ATM BASICS - Chapter 4 docx

Table 4-1, QoS Categories and Service Classes ATM Forum defined also an updated version of UBR QoS category called Differentiated UBR... The ATM Forum QoS categories are list-ed here: •C

ATM Services

The chapter explains and summaries a range of concepts and mechanisms,

which allow ATM for supporting of large variety of services It presents the

concept of Quality of Service in ATM from the services perspective It

describes QoS categories and traffic classes, QoS parameters as well as

traf-fic parameters that are used in ATM devices The chapter also explains

basic traffic management mechanisms, which ensure that requested QoS is

delivered to the customers The reading of the chapter is advised prior to

discussing ATM applications

Before presenting different ATM services and applications, one has to

real-ize the great capabilities of ATM technology in terms of provisioning

guar-anteed bandwidth and strict QoS conditioning In fact whenever ATM

ser-vices are discussed, the QoS objectives and mechanisms are concerned ATM

was designed to deliver the QoS in a way incomparable to any former

tech-nology The state-of-the-art transmission and switching constraints

impli-cated a very detailed and coarse definition of the ATM QoS The unique

com-bination of capabilities and mechanisms allow for ensuring that all types of

applications can be given a suitable set of parameters Therefore, different

services can be supported simultaneously with adequate QoS treatment

4.1 Quality of Service Categories

Since ATM supports transport of several different connection and traffic types, as well as service types, a service classification was developed Initially, ITU-T took into account three factors that are used to differentiate between services Service classes introduced by ITU-T are dependent on the timing relationship between source and destination, bit rate, and the con-nection mode Next ATM Forum presented its own classification of so called QoS service categories The relations between two classification models are given in the Table 4-1

Table 4-1, QoS Categories and Service Classes ATM Forum defined also an updated version of UBR QoS

category called Differentiated UBR.

QoS mainly relates to the needs of the user for the particular application.

Therefore, a number of categories have been defined Voice services, for

example, require a minimal end-to-end delay to minimize the need for echo

cancellers They also require a minimum level of delay variation On the

other hand, data services are more tolerant of delays and delay variation but

can hardly tolerate any data loss The ATM Forum QoS categories are

list-ed here:

•CBR (Constant Bit Rate) is intended to support constant bit rate

connection-oriented traffic where end-to-end synchronization is

required This category is equivalent to ITU-T Class A performance

requirements No error checking, flow control, or other processing is

done This service should meet the current requirements for

perfor-mance comparable to digital private line services such as E1 All

real-time streaming applications can be considered as the CBR traffic Hence,

the CBR services are used to emulate synchronous or plesiochronous

transmission This capability is called Circuit Emulation Services (CES)

•VBR-RT (Variable Bit Rate – Real-Time) has been defined to

sup-port variable bit rate connection-oriented traffic where end-to-end

syn-chronization is required This is otherwise known as ITU-T Class B

per-formance requirements This service is intended for packetized video

and voice applications, such as video conferencing systems Compressed

voice with silence detection/suppression can be given as the example of

traffic that is well fitted into this category

•VBR-NRT (Variable Bit Rate – Non Real-Time) is for types of

traf-fic which behavior is predictable, yet does not require as strict timing

relationship to be maintained on end-to-end basis This service can be

used, for instance, for interconnecting LANs where multimedia email

are exchanged and any variation in cell delivery times will be

eliminat-ed before the email is vieweliminat-ed

•ABR (Available Bit Rate) service is designed for economical support

of applications with vague requirements for throughputs and delays It

has been introduced and covered under the UNI V4.0 ABR has an

embedded congestion control mechanism that is based on the feedback

ingly to the current network conditions A typical example might be for use in a company, which requests for the guaranteed minimum load dur-ing a day and which wishes to benefit from additional capacity durdur-ing idle periods of the day

•UBR (Unspecified Bit Rate) it’s the ATM version of the ‘best effort’

service, with no reservation of bandwidth and no QoS parameters guar-anteed All UBR cells are accepted and if there is capacity left, they are delivered Under congestion conditions, UBR cells are discarded, without any notification sent to the sender Signaling used to set up and clear down calls is normally transmitted as UBR, as is Local Area Network Emulation (LANE) traffic Recently the definition of UBR has been updated with Differentiated UBR, which is the ATM Forum response to the growing popularity of DiffServ model deployed in IP networks

