VISVESVARAYA TECHNOLOGICAL UNIVERSITYSanthibastawad Road, Belgaum-590 014

MAC for Optical Networks

SEMINAR REPORT Submitted by SHRUTHI.NK

(USN: 1VE10EC124 )

In partial fulfillment for the award of the degree of

BACHELOR OF ENGINEERING in

ELECTRONICS AND COMMUNICATION ENGINEERING

Under the Guidance of

Dr. Thippeswamy MNProf and HoD,Dept of ECE,

SVCE, Bangalore

SRI VENKATESHWARA COLLEGE OF ENGINEERINGDepartment of Electronics and Communication Engineering

Vidyanagar, Bettahalusur Post, Kempegowda International Airport Road Bangalore - 562 157

2013--2014

SRI VENKATESHWARA COLLEGE OF ENGINEERINGDepartment of Electronics and Communication Engineering

Vidyanagar, Bettahalusur Post, Kempegowda International Airport Road Bangalore - 562 157

CERTIFICATEThis is to certify that the Seminar titled

NAME OF THE SEMINARwas prepared and presented by

SHRUTHI.N.K (USN: 1VE10EC124)

of the Eight Semester Electronics and Communication Engineering in partial fulfillment of requirement for the award of

Degree of Bachelor of engineering in Electronics and Communication Engineering under the

VISVESVARAYA TECHNOLOGICAL UNIVERSITYDuring the year 2013-14.

SEMINAR GUIDE HEAD OF THE DEPARTMENT

(Dr. Thippeswamy M.N) (Dr.Thippeswamy.M.N) Prof and HoD, Dean of Academics Prof and HoD, Dean of Academics

Place: BANGALORE Date:

ACKNOWLEDGMENT

I take this opportunity to express my profound gratitude and deep regards to my guide Dr. ThippeswamyM.N Prof and HOD Dept of ECE for his exemplary guidance, monitoring and constant encouragement throughout the course of this seminar. The blessing, help and guidance given by him time to time shall carry me a long way in the journey of life on which I am about to embark.

 I also take this opportunity to express a deep sense of gratitude to Dr. C. Prabhakar Reddy, Head of the In-stitution for his cordial support.

Lastly, I thank almighty, my family and friends for their constant encouragement without which this assign-ment would not be possible.

ABSTRACT

Optical fiber has huge transmission capacity. On the other hand optical technology is still in its infancy, and conversions between electrical and optical environment are relatively slow compared to the transmission ca-pacity. Thus, in optical networks the processing power, instead of bandwidth, is the limiting factor. Therefore, the requirements for the MAC protocol are different in the optical network than in the traditional electronic network. In this report these basic requirements are discussed and the MAC protocols proposed for optical packet/burst switching networks are introduced. Additionally, MAC protocols for different types of topology are discussed and compared in more detail. Additionally particular attention is devoted to some of the case studies of MAC protocols for Wavelength Division Multiplexing (WDM) optical networks

1) INTRODUCTION

All optical networks are very often considered to be the main candidate for constituting the backbone that will

carry global data traffic whose volume has been growing at astounding rates that are not expected to slow

down in the near future.In optical networks the theoretical capacity is huge. Potential bandwidth is more than

50 terabits per second. The problem is that while the signals are converted into electronic form in the nodes, a

part of this capacity is lost. The capacity of electronic devices is a few gigabits and thus the end user can trans-

mit at this rate. Dividing the bandwidth to multiple users is thus needed in order to use the resources effi-

ciently.

According to the physical technology employed, one can identify three generations of networks

1) Networks built before the emergences of optical fiber technology are the first generation networks. The first

generation optical network is identical to the electrical communication network except that the transmission

medium is fiber instead of copper. Functions such as switching, routing and processing are still done electroni-

cally. There is an improvement in speed and bit error rates. Since central functions are done electronically, the

system uses a large number of electrical to optical and optical to electrical conversions. All such networks use

a single wavelength transmission.

