Chapter12 ATM

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Datacom Chapter 12

Chapter 12Asynchronous Transfer Mode

Learning Objectives

Understand the background information on ATM technology and protocols

used for broadband networking.

Understand the BISDN UserNetwork Interface concept.

Understand the BISDN Protocol Reference Model.

Understand the functions performed by physical layer.

Understand the ATM cell structure and ATM layer functions.

Understand the ATM Adaptation Layer concept.

Understand how ATM works ? Understand Why ATM service categories are defined ?

Understand ATM Service Architecture and Applications.

IntroductionIn the emerging field of highspeed virtual networking, Asynchronous Transfer Mode

(ATM) is a key component. ATM is a telecommunications concept defined by ANSI

and ITU (formally CCITT) standards for carriage of a complete range of user traffic,

including voice, data, and video signals, on any UsertoNetwork Interface (UNI). As

such, ATM is extremely well suited to highspeed networking. ATM technology canbe used to aggregate user traffic from existing applications onto a single UNI (e.g.

PBX tie trunks, hosttohost private lines, video conference circuits), and to facilitate

multimedia networking between high speed devices (e.g. workstations,

supercomputers, routers or bridges) at multimegabit speeds (e.g. 100s of Mbit/s) and

higher.

On the basis of its numerous strengths, ATM has been chosen by standards

committees (e.g. ANSI T1, ITUT SG13) as an underlying transport technology

within Broadband Integrated Service Digital Network (BISDN) protocol stacks. In

this context, transport refers to the use of ATM switching and multiplexing

techniques at the data link layer (i.e., OSI Layer 2) to convey enduser traffic fromsource to destination within a network. While BISDN is a definition for public

networks. ATM can also be used within private networking products, in recognition

of this fact, and for clarity, here we first define two distinct forms of ATM UNI :

Public UNIwhich will typically be used to interconnect an ATM user with an ATM

switch deployed in a public service providers network.

Private UNI which will typically be used to interconnect an ATM user with an

ATM switch that is managed as part of the same corporate network.

R.T.T.C., Hyderabad 1


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The primary distinction between these two classes of UNI is physical reach. Both

UNIs share an ATM layer specification, but may utilize different physical media.

Facilities that connect users to switches in public central offices must be capable of

spanning long distances. In contrast, private switching equipment can often be located

in the same room as the user device (e.g. computer, PBX), and hence can use limiteddistance technologies.

The term ATM user represents any device that makes use of an ATM network, via

an ATM UNI, as illustrated in Fig.1.

Fig.1

Implementations of the ATM UNI

For example, an ATM user device may be either of the following :

An Intermediate System (IS), such as an IP router, that encapsulates data into ATM

cells, and then forwards the cells across an ATM UNI to a switch (either privately

owned, or within a public network),

A private network ATM switch, which uses a public network ATM service for the

transfer of ATM cells (between public network UNIs) to connect to other ATM userdevices.

The carriage of user information within ATM format cells is defined in standards as

the ATM Bearer Service.

What is ATM Bearer Service ?

The ATM bearer service as defined by ANSI and ITU standards, provides a

sequencepreserving, connectionoriented cell transfer service between source and

destination with an agreed Quality of Service (QoS) and throughput. The ATM bearer

service involves at a minimum the two lower protocol layers (ATM, Physical) of the

BISDN protocol stack. These two layers are serviceindependent and contain


ATM

User

ATM

User

ATM

User

Private

ATMSwitch

Public ATM

Network

Private

UNI

Public

UNI


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functions applicable to all upper layer protocols (i.e. they are independent of user

applications). Additionally, the ATM bearer service may involve the CPlane

adaptation layer and signaling protocol for SVC service, UPlane adaptation layers,

which reside above the ATM layer, have been defined in standards to adapt the ATM

bearer service to provide several networking classes of service including ConstantBitRate (CBR) and Variable BitRate (VBR) services.