•GFR (Guaranteed Frame Rate) is a service category that was

defined in the late 90’s It is intended to provide a mechanism that will make certain guarantee at the frame level What is guaranteed is a frame rate rather than a cell rate This approach was thought to offer an efficient tool for transmission of network layer packets (e.g IP packets) Note, that AAL discussed in the previous chapter are correlated with service classes and QoS categories In fact different types of AAL were designed to

be capable of serving traffic described by a specific service class For instance Class A traffic (CBR QoS category) should be transported with the AAL 1 (as the requirement) or AAL 5 (as an option)

4.2 Quality of Service Parameters

QoS in an ATM network is defined by the set of six parameters that

char-acterize the performance of a given virtual connection The parameters

quantify the performance of the connection all the way across the ATM

net-work, but excluding the end stations Three of the six parameters are

nego-tiable between the end-stations and the network as part of the traffic

con-tract The remaining three of the parameters is non-negotiable

4.2.1 Negotiable QoS Parameters

The values of the negotiable parameters are signaled during the setup

process between user equipment and a switching device and further within

the ATM network The user can request for QoS specifying values for these

parameters prior to the connection setup The network may accept a new

call and establish the connection Alternatively, the network may reject the

call due to the negative result of connection admission control process

One of the negotiable QoS parameters relates to the potential loss of

trans-mitted cells Cell loss occurs due to the buffer overflow and component or

link failure ATM switch buffers may overflow when several bursts of cells

arrive simultaneously from different virtual connections The Cell Loss

Ratio (CLR) is defined as shown in the equation shown in the Fig 4-1 Lost

cells include cells that did not reach the destination, cells received with an

invalid header, cells corrupted by errors In brief, lost cells include only

those which are sent but do not arrive within expected time

The total transmitted cells figure in the equation is the total number of

con-forming cells, that is, cells that conform to the traffic descriptor Cells that

are lost due to violation of their traffic descriptors are not counted when

cal-culating the CLR Hence, only cells, which were admitted to the network,

are counted The CLR, which in UNI 4.0 became a negotiable parameter,

has a value in the range 10-15 to 10-1 in increments of 10-x

For many services it is extremely important that the delay on end-to-end basis is minimized and has a stable value Although ATM is an asynchro-nous transmission method, it can monitor and guarantee the delay for the transmitted traffic The second negotiable QoS parameter, Cell Transfer

Delay (CTD) is influenced by a number of factors The factors include

prop-agation delay, switch design, buffer capacity and traffic load An application may require a service that can set an upper limit on the value of CTD This

limit is referred to as Maximum Cell Transfer Delay (maxCTD), which can

be requested in the range of 10 ms to 16,777,215 ms with increments of 10

ms Please note that CTD is a variable that changes its value randomly with

every cell received

Fig 4-1, Cell Loss Ratio

The control of the maxCTD parameter does not ensure that every real-time

application can operate properly Some services and applications require

that the delay the delay should not change its value during transmission of

sensitive data For instance, the transmission of voice services over packet

switched networks is possible as long as the end-to-end delay fluctuates

within defined limits The major source of variation of the CTD is related to

one of the fundamental ATM mechanisms - multiplexing of cells from

dif-ferent users The processes of buffering and switching inevitably introduce

certain level of uncertainty in terms of delivery time for a single cell

Fig 4-2, Cell Delay Variation

Consequently, there was a need for the definition of another QoS

parame-ter: the Peak-to-Peak Cell Delay Variation (peak-to-peak CDV) It is the dif-ference between the maximum CTD and the minimum CTD Note that the minimum CTD is made up of the fixed-delay components, that is, the

unavoidable delays such as a propagation, transmission and switching delay Peak-to-peak Cell Delay Variation, has been a negotiable parameter since UNI 4.0 was introduced, is mainly affected by the ATM switch design,

buffer capacity, traffic load and the number of nodes in a system CDV is

measured in the range 10 ms to 655350 ms, with an increment of 10 ms

The relation between maxCTD and peak-to-peak CDV is explained in the

Fig 4-3, where the cell arrival distribution as a function of arrival time is presented As can be easily noticed, for a given source, destination and route, some minimum (fixed) delay always exists due to the physical char-acteristics of the transmission path However, only some cells make it in the minimum time Most of the cells, according to the probability density