In terms of the number of users, it is conventional to broadly define three types of network. They are

a) Local Area Network or LAN, which is used to connect nodes within a small geographic area

b) Metropolitan Area Network or MAN covering users within a city or a metropolis. Typical distance could

be about a hundred kilometers with applications such as a telephone exchange

c)Wide Area Network or WAN, which can cover a much wider geographic area.

2) The second generation networks employ fiber in traditional architectures. The choice of fiber is due to its

large bandwidth, low error rate, reliability, availability, and maintainability. Although some performance im-

provements can be achieved by employing fiber, the performance for this generation is limited by the max-

imum speed of electronics (a few gigabits per second) employed in switches and end-nodes. This phenomenon

is called an electronics bottleneck. In order to satisfy the increasing bandwidth requirements of emerging ap-

plications, totally new approaches are employed to exploit vast bandwidth (approximately 30THz in the low

loss region of single mode fiber in the neighbourhood of 1500nm) available in fiber.

3) Therefore, the third generation networks are designed as all-optical to avoid the electronics bottleneck. That

is, information is conveyed in the optical domain (without facing any electro optical conversions) through the

network until it reaches its final destination. The emergence of single mode fiber, all-optical wide-band ampli-

fiers, optical couplers, tuneable lasers (transmitters)/filters (receivers), an all -optical cross-connects enable the

realization of third generation networks.

Figure 1: The Three Generations of Networks

1.1 FIRST GENERATION NETWORKS

Fiber Distributed Data Interface (FDDI)

Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH)

1.1.1 FIBER DISTRIBUTED DATA INTERFACES

FDDI is an ANSI standard, which defines several layers providing for control of the ring. Geometrically, it

consists of two counter rotating rings operating at a data rate of 100 Mb/s. A maximum of 500 nodes may be

connected by multimode or single mode fiber which could be 100 km in length. The maximum distance be-

tween two stations may be 2 km for LED sources and multimode fiber. Using laser sources and single mode

fiber, this distance may be increased to 40 km. To avoid long streams of zeros and ones, FDDI encodes each

four bit as a five bit symbol, i.e. it encodes information as bit streams from 00000 to 11111. Out of these 32

symbols, 16 stand for data in hexadecimal format from 0 to F, while the remaining 16 act as control and code

violation symbol. FDDI provides an example of layered architecture. A network is a complicated collection of

components which performs different functions. It is simple to logically think of a network as consisting of

several layers which perform a class of functions and provide a set of service to the layer higher in hierarchy.

FDDI defines several layers which provide for control of the ring. the various layers are as shown in the figure.

The functions of various layers are as follows: 

Station Management (SMT) layer determines the logical connection between nodes and montitor, man-

age and configure the ring. 

Media Access Control (MAC) layer controls passing of token and performs operations like packet

framing and interpretation. 

Physical Media Dependent (PMD) layer specifies physical elements of the network such as sources,

detectors, cables and performs electro-optic conversions. 

Physical (PHY) layer performs functions like coding and decoding of information received from PMD

layer.

1.1.2 SONET/SDH

Synchronous Optical network or SONET is a high speed and high bandwidth standard for WAN. It has a base

rate of 155 Mb/s with an expansion capability to gigabit speeds. Synchronous Digital Hierarchy or SDH is

simply the international version of the same. SONET standard specifies the following optical parameters: Op-

erating wavelength is in the range of 1.28 mm to 1.34 mm with a maximum spectral width of 0.009 mm. The

ratio of the optical power level for "1" bit to that for "0" bit is 10:1. This is known as the extinction ra-

tio. NRZ coding of data. In SONET/SDH, there is a master clock synchronizing clocks from different sources.

There is, therefore, no need to add extra bit and the speeds are now multiples of the base rates.