An ATM bearer service at a Public UNI offers pointtopoint, bidirectional or

pointtomultipoint unidirectional virtual connections at either a virtual path (VP)

level and/or a virtual channel (VC) level. Networks can provide either a VP or VC (or

combined VP and VC) level service. For ATM users that desire only a VP service

from the network, the user will be able to allocate individual VCs (which are not

reserved or allocated for ILMI) within the VP connection (VPC) as long as none of

the VCs is required to have a higher QoS than the VP connection. QoS of a VPC can

be either explicitly specified at subscription time or implicitly specified (through a

variety of mechanisms) and is selected to accommodate the most demanding QoS of

any VC to be carried within that VPC. For VC level service at the UNI, the QoS andthroughput are configured for each virtual channel connection (VCC) individually.

The virtual connection (VPC or VCC) will be established or released via the signaling

protocol or on a subscription basis.

User Network Interface Configuration

Figure 2 illustrates how equipment at both the Private UNI and Public UNI, map into

the BISDN access reference configuration shown in standards. The Public UNI is

modeled after the BISDN UserNetwork Interface defined in ITU Recommendations

and ANSI Standards. It embraces the physical characteristics corresponding to

reference points.

Two elements can be used to describe a reference configuration of the UserNetwork

access of BISDN.

They are

Functional groups

Reference points.

BNT1 functions are similar to Layer 1 o f the OSI Reference model and some of the

functions are

Line Transmission Termination Transmission Interface handling

OAM functions.

BNT2 functions are similar to layer 1 and higher layers of the OSI model. Some

functions of BNT2 are

Adaptation functions for different interface media and topology

Multiplexing and demultiplexing and concentration of traffic

Buffering of TM cells

Resource allocation and usage parameter control



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Signaling protocol handling

Interface handling

Switching of Internal connections.

SB and TB indicate reference points between the terminal (BTE) and the BNT2 and

between BNT2 and BNT1, respectively.Fig. 2

UserNetwork Interfaces Configuration

BISDN : ATM Protocol Reference Model

The BISDN protocol reference model defined in ITUT Recommendation 1.121 is

shown in Fig.3. The reference model is divided into multiple planes as follows :

User plane (Uplane)

with its layered structure, provides for user information flow transfer, along with

associated controls ranging from flow control to error recovery, etc. It contains

Physical Layer, ATM layer and multiple ATM Adaptation Layers required fordifferent service users (e.g. CBR service, VBR service, etc.).

Control plane (Cplane)

has a layered structure and performs the call control and connection control functions;

it deals with the signaling necessary to set up, supervise and release calls and

connections. Thus, Control plane protocols deal with call establishment and release

and other connection control functions necessary for providing switched services. The

Cplane structure shares the Physical and ATM layers with the Uplane as shown in



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Fig.3. It also includes ATM adaptation layer (AAL) procedures and higher layer

signaling protocols.

Management plane (Mplane)

provides two different types of functions : Plane management functions

o not layered, that are related to a system as a whole and provide co

ordination between all the planes.

Layer management functions

o which are related to resources and parameters residing in its protocol

entities; layer management handles the operation and maintenance

(OAM) information flows specific to the layer concerned.

Thus, Management plane provides management functions and the capability to

exchange information between Uplane and Cplane. The Layer Management

performs layerspecific management functions while the Plane Management performs

management and coordination functions related to the complete system.

Fig. 3

BISDN : ATM Protocol Reference Model

The UNI specification involves those protocols, which are either terminated ormanipulated at the usernetwork interfaces. Based on the ATM bearer service

capabilities defined earlier, the protocol layers involved at both UNIs are limited to

the physical and ATM layers, Cplane higher protocol layers for SVC support and

other protocols required for UNI management. Many physical layers (e.g. SDH :

STM1/SONET : DS3) can be specified at both the private or public UserNetwork

Interfaces

Additional physical layers (e.g. blockcoded) are specified for the private UNI. The

applicability of any physical layer at a given interface will depend on technology



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limitations (e.g. maximum reach) or cost effectiveness (e.g. complexity). The UNIs

may also contain Physical Layer Management functions (e.g. SDH/SONET OAM)

and ATM Layer Management functions.

