. Once the transmission process begins, it is still possible for the actual transmission of application data to not occur

• Timing in CSMA/CA

• Interframe Space (IFS)– When an idle channel is found, the station does not send

immediately but waits for a period of time call IFS– The IFS time allows the front of the transmitted signal by the distant

station to reach this station– The IFS variable can also be used to prioritize stations or frame types

• e.g., DIFS(Distributed IFS) , SIFS((Short)

CSMA/CA is a modification of carrier sense multiple access.

Collision avoidance is used to improve CSMA performance by not allowing wireless transmission of a node if another node is transmitting, thus reducing the probability of collision due to the use of a random truncated binary exponential backoff time.

b. Controlled AccessControlled access protocols grant permission to send only to one node at a time, avoiding collisions of messages on the shared medium. Predetermined channel allocation schemes assign access rights to the nodes statically. Bandwidth requirements need to be known in advance and the system cannot adapt to changing needs at run time. Demand adaptive protocols take the current demands of the nodes into account and grant network access accordingly. This class can be subdivided into reservation,Polling and token -based schemes. If network traffic is bursty, controlled access protocols are potentially inefficient, because in that case they frequently grant transmission rights to nodes that do not want to send. Even if such nodes immediately give up their rights, these schemes may cause considerable overhead.

I. Reservation

• Features– Time is divided into intervals

• In each interval, a reservation frame precedes the data frames sent in that interval

– N stations è N reservation minislots in the reservation frame• When a station needs to send a data frame è it makes a

reservation in its own minislot

II. Polling • Polling uses a centralized controller

– which cycles through stations on the network and gives each an opportunity to transmit a packet

• During selection a controller can choose which station to poll at a given time

• Select and Poll

• There are two general polling policies:

– Round robin order

• Round-robin means each station has an equal opportunity to transmit packets

– Priority order

• Priority order means some stations will have more opportunity to send

For example, priority order might be used to assign an IP telephone higher priority than a personal computer

III. Token Passing

Token passing is a channel access method where a signal called a token is passed between nodes that authorizes the node to communicate. The most well-known examples are token ring and ARCNET.

Token passing schemes provide round-robin scheduling, and if the packets are equally sized, the scheduling is max-min fair. The advantage over contention based channel access is that collisions are eliminated, and that the channel bandwidth can be fully utilized without idle time when demand is heavy. The disadvantage is that even when demand is light, a station wishing to transmit must wait for the token, increasing latency.

• Features– A station is authorized to send data when it receives a special frame

called a token– Stations are arranged around a ring (physically or logically)

• A token circulates around the ring– If a station needs to send data, it waits for the token– The station captures the token and sends one or more frames as long

as the allocated time has not expired– It releases the token to be used by the successor station

c. Channelization

• Definition– A Multiple-access method in which the available BW of a link is

shared in time, frequency, or through code, between different stations• Features

– FDMA, TDMA are based on FDM and TDM in the physical layer– CDMA is a data link multiple-access protocol

I. FDMA

Frequency Division Multiple Access or FDMA is a channel access method used in multiple-access protocols as a channelization protocol. FDMA gives users an individual allocation of one or several frequency bands, or channels. It is particularly commonplace in satellite communication

• Approach and features– A data link layer protocol that uses FDM at the physical layer– The available BW is shared by all stations– Each stations uses its allocated band to send its data– Ex. AMPS (1G cellular networks)

II. TDMA

Time division multiple access (TDMA) is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This allows multiple stations to share the same transmission medium while using only a part of its channel capacity. TDMA is used in the digital 2G cellular systems such as Global System for

Mobile Communications (GSM) , Personal Digital Cellular (PDC), and in the Digital Enhanced Cordless Telecommunications (DECT) standard for portable phones. It is also used extensively in satellite systems, combat-net radio systems, and PON networks for upstream traffic from premises to the operator. For usage of Dynamic TDMA packet mode communication, see below.

• Approach and features– A data link layer protocol that uses TDM at the physical layer– The stations share the capacity of the channel in time– Each station is allocated a time slot during which it can send data– Ex. GSM (2G cellular networks - Europe)

III. Code Division Multiple Access (CDMA)

• used in several wireless broadcast channels (cellular, satellite, etc) standards• unique “code” assigned to each user; i.e., code set partitioning• all users share same frequency, but each user has own “chipping” sequence

(i.e., code) to encode data• encoded signal = (original data) X (chipping sequence)

• decoding: inner-product of encoded signal and chipping sequence• allows multiple users to “coexist” and transmit simultaneously with minimal

interference (if codes are “orthogonal”)

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