In SDH, there is no speed corresponding to OC-1. The base rate for its lowest level STM-1 is 155.52 Mb/s and

the higher STM levels are multiples of this bandwidth. It is also possible to transport smaller payloads which

require less bandwidth. This is done by breaking up the base rate channel to smaller components called virtual

tributaries or VT. The corresponding quantity in SDH is called it virtual container. In SONET, there are four

such virtual tributaries called VT-1.5, VT-2, VT-3 and VT-6 with operating bandwidths of 1.724, 2.304, 3.456

and 6.912 Mb/s respectively. The virtual containers of SDH are termed TU-11, TU-12, TU-2 and TU-3 with

bandwidths of 1.728, 2.304, 6.912 and 49.152 Mb/s respectively. Like FDDI, SONET consists of two counter

rotating optical rings, the second ring providing a backup in case of a failure.

FIGURE 2:SONET/SDH Network

1.2 THE SECOND GENERATION NETWORKS

The second generation networks employ fiber in traditional architectures. The choice of fiber is due to its large

bandwidth, low error rate, reliability, availability, and maintainability. Although some performance improve-

ments can be achieved by employing fiber, the performance for this generation is limited by the maximum

speed of electronics (a few gigabits per second) employed in switches and end-nodes. This phenomenon is

called an electronics bottleneck. In order to satisfy the increasing bandwidth requirements of emerging applic-

ations, totally new approaches are employed to exploit vast bandwidth (approximately 30THz in the low loss

region of single mode fiber in the neighbourhood of 1500nm) available in fiber.

3) Therefore, the third generation networks are designed as all-optical to avoid the electronics bottleneck. That

is, information is conveyed in the optical domain (without facing any electro optical conversions) through the

network until it reaches its final destination. The emergence of single mode fiber, all-optical wide-band ampli-

fiers, optical couplers, tuneable lasers (transmitters)/filters (receivers), an all -optical cross-connects enable the

realization of third generation networks.

In order to make use of the vast bandwidth available without experiencing electronics bottleneck, concurrency

among multiple user transmissions can be introduced. In all-optical networks, concurrency can be supplied

through time slots (OTDM-optical time division multiplexing), wave shape (CDM-code division multiplexing)

or wavelength (WDM-wavelength division multiplexing)

In optical time division multiplexing (OTDM), many low-speed channels, each transmitted in the form of ultra-

short pulses, are time interleaved to form a single high-speed channel. By this method, the information carry-

ing capacity of the network can be improved to 100 Gigabits/sec or higher without experiencing electronics

bottleneck. In order to avoid interference between channels, transmitters should be capable of generating ultra-

short pulses, which are perfectly synchronized to the desired channel (time slot), and receivers should have a

perfect synchronization to desired channel (time slot).

In code division multiplexing (CDM) each channel is assigned a unique code sequence (very short pulse se-

quence), which is used to encode low-speed data. The channels are combined and transmitted in a single fiber

without interfering with each other. This is possible since the code sequence of each channel is chosen such

that its cross-correlation between the other channels' code sequences is small, and the spectrum of the code se-

quence is much larger than the signal bandwidth. Therefore, it is possible to have an aggregate network capa-

city beyond the speed limits of electronics. Like OTDM, CDM requires short pulse technology, synchroniza-

tion to one chip time for detection

In WDM, the optical spectrum (low loss region of fibers) is carved up into a number of smaller capacity chan-

nels. Users can transmit and receive from these channels at peak electronic rates, and the different channels

can be used simultaneously by many users. In this way, the aggregate network capacity can reach the number

of channels times the rate of each channel. In order to develop an effective WDM network, each user may be

able to transmit and receive from multiple channels. That is why, the tunable transmitter (laser)/tunable receiv-

ers (filter) and/or multitude of fixed transmitters/receivers are employed at end-nodes.WDM is the favorite choice over OTDM, and CDM. This is due to the complex hardware requirements, and

synchronization requirements of OTDM and CDM (synchronization within one time slot time and one chip

time respectively). OTDM and CDM are viewed as a long-term network solution, since they rely on different