In terms of ATM protocol stack, above the Physical layer there is the ATM Layer that

provides cell transfer for all services and the ATM Adaptation Layer (AAL) providingservicedependent functions to the layers above (indicated as higher layers). The layer

above AAL in the control plane provides call control and connection control; the

management plane provides network supervision functions. The functions of each

layer are detailed in Table 1, which also shows sublayers : Convergence Sublayer

(CS) and Segmentation and Reassembly Sublayer (SAR) for the AAL, Transmission

Convergence (TC) and Physical Medium (PM) for the Physical Layer.

ATM Physical Layer

Physical layer controls transmission and receipt of bits on the physical medium. It

also keeps track of ATM cell boundaries and packages cells into the appropriate type

of frame for the physical medium being used.

The ATM physical layer is divided into two parts :

Physical medium sublayer

Transmission convergence sublayer.

Table 1

Functions of the BISDN : ATM in relation to the Protocol Reference Model

Layer

Management

Higher layer Higher layers

Convergence CSAAL

Segmentation and Reassembly SAR Generic flow control

ATMCell header generation/extraction

Cell VIP/VCI translation

Cell multiplex and demultiplex

Cell rate decoupling

TCPhysical

layer

HEC head. sequ. generation/verification

Cell delineation

Transmission frame adaptation

Transmission frame generation/recovery

Bit timing PM PMPhysical medium

The physical medium sublayer is responsible for sending and receiving a continuous

flow of bits with associated timing information to synchronize transmission and

reception. Because it includes only physicalmediumdependent functions, its

specification depends on the physical medium used. It provides bit transmission

capability including bit alignment. It performs Line coding and also electrical/optical

conversion, if necessary.



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ATM can use any physical medium capable of carrying ATM cells. Some existing

standards that can carry ATM cells are SONET (Synchronous Optical

Network)/SDH STM 1, DS3/E3, 100Mbps local fibre [Fibre Distributed Data

Interface (FDDI) physical layer), and 155Mbps local fibre (Fiber Channel Physical

Layer). Various proposals for use over twistedpair cables were under considerationand Physical layer specification for UTP Cat 5 and Cat 3 have already been

developed.

The transmission convergence sublayer is responsible for the following five functions

Cell rate decoupling

Inserts or suppresses idle (unassigned) ATM cells to adapt the rate of valid ATM cells

to the payload capacity of the transmission system, i.e. in cell rate decoupling it

inserts the idle cells in transmitting direction in order to adapt the rate of the ATM

cells to the payload capacity of the transmission system. It suppresses all idle cells in

the receiving direction. Only assigned and unassigned cells are passed to the ATM

layer.

Header Error Control (HEC) sequence generation and verification

Generates and checks the header error control code to ensure valid data. HEC

sequence generation is done in the transmit direction and its value is recalculated and

compared with the received value and thus used in correcting the header errors. If the

header errors cannot be corrected, the cell will be discarded.

Cell delineation

Maintains ATM cell boundaries. This function enables the receiver to recover the cell

boundaries. Scrambling and Descrambling are to be done in the information field of a

cell before the transmission and reception respectively to protect the cell delineation

mechanism.

Transmission frame adaptation

Packages ATM cells into frames acceptable to the particular physicallayer

implementation, i.e. transmission frame adaptation takes care of all actions to adapt

the cell flow according to the used payload structure of the transmission system in the

sending direction. It extracts the cell flow from the transmission frame in the

receiving direction. The frame can be a synchronous digital hierarchy (SDH) envelope

or an envelope according to ITUT Recommendation G.703.

