Chapter 4

Managing Multiple Access

Introduction

• Look at:– Design Issues (4.1)– Implementation Issues (4.2)– Centralized Access (4.3)– Distributed Access (4.4)– Hardware Addressing (1.5)

Introduction

• Multiple access allows more than one device to communicate

• Network data congestion is a consistent issue

• Several types of multiple access links are available, such as point-to-point, broadcast, and switched

Design Issues

• Based on network architecture, networks can be logically divided into two classes: – In a distributed design, all nodes have

approximately the same kinds and amounts of information

– In a centralized design, there is a central node or a manager, which manages all communication decisions

Design Issues

• Circuit mode is used to send steady streams of data at a continuous bit rate

• Circuit mode provides a physical, dedicated path called a time slot for the data

• Because this time slot is dedicated, no other node can use its path

Design Issues

• Packet-mode design provides increased network efficiency by providing system resources to system users only when the user has data to send– With packet mode, the nodes contend for

every packet to avoid wasting bandwidth – Packet mode is shared, the network media

serves many more users with the same system resources

Implementation Issues

• Implementation issues that need to be addressed where multiple access is concerned: – The first is spectrum scarcity – The next issue is the properties that radio

links possess– In wireless systems it is very difficult to

receive data when the transmitter is sending data

Implementation Issues

• Implementation issues that need to be addressed where multiple access is concerned:– When a node is transmitting data, a large

fraction of the signal energy leaks into the receive path

– There also has to be a method devised for communication that can help control errors, fading, and interference

Implementation Issues

• The four factors that affect performance are: – Normalized throughput in which a fraction

of the link capacity is used to carry non-retransmitted packets

– Mean delay which is the amount of time a node has to wait before it successfully transmits a packet

Implementation Issues

• The four factors that affect performance are: – Stability to counter when a load becomes

heavy and the majority of time is spent on resolving contentions

– Fairness so that each contending node receives an equal share of the bandwidth and an equal chance to send data

Implementation Issues

• Base technologies isolate data from different sources

• A base technology uses a line driver to introduce voltage shifts in digital signals onto a channel

• The channel acts a a transport mechanism for the digital voltage pulses as they travel through the channel

• In general, only one communication channel is available at any given time

Implementation Issues

• The wireless industry began to explore converting the existing analog network to digital as a means of improving capacity

• Currently there are three choices : – FDMA– TDMA– CDMA

Implementation Issues

• FDMA works by dividing the frequency spectrum allocated for wireless cellular telephone communication into channels of equal bandwidth

• TDMA is analog’s FDMA with a built-in, time-sharing component

Implementation Issues

• CDMA combines spread spectrum technology with analog-to-digital conversion– Audio input is first digitized into binary

elements– The data is then scattered across the

frequency band in a random pattern

Centralized Access

• A centralized design, there is a central node or a manager that keeps important information and makes all decisions

• Centralized access is simple, and the master controller provides a single point of management and synchronization

• The master is the single point of failure and it needs a re-election protocol

Centralized Access

• The master controller is involved in every single data transfer, causing added delay and slowing the network

• Several different methods of centralized control include:– circuit-mode– polling or packet mode – reservation based

Centralized Access

• The way circuit-mode data transfer works is as follows:– When a station wants to transmit data, it

sends a message to the master– The master allocates transmission

resources to the slave– The slaves use the allocated resources

until it completes transmission

Centralized Access

• Radio link protocols for circuit-mode data transmissions on cellular systems are based on the premise that the error-prone link must be reliable

• The first protocol, Protocol S, uses a flexible segmentation and recovery sub layer to package data frames into multiplexed physical layer bursts

Centralized Access

• The second one, Protocol T, consists of two levels of recovery

• Because the partial recovery provided by the first level usually recovers the data, the second recovery level is seldom used

• This type of centralized access is used primarily in cellular home systems

Centralized Access

• In polling- or packet-mode access, a user's data stream is broken down into smaller segments, called packets

• Each packet then has network control information added before it is transmitted through the network

• Once the packet transmission is complete, that resource becomes available for use by other nodes to transmit packets

Centralized Access

• Polling is the process in which the master broadcasts a query to every node in the network asking each node in turn whether it has anything to communicate

• Polling can be thought of as a combination of broadcasting and gathering information from the nodes

• To ensure that only one message is transmitted at any time, the master either polls or selects each terminal connected to the line in a specific sequence

Centralized Access

• The poll control message is used to request a specific terminal to send any waiting data message it may have

• Usually, the master polls all the nodes in round-robin fashion, but in some circumstances some nodes may get more than one poll per cycle

• On half-duplex lines each poll requires two line turnarounds, one for the master to send, and one for the node to send

Centralized Access

• Polling is considered an inefficient method especially if only a few stations are active

• Because it is centrally controlled, the network will slow down if the system has many terminals

• The overhead for polling messages is high because all nodes have to be polled and will cause network performance to suffer when the controllers on the link become overloaded

Centralized Access

• If it takes a long time for the data to reach the farthest station, the master coordinates access to links using reservations

• The master assigns slots that are devoted to sending just these reservation messages

• The nodes either contend for a slot or they own one

• Packet collisions are limited only to slots, so the overhead on contention is reduced

Distributed Access

• The premise behind distributed access is similar to that of a distributed design

• There is no central node or manager; all nodes have equal access

• A distributed scheme is more reliable, has lower message or propagation delays, and often allow higher network bandwidth utilization