and immature technology whereas it is possible to realize WDM systems using components that are already (or

very nearly) available commercially. Moreover, WDM has an inherent property of transparency. Since there is

no electronic processing involved in the network, channels act like independent fiber(transparent pipes)

between the end nodes provided that channel bandwidths are not exceeded. Once a connection is established

between the end-nodes on a WDM channel, the communicating parties have the freedom to choose the bit rate,

signalling and framing conventions, etc. (even analog communication is possible). This transparency property

makes it possible to support various data formats and services simultaneously on the same network. In addition

to this great flexibility, transparency protects the investments against future developments. Once deployed,

WDM networks will support a variety of future protocols and bit rates without making any changes to the net-

work.

FIGURE 3: WDM Network

The commonly used architectural forms for WDM networks are

WDM Link

Passive Optical Network (PON)

Broadcast and Select Networks

Wavelength Routing Networks.

1.2.1 WDM LINK

To increase information carrying capacity, second generation networks employ parallel fibers for individual

channels. In the WDM Link approach parallel fibers are replaced by wavelength channels on a single fiber . In

long haul WDM links, all channels are amplified together by a single wideband optical amplifier (no separate

amplifier for each channel), and existing fibers are utilized efficiently by integrating more than one channel in

a single fiber. Therefore, WDM link offers a very cost-effective system. The other factors that make WDM

links very popular are the maturity of technology and simplicity of integration with legacy equipment.

FIGURE 4: WDM Link

1.2.2 PONs

The main feature of PON is to share fiber between the Central Office and Optical Network Units (ONU).The

PON establishes a tree structure that enables bi-directional communication between a server (central office)

and multiple customers (ONUs) with centralized control and routing at central office. This architecture is a

good network choice for regional communication providers. The main technological problem for the PONs is

to design cheap, simple, and durable equipment for the ONUs.

FIGURE 5: PONs

1.2.3: BROADCAST AND SELECT NETWORKS

Broadcast and Select Networks are based on the simplest all-optical organization that enables WDM. In this

organization, the network infrastructure is totally made up of glass material, which acts as a propagation me-

dium that broadcasts the individual transmissions to the whole network. The broadcast and select network

model is illustrated in. In this model, the transmitters on the end-nodes transmit the signals on distinct

wavelengths to the network, and the network combines these signals and distributes the aggregate signal to the

receivers. In this figure, instead of the end-nodes, only the transmitters and receivers are shown, since each

end-node in a broadcast and select network has one or more transmitters or receivers. For proper network oper-

ation, each end-node should be able to make connections with any of the other end-nodes. For this reason,

each end-node should have access to multiple WDM channels. This can be accomplished by making the trans-

mitters and/or receivers tunable over the multiple channels (wavelengths), or having a multitude of fixed tuned

transmitters and/or receivers each assigned to a different channel on an end-node. In order to avoid inter-chan-

nel interference, the transmitters should have narrow line width, and receivers can be able to filter each chan-

nel individually (i.e., narrow bandwidth filters are required). In addition, to improve efficiency, tunable com-

ponents should cover all of the channels.

FIGURE 6 :BROADCAST AND SELECT NETWORK

These networks can transmit as

Circuit switching

Packet switching

a) Optical Circuit Switching

OCS is an optical networking technology. In OCS, the network is configured to establish a circuit, from an en-

try to an exit node, by adjusting the optical cross connects circuits in the core routers in a manner that the data

signal, in an optical form, can travel in an all-optical manner from the entry to the exit node. This approach

suffers from all the disadvantages known to circuit switching - the circuits require time to set up and to de-

stroy, and while the circuit is established, the resources will not be efficiently used to the unpredictable nature

of network traffic.

b) Packet switching

Packet switching in a broadcast and select network requires a significant amount of dynamic coordination

between the end-nodes. In order to enable packet switching, one of the transmitters of the source and one of

the receivers of the destination end-nodes should be tuned to the same wavelength channel during packet trans-

mission. In addition, wavelength agile transmitters and/or receivers should be used, or extra mechanisms are

required for the effective use of networks resources.