Transmission frame generation and recovery

Generates and maintains the appropriate physicallayer frame structure. This is the

lowest functions of TC sublayer.

Service expected from the Physical Layer

The ATM layer expects the Physical layer to provide for the transport of ATM cells

between communicating ATMentities. The information exchanged between the

ATM layer and the Physical layer across the PHYSAP includes the following

primitives :



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Primitive Request Indicate Confirm Respond1

PHYUNITDATAX X

1 : The ATMentity passes one cell per PHYUNITDATA request and accepts one

cell per PHYUNITDATA indicate.

Fig. 4

PHYSAP Services required by the ATM Layer

Physical Layer UNI Interfaces

Because of the different kinds of details in the coupling between the fibre or other

physical medium, the transmission convergence sublayer is different, depending on

the physical layer.

155 Mbps SONET STS3c/SDH STM 1

Lets start with SONET/SDH, which is probably the physical layer most often

associated with ATM.

The essential feature of SONET/SDH is to keep track of boundaries of streams that

dont really depend on the particular medium. So, although we typically think about it

as fibre, it will in fact operate over other media. Some of the work going on currently

in the ATM Forum on a physical specification for using (copper) unshielded-twisted

pair will be using the SONET type framing.

155 Mbps, SONET STS3c/SDH STM1


1 Synchronous

Payload Envelope

(1 column of overhead)

Maintenance

and

Operations

9

R

O

WS

125 sec

270 columns

9 X 260 X (8/125) msec = 149.76 Mbps payload9 bytes


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This is the SONET frame at 155 Mbps. To read this chart, let us start in the upperlefthand corner. The bytes are transmitted across the medium a row at a time

wrapping to the next row. By the time we go through all nine rows, the elapsed time is

nominally 125 microseconds.

The first 9 bytes of each row have various overhead functions. For example, the first

two bytes here are used to identify where the beginning of this frame is so the receiver

can lock on to the frame.

In addition, although not shown here, there is another column of bytes, which are

included in the Synchronous Payload Envelope that is additional overhead, with the

result that each row has 260 bytes of information. Consequently, 260 bytes per rowtimes 9 rows times 8 bits divided by 125 microseconds, we get 149.76 Mbps of

payload.

This is called the STS3C. It is also known as the STM1 because in the international

carrier networks, this will be the smallest package that we see available in terms of the

Synchronous Digital Hierarchy (SDH), the international flavor of SONET. The bit

rates for SDH STMn are three times the bit rates for SONET STSn for the same n

example.

HEC Cell Delineation

The cells within the SONET/SDH STM1 payload are delineated by using the Header

Error Check (HEC) in the ATM cell.

The receiver, when its trying to find the cell boundaries, takes five bytes and says, I

wonder if this five bytes is a header. It does the HEC calculation on the first four

bytes and matches that calculation against the fifth byte. If it matches, the receiver

then counts 48 bytes and tries the calculation again. And if it finds that calculation

correct several times in a row, one can probably safely assume that in fact its found

the cell boundaries. If it tries the calculation and it fails, you just slide the window and

try the calculation again.

This kind of process must be used because, of course, we dont really know whats inthe 48 bytes of payload, but the chances that the user data would contain these

patterns separated by 48 bytes is essentially zero for any length of time.

Consider for a moment what happens if you come across a series of empty cells. Then

how do you determine the cell boundaries ? This is especially important since the

CRC for an all zero (empty cell) header would be all zeros. Consequently, the HEC

must be based on something other than a simple CRC.

The answer is that the HEC is calculated by first calculating the CRC value, then

performing an exclusive or operation of the CRC value with a bit pattern called the

coset, resulting in a nonzero HEC. Thus, the HEC is unique from the zeros in the



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empty cells, and the HEC may still be used for cell delineation. At the receiving end,

another exclusive or operation is performed, resulting in the original CRC for

comparison.