Distributed Access

• Almost all distributed access methods are packet mode

• In distributed access, a station starts transmission after it satisfies a set of network requirements by using a random access Media Access Control (MAC) low-level protocol

• This is the preferred method of access for LANs and many WANs also use it

Distributed Access

• There are several methods of distributed access:– Decentralized polling– Carrier sense multiple access– Busy tone multiple access– Multiple access collision avoidance – Token passing– ALOHA

Distributed Access

• The principle behind decentralized polling is similar to centralized polling except there is no master to control the transmitting and collection of packets

• All stations must share a time base• The access time is divided into slots

and then each station is assigned a slot which it uses for transmissions

Distributed Access

• Ethernet networks use carrier sense multiple access (CSMA) to improve performance and reduce traffic

• When a node has data to transmit, it first listens on the cable by using a transceiver to check and determine if a signal is being transmitted by another node

• If the channel is sensed as idle, it transmits. If the channel is sensed as busy, it holds off the transmission until it can sense that the medium is idle

Distributed Access

• There are two types of CSMA:– Persistent: The transmission is immediately

retried based on a probability (p) of when the transmission medium will become available

– Non-Persistent: The transmission is retried after random intervals

Distributed Access

• Due to propagation delay, two nodes may not hear each other's transmission

• The role that distance and propagation delay play in collisions is important because these factors determine the probability of a collision

• When a collision occurs, the entire packet transmission time is wasted

• Obviously if a collision happens, we want to detect and resolve it

Distributed Access

• In carrier sense multiple access with collision detection (CSMA/CD), if a node observes a collision, it stops transmission immediately

• Instead, it sends a random 32-bit pattern for a short period of time, referred to as jam sequence or a jam signal

• This causes the other nodes to discard the frame and assures that all stations are aware of a collision so that they can increment the timeout range

Distributed Access

• When two or more transmitting nodes each detect a collision, each responds in the same way by transmitting the jam sequence

• CSMA/CD uses exponential backoff • When a collision occurs, a timeout is

randomly chosen from a doubled range• The backoff range then adapts to number of

contending stations

Distributed Access

• In a wireless environment, it is very important that the number of collisions be limited to the absolute minimum

• This can be achieved by a protocol called carrier sense multiple access with collision avoidance (CSMA/CA)

• CSMA/CA is actually a variation of CSMA/CD

• The idea behind CSMA/CA is to prevent collisions at the moment they are most likely to occur

Distributed Access

• In collision avoidance, when a node wants to send a data packet, it sends a message called a Request to Send (RTS) to the destination device

• If the node receives a Clear to Send (CTS) message back from the device, it sends its data

• When the data packet has been received, the receiving device sends an acknowledgement (ACK) packet

Distributed Access

• Packet-radios or stations use the MAC protocol for sharing a common broadcast channel

• Busy tone multiple access (BTMA) is designed for station-based networks and divides the channel into two categories:– Message channel– Busy-tone channel

Distributed Access

• Multiple access collision avoidance (MACA) attempts to detect collisions at the receiver by establishing a request-response channel between the sender and receiver

• Instead of using two channels like BMTA, it uses a single frequency band, and uses explicit messages to tell others that the receiver is busy

• This protocol was designed to solve the hidden and exposed terminal issues, but it doesn't completely eliminate them

Distributed Access

• Other distributed access protocols:– MACAW uses a message exchange that

consists of five steps and includes a backoff algorithm

– The floor acquisition multiple access (FAMA) protocol is used when a station acquires control of a channel before sending a packet, yet it ensures no collisions happen at the receiver

Distributed Access

• Other distributed access protocols:– In the DBTMA protocol, two narrow-

bandwidth tones are implemented in a single channel

– DBTMA resolves the hidden and exposed terminal issues, but extra hardware is required

Distributed Access

• Token passing is an alternative to CSMA/CD Ethernet networks

• Possession of the token allows exclusive access to the network for transmission

• When the transmission is finished, the node passes the token to next station

• Every computer in the network is responsible for either passing the token or creating a new one

Distributed Access

• Token-passing networks, also called token Ring networks, are either single or double rings

• With a single ring, the failure of a single link or station breaks the network

• With a double ring, if there is a failure on one ring, the second ring can be used to avoid a single point of failure

Distributed Access

• ALOHA is one of the earliest multiple access schemes

• It is a simple communications scheme where each transmitter in a network sends data whenever there is a frame to send

• If the frame successfully reaches the receiver, the next frame is sent

• If the frame is not received, it is sent again

Distributed Access

• There are several variants of ALOHA:– Pure ALOHA can work well when the

medium has a low bandwidth utilization– Slotted ALOHA is commonly used in

cellular phone uplinks– Reservation ALOHA is a combination of a

slot reservation design with slotted ALOHA

Hardware Addressing

• Most network technologies have a unique physical address scheme that identifies computers on the network

• This unique physical address is called a hardware address or Media Access Control (MAC) address

• This address is the MAC hexadecimal address of the system's network interface card (NIC) and is 48 bits long

Hardware Addressing

• A MAC address looks like this: 00-10-A4-A8-DA-D0

• IEEE designates an addressing scheme and assigns unique blocks of addresses to NIC manufacturer

• The NIC transmits outgoing data in frames and accepts incoming data

• The NIC is a link between a computer and a network