Broadcast and select can be divided further into

Single hop

Multi hop

Therefore, the most important part of the design of packet switched broadcast and select network is to develop

a good protocol to coordinate packet transmissions. The main task of these protocols is to coordinate packet

transmissions to inform destinations to tune to the proper wavelength channel and avoid or handle collisions.

Hence in order to access the media MAC (Media/Medium Access Control) protocols were developed for the

optical networks in order to obtain a reliable transmission and communication

The following section gives a detailed description of MAC PROTOCOLS FOR OPTICAL NETWORK based

on technique or topology used and some case studies.

CHAPTER 2: MEDIA ACCESS CONTROL (MAC) LAYER

In the seven-layer OSI model of computer networking, media access control (MAC) data communication

protocol is a sub layer of the data link layer, which itself is layer 2. The MAC sub layer provides addressing

and channel access control mechanisms that make it possible for several terminals or network nodes to com-

municate 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) sub layer 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, mul-

ticast or broadcast communication service.

According to IEEE Functions performed in the MAC sublayer are :

Frame delimiting and recognition

Addressing of destination stations (both as individual stations and as groups of stations)

Conveyance of source-station addressing information

Transparent data transfer of LLC PDUs, or of equivalent information in the Ethernet sublayer

Protection against errors, generally by means of generating and checking frame check sequences

Control of access to the physical transmission medium

FIGURE 7: MAC LAYER IN OSI MODEL

2.1: NETWORK TOPOLOGIES

There are several different possible network topologies, as illustrated in Figure. Bus, star and ring topologies

are quite simple to implement, while the mesh topology is in general more complex but on the other hand also

more efficient. Because of the infancy of the optical technology, mesh topology is not used. The bus topology,

on the other hand is inefficient compared to two other remaining alternatives. Most of the MAC protocols

proposed for optical networks are meant either for star topology or for ring topology. The main different

between these two is that the star topology is centralized, while in the ring topology the network control is

distributed.

2.2 : BASIC DEVICES

Properties of optical components are out of the scope of this study. However, to understand the descriptions of

the MAC protocols, it is important to be familiar with a few basic devices used in optical switches. Optical

transmitters and receivers can be either fixed or tunable. Fixed transmitters can transmit only with one specific

wavelength, while tunable transmitters can be tuned to different wavelengths. Similarly, fixed receivers can

receive with one specific channel, while tunable receivers can receive packets from several different channels

consecutively.

Tunable devices are more flexible and in most case also more efficient. On the other hand, if the receiver has

to receive packets from different channels consecutively, it has to be tuned from one wavelength to another

really fast. Thus, tunable devices are complex and costly to implement.

Responsible for error and flow

Responsible framing and MAC address and Multiple Access Control

It is not easy to decide, whether to use tunable or fixed devices. For instance, in some optical ring networks

there are W+ 1 channel. One channel is allocated for control signalling. It is clear that one fixed transmitter and

one fixed receiver should be used at each node for this purpose. In addition to this, however, every node needs

one of the following combinations.

Fixed transmitter fixed receivers (FTFR)

Transmitter fixed receivers (TTFR)

Fixed transmitter tunable receiver (FTTR)

Tunable transmitter tunable receiver (TTTR)

FIGURE 8: NETWORK TOPOLGIES

If receivers are fixed, every receiver has one channel allocated for it. All packets that are sent with that

wavelength and only these packets are directed to the receiver. In this case, if the channel is free, when a

transmitting node tries to send, the transmission is successful because there are no destination conflicts. If

transmitters are fixed, the payload channels are allocated for the source nodes instead of destinations. In this

case, every node can transmit without worrying about transmission conflicts, because no other node can

transmit with the same wavelength. However, it is possible that several packets arrive at the same node at the

same time. The receiver can be tuned to one wavelength at the time, and thus all the arriving packets but one

are lost. If an array of fixed receivers is used instead of the tunable receiver, the conflicts are solved. However,

using W receivers at each of the nodes becomes very expensive, when the number of wavelength channels

increases. Additionally, more electronic components are needed.