2.048 Mbps E1The 2.048 Mbps interface is particularly important in Europe, where this speed (E1),

is the functional equivalent of North American DS1 interfaces.

2.048 Mbps E1

The diagram here shows the basic E1 framing format. The 2.048 Mbps rate is an exact

multiple of 64 kbps.

The basic E1 frame consists of a collection of 32 bytes, recurring every 125microseconds. Instead of using framing bits, this format uses the first (Byte 0) and

seventeenth (Byte 16) for framing and other control information. The receiver uses

the information within the framing bytes to detect the boundaries of the physical layer

block, or frames. The remaining 30 bytes are used to carry ATM cells.

Consequently, the physical layer payload capacity for the E1 interface is 1.920 Mbps.

The HEC is used to find the cell boundaries.


31 Bytes15 16 171 2 3

32 Bytes/125 s

Cell Carrying Bytes

Framing and Overhead Bytes

32 x 8/125 s = 2.048 Mbps HEC cell delineation used


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34 Mbps E3

The diagram shows a single 125 microsecond frame, so this pattern time each second.

It consists of 9 rows of 59 bytes each, plus 6 extra framing and overhead bytes. The

result, once we do the arithmetic, is 34.368 Mbps of physical layer capacity. Theactual capacity available for carrying cells is 33.92 Mbps, once the overhead bytes are

subtracted.

The HEC is used to find the cell boundaries.

25.6 Mbps UTP3

Turning to the private UNI, the lowest speed interface is the 25 Mbps interface over

UTP3. This is designed as a physical layer that can use the typical existing wiring

within the office environment, such as between the wiring closet and the desktop.

Thus, this is targeted at desktop ATM.

Use IEEE 802.5 physical layer with 4B/5B coding.

32 Mbaud x 4/5 = 25.6 Mbps

Cells delineated by special symbol pairs

25.6 Mbps UTP3


59 Columns

125 s Frame

9 Rows

Framing and Overhead Bytes

[(59x9) + 6] x 8/125 s = 34.368 MbpsHEC Cell Delineation used

CellX X

Reset Scramble

CellX 4

No Scramble Reset

or


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In fact, this actually just takes the Token Ring physical layer and does a couple of

interesting tricks with it. In particular, what it does is it uses whats called a 4B/5B

block code. Every five bits in the physical layer are considered a fivebit block and

this actually represents a fourbit pattern. Thus, we have 32 possible fivebit

symbols. Sixteen of the symbols will be for data and 16 of the symbols can be usedfor other things such as control. The reason of doing this is it effectively takes the 16

Mbps Token Ring rate and makes it 25 megabits.

Defining, or declining, cells is very easy here. We define a brandnew symbol called

the X symbol, which will never show up in the cell because the cell is all data and

always uses symbols from the 16 data symbols. So, whenever a receiver sees an X

symbol actually sees two X symbols it knows that what follows is the symbols for

the rest of the cell.

It turns out theres another technical detail here. Theres a scrambling technique

which helps make the spectrum of frequency a littler smoother. There are two ways todo this scrambling.

One is to not reset the scrambling. In this case, the transmitter will use two Xs. If the

receiver is to reset the scrambler, we put an X followed by a 4 data symbol. But again,

well never find the X within the cell because its not a data symbol, so its very easy

to lock onto the cell boundaries.

100 Mbps, 4B/5B Coding

Theres also a 100 Mbps physical layer. One of the reasons this exists is that it is

basically reusing the FDDI technology. FDDI uses a block coding technique using 5

bit (baud) blocks to encode 4 bits of information. There are 16 symbols used for data,

and there are 16 remaining symbols used for control.

5 bit symbols are used to encode 4 bits of data and certain control information

16 used for data.

16 symbol pairs defined in the FDDI standard

Operates at 125 Mbaud or 100 Mbps data rate over multimode fibre.

Cells delimited with TT symbol pair.

108 symbols needed per cell and 25 Msymbols per second implies 85.89 Mbps of cell

payload.