func-tion are delivered within ranges set by the value of the maxCTD But the

probability density function says that it is also possible that some cells can reach the destination later than it is desired Such cells will not be counted

as delivered properly and they are treated as lost cells By choosing a value

of CTD, the customer and the carrier make an agreement on how late a cell can be delivered and still count as a correctly delivered cell Normally, CDV

is chosen so that, the fraction of cells that rejected for being too late be on the order of 10-10 or less

4.2.2 Non-negotiable QoS Parameters

The remaining three QoS parameters refer to the characteristics of the

vir-tual connection in terms of errors that can be caused at the time of

trans-mission and switching processes The parameters’ definitions are given in

the Fig 4-4

Fig 4-3, The relation between CTD and CDV

CER (Cell Error Ratio) is defined as the number of ‘errored’ cells over the

number of successfully delivered cells plus the number of errored cells An errored cell is one that has been modified incorrectly, in such a way that it cannot be efficiently recovered by error correction mechanisms The CER ratio is affected by the characteristics of the physical media being used Hence, CER has typically lower value if the physical medium is fiber as opposed to a copper wire

SECBR (Severely Errored Cell Block Ratio) is the fraction of N-cell blocks

of which M or more cells contain an error Hence, it is defined as the

num-ber of severely errored cell blocks over the total transmitted cell blocks A cell block is the number of user cells transmitted between successive O&M cells The knowledge of this parameter can be helpful when there is a need

to investigate the nature of errors

Fig 4-4, Non-negotiable QoS parameters

CMR (Cell Misinsertion Rate) is defined as the number of misinserted cells

over a particular time interval (the unit of the time interval is typically an

hour) A misinserted cell is one that is switched to a VC to which it does not

belong This phenomenon is caused by undetected or miscorrected errors in

the cell header, which imply wrong switching decisions The time interval is

usually taken to mean the lifetime of the virtual connection

The non-negotiable parameters mentions are not signaled between user and

ATM network Their meaning is descriptive, which means they can be

mea-sured, collected and analyzed, but they cannot be requested from the ATM

network as QoS objectives

4.3 Traffic Descriptors

When a virtual circuit is established, both the user (typically the process in

the user device) and the network operator (the ATM network layer) must

agree that certain conditions must be applied to this circuit This

bi-direc-tional agreement is called a traffic contract (similar to the SLA in IP

net-works) and may even have legal implications in the case of a public network

The traffic contract consists of three elements: the specification of offered

traffic characteristics, the set of QoS parameters, and the conformance

def-inition

The first part of the traffic contract is the traffic descriptor containing

fic parameters The user needs to specify his requirements in terms of

traf-fic characteristics Depending on the QoS category different traftraf-fic

parame-ters can be included in the contract The whole set of parameparame-ters that can

be set at the user side includes:

•PCR (Peak Cell Rate), which is the maximum rate at which the

sender is planning to send cells The upper limit for this parameter is

constrained by the physical medium capabilities The unit of this

para-meter is a cell per second [cps] The opposite of the PCR, which is 1/PCR,

indicates how often cells are generated at the transmit side

•SCR (Sustainable Cell Rate) is the expected or required average

cell rate that is measured over a long time interval In case of the CBR traffic, SCR is equal to PCR, but for other QoS categories is always lower than PCR

•MCR (Minimum Cell Rate) is the minimum rate the user considers

acceptable during the whole time of the connection Due to the conges-tion control mechanism that is deployed in the ABR QoS category, the

actual bit rate varies dynamically between MCR and PCR.

•MBS (Maximum Burst Size) is the maximum number of cells that

can be transmitted at the PCR MBS is expressed in the unit of cells

This parameter has its equivalent parameter called BT (Burst

Tolerance), which is expressed in seconds and employed as the measure for conformance checking of the SCR

Each type of AAL uses its own set of traffic parameters The relation between QoS Categories and traffic parameters is given in the Fig 4.5

Fig 4-5, Relations between QoS categories and traffic parameters