CHAPTER 3: MAC PROTOCOLSIn this chapter several MAC-protocols proposed for optical networks are introduced. The MAC protocols are

classified into several different categories. The most important factors of the performance of the MAC protocols

in optical networks are:

Throughput

Delay

Fairness

Buffer requirements

Number and cost of components needed

A good MAC protocol should be:

Efficient. There should be high data throughput and packets should not experience large transfer

delays.

Fair. Each station should have equal access to the medium.

Simple. The implementation of the MAC protocol should not be so complex that it requires powerful

hardware or long processing times that impair performance.

These conflicting requirements have been a challenge to MAC protocol de- signers ever since the development

of the Aloha protocol in the 1960's. A large amount of research has been performed in this area, which has led

to many solutions, implementations and standards. Despite this extensive research effort, there is still a strong

need for MAC research. The fundamental trade of between efficiency and simplicity has been analysed and

debated for the last thirty years. Traditionally, MAC protocols were designed to be simple. The need for speed

and simplicity in the MAC layer outweighed the benefit of efficiency. However, with the incredible growth in

computing speed, it is becoming plausible to design more complex MAC protocols in order to improve effi-

ciency, such as achieving better utilisation and meeting quality of service (QoS) requirements. The MAC

problem of scheduling transmissions in a broadcast-and-select network can be posed as a matrix-clearing prob-

lem, in which a traffic matrix must be processed time slot by time slot until all entries in the matrix are clear.

Since this problem has been proven to be NP-hard, only heuristics have been proposed.

A current trend in the design of MAC protocols for WDM local area networks is to employ a centralised archi-

tecture rather than a distributed one. Requests are sent by users to a central controller that allocates bandwidth

according to a given scheduling algorithm. The use of such a scheduler allows control over the bandwidth al-

located to different services, and hence the provision of QoS requirements. Many different MAC protocols

have been proposed, ranging from random access schemes, fixed-access schemes such as time division mul-

tiple accesses, to reservation schemes where nodes reserve communication sub-channels.

CHAPTER 4: CLASSIFICATION OF MAC PROTOCOLS

There are several parameters that can be used when classifying the MAC protocols:

Topology: Is the protocol proposed for bus, star or ring networks, or can it be used in all of them.

Use of transmitters and receivers: How many tunable/fixed devices are needed.

Use of channels: How many control channels and how many wavelength channels are used. Is the number

fixed or can it be changed.

Tell and go: Can the data be transmitted immediately, or is there some kind of waiting time.

Access strategy: Is access strategy a priori or a posterior.

Channel contention: Is it possible that the packets try to allocate for the same time slot.

Destination conflicts: Is it possible that several packets arrive at the same destination node at the same time.

In this chapter the MAC protocols are classified into several different categories. Some of the protocols belong to

more than one class. However, they are added only to one of the classes. Thus this classification is not the only

possible one. The purpose of this chapter is not to classify the different protocols, but to give the view of the

different kinds of MAC protocols that can be used in optical networks

Figure 8: Basic classification of MAC

Figure 9: broadcast and select MAC classification

The proposed protocols can be divided into two categories:

(1) Pretransmission coordination required

(2) No pretransmission coordination required.

Pretransmission coordination is generally required when TRs are used. The receiver must be informed of the

wavelength of each incoming transmission. All of the protocols based on pretransmission coordination employ

one or more control channels, possibly embedded on the data channels, for accomplishing this purpose. Note

that this coordination is not required if the transmission schedule is fixed or pre-determined in a distributed

manner at each station. With FRs, no pretransmission coordination is required since each receiver is at a fixed

wavelength for reception of incoming traffic. A review of the proposed protocols based on pretransmission co-

ordination is given in the next section.