Cells can be separated by JK idle symbols.

25.6 Mbps UTP3

The technique for finding the cells is to define a symbol called the TT symbol and

thats inserted in front of every cell. This becomes very easy. Since this symbol pair

cannot appear in any data, it will not appear anyplace else in a stream. The receiver

needs only to scan for the first TT symbol. It then has locked into the cell

boundaries immediately. If we go through the calculations of cell payload, this is

effectively 6 bytes of overhead for every 48 bytes of payload. Thus, this yields a little

less than 89 Mbps for the cell payloads.


TTTTCell Cell Cell


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ATM LayerThe ATM layer is responsible for establishing connections and passing cells through

the ATM network. To do this, it uses the information contained in the header of each

ATM cell.

ATM Layer Functions

ATM layer is the layer above the physical layer. As shown in the Fig.3, it does the 4

functions, which can be explained as follows :

Cell header generation/extraction

This function adds the appropriate ATM cell header (except for the HEC value) to the

received cell information field from the AAL in the transmit direction. VPI/VCI

values are obtained by translation from the SAP identifier. It does opposite, i.e.

removes cell header in the receive direction. Only cell information field is passed to

the AAL.

Cell multiplex and demultiplex

This function multiplexes cells from individual VPs and VCs into one resulting cell

stream in the transmit direction. It divides the arriving cell stream into individual cell

flows with respect to VC or VP in the receive direction.

VPI and VCI Translation

This function is performed at the ATM switching and/or crossconnect nodes. At the

VP switch, the value of the VPI field of each incoming cell is translated into a new

VPI value of the outgoing cell. The values of VPI and VCI are translated into new

values at a VC switch.

Generic Flow Control (GFC)

This function supports control of the ATM traffic flow in a customer network. This is

defined at the BISDN Usertonetwork interface (UNI).

The ATM layer provides for the transparent transfer of fixed size ATM layer Service

Data Units (ATMSDUs) between communicating upper layer entities (e.g., AAL

entities). This transfer occurs on a preestablished ATM connection according to a

traffic contract. A traffic contract is comprised of a QoS class, a vector of traffic

parameters, a conformance definition and other items. Each ATM endpoint is

expected to generate traffic, which conforms to these parameters. Enforcement of thetraffic contract is optional at the Private UNI. The Public Network is expected to

monitor the offered load and enforce the traffic contract.

Two levels of virtual connections can be supported at the UNI

A pointtopoint or pointtomultipoint Virtual Channel Connection (VCC)

which consists of a single connection established between two ATM VCC

endpoints.

A pointtopoint or pointtomultipoint Virtual Path Connection (VPC)

which consists of a bundle of VCCs carried transparently between two ATM

VPC endpoints.



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ATM layer performs no retransmission of lost or corrupted information. The ATM

layer also provides its users with the capability to indicate the loss priority of the data

carried in each cell. The information exchanged between the ATM layer and the upper

layer (e.g., the AAL) across the ATMSAP includes the following primitives :

Primitive Request Indicate Confirm Respond

ATMDATA X X

Fig. 5

ATM Service Access Point (SAP) Primitives

ATM Cell StructureEquipment supporting the UNI shall encode and transmit cells according to the

structure (see Fig.6).

GFC VPI

VPI VCI

VCI

VCI PT CLP

HEC

Cell Payload

(48 octets)

GFC : General Flow Control VPI : Virtual Path Identifier

VCI : Virtual Channel Identifier PT : Payload Type

CLP : Cell Loss Priority HEC : Header Error Check

Fig. 6

ATM Cell Structure at the UNIThe structure of the ATM cell shown in Fig.6 contains the following fields :

Generic Flow Control (GFC)

This field has local significance only and can be used to provide standardized local

functions (e.g. flow control) only the customer site. The value encoded in the GFC is

not carried endtoend and will be overwritten by the ATM switches. The GFC is

envisaged to provide contention resolution and simple flow control for shared

medium access arrangement at the customer premises equipment (CPE). Thus, the

GFC field is present at the cells between the users and the network that can be used to


BIT

8 7 6 5 4 3 2 1

1

2

3

4

5

6

.

.

53

OC

T

ET


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provide local functions, such as identifying multiple station that share a single ATM

interface.

Two modes of operation have been defined for operation of the GFC field. These are

uncontrolled access and controlled access. The uncontrolled access mode ofoperation has been used in early ATM environment. This mode has no impact on the

traffic that a host generates. CPE at the UNI shall encode the GFC value to all zeros

(0000).

Public network equipment at the public UNI shall encode the GFC value to all zeros

(0000).

Virtual Channel Identifier

The VCI is used to establish connection using translation tables at switching nodes

that map an incoming VCI to an outgoing VCI. Circuits established using VCIs

connections are referred to as virtual circuits, and VCIs endtoend connection is

called a virtual connection. In this sense, that bandwidth is not utilized unless userinformation is actually transmitted. The VCI field in the header of the ATM has 16

bits.

Virtual Path Identifier

The VPI is used like VCI to establish a virtual path connection for one or more

logically equivalent VCIs in terms of route and service characteristics. The VPI

allows simplified network routing functionality and management. The VPI field has 8

bits in case of UNI, or 12 bits in case of NNI, i.e. depending on the location of the

ATM cell. The VPI is used in setting up the endtoend virtual path connection of

multiple virtual path segments. A virtual path contains multiple virtual channels.

The bits within the VPI and VCI fields used for routing are allocated using the

following rules :

The allocated bits of the VPI subfield shall be contiguous;

The allocated bits of the VPI subfield shall be the least significant bits of the

VPI subfield, beginning at bit 5 of octet 2;

The allocated bits of the VCI subfield shall be contiguous;

The allocated bits of the VCI subfield shall be the least significant bits of the

VCI subfield, beginning at bit 5 of octet 4;

Any bits of the VPI subfield that are not allocated are set to 0. For a given VP,

any bits of the VCI subfield that are not allocated are set to 0.

Payload Type (PT)

This is a 3bit field used to indicate whether the cell contains user information or

Connection Associated Layer Management Information (F5 flow). It is also used to

indicate a network congestion state or for network resource management.

The first bit is used to discriminate cells of data from cells of maintenance and

operation. Assuming that the cell is a data cell, the second bit is called the Explicit

Forward Congestion Indication (EFCI) bit. If a cell passes through a point in the

network that is experiencing congestion, this bit is set. At this point, this bit is used in

congestion control for Available Bit Rate (ABR). Again, assuming data cells, the third



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bit is carried transparently by the network. Currently, its only defined use is in one of

the ATM Adaptation Layers AAL5.

Cell Loss Priority (CLP)

This is a 1bit field which allows the user or the network to optionally indicate theexplicit loss priority of the cell, i.e. it indicates whether the cell should be discarded if

it encounters extreme congestion as it moves through the network.

Header Error Control (HEC)

The HEC field is used by the physical layer for detection/correction of bit errors in the

cell header. It may also be used for cell delineation.

The last eight bits in the header are the header error check (HEC). HEC is needed

because if a cell is going through a network and the VPI/VCI values get errored, it

will get delivered to the wrong place. As a security issue, it was deemed useful to put

some error checking on the header. Of course, the HEC also is used, depending on the

physical medium, e.g. in SONET, to delineate the cell boundaries.HEC actually has two modes. One is a detection mode where if there is an error with

the CRC calculation, the cell is discarded. The other mode allows the correction of

onebit errors. Whether one or the other mode is used depends on the actual medium

in use. If fibre optics is used, the onebit error correction may make a lot of sense

because typically the errors are isolated. It may not be the right thing to do in a copper

medium because errors tend to come in bursts. When the onebit error correction is

used, you increase the