3.1.1: PROTOCOLS BASED ON PRETRANSMISSION COORDINATION

In pre-transmission coordination based protocols, one of the wavelength channels is chosen as the control

channel through which end-nodes broadcast the information about their packet transmissions. Packet transmis-

sion takes place on the remaining channels. Each end-node may have a separate transmitter o receiver (which

can be fixed) tuned to the control channel or may use transmitter and/or receiver used for data transmissions.

However, each end-node should have access to the control channel in any case. In a pre transmission coordina-

tion based network, any end-node which wants to transmit packets should select a data channel, and transmit

channel reservation information in a control packet (which includes destination address, chosen channel,

packet length etc.) on the control channel. Then the data packet can be transmitted on the selected data chan-

nel.

The protocols requiring pretransmission coordination are summarized in Table 1 using the following metrics.

Equipment: This lists the components required at each station. Note that C is defined to be the number

of channels.

Channels: The first number of the pair is the number of control channels required by the protocol. The

second number is the number of data channels. N is defined to be the number of stations in the net-

work.

Processing: This gives a relative measure of the processing requirements. Monitoring of a single con-

trol channel with packet headers for all the network traffic can become an electronic processing bottle-

neck. Distributed algorithms executed to avoid collisions and/or destination conflicts further increase

the processing requirements.

Tell and go: This feature allows a station to inform the destination it is transmitting a packet and then

transmit it without waiting. In reservation protocols, a station must wait for at least one round-trip

delay before transmitting.

Throughput: This is a relative measure of the maximum achievable throughput. Collisions and/or des-

tination conflicts limit the throughput as the offered load increases. In general, dynamic reservation

schemes that avoid collisions and destination conflicts can achieve the highest throughput.

These pre transmission based protocols includes ALOHA, Slotted ALOHA, CSMA,CSMA/CD

Table 1: Comparison of MAC Protocols Based on Pretransmission Coordination

3.1.1 a) ALOHA

The ALOHA-based protocols are well known, because they have been used in electrical networks. The

operational principle of the basic ALOHA protocol is that the source randomly and sends a packet to the

network whenever a packet arrives. If none of the other nodes tries to use the same time slot, the transmission

is successful.

In WDM network, the arriving packet is sent to a randomly chosen channel. Each packet is connected to a

control packet that contains information about the sender and the receiver. According to this information the

destination node identifies the packets that are sent to it. And can tune it receiver to the right wavelength.

Slotted ALOHA is otherwise similar to the basic ALOHA protocol, but the channels are slotted into fixed

length time slots. The packets can be sent only at the beginning of a time slot and their length have to be an

integer multiple of the slot length.

The main benefit of the ALOHA protocol is its simplicity. Additionally, the number of the channels can be

freely chosen. On the other hand, the basic ALOHA has low throughput, tunable receivers and tunable trans-

mitters are needed at each node and each node has to read and process all the control signalling information,

because it does not know, when or at which channel the is packets for it.

INTERLEAVED SLOTTED ALOHA (I-SA) AND ITS VARIATION INTERLEAVED SLOTTED ALOHA *(I-SA*) These protocols differ from the basic ALOHA protocols in that that after sending the control packet they wait

for acknowledgement from the destination node before sending the data packet (or any other data packets in

the same queue). If the acknowledgement does not arrive fast enough, it is assumed that the transmission has

failed. In I-SA the data packets wait for the transmission in a single queue, while in I-SA* each node has N-1

queues – one for each destination node. The benefit of I-SA* is the possibility to send packets to other nodes

while waiting for the acknowledgement for one destination node.

As the basic ALOHA protocols, these two protocols are simple. I-SA and I-SA* need relatively little process-

ing. One tunable transmitter and one fixed receiver per node are needed. Because of waiting for the acknowl-

edgements, the protocols have long delay compared to many others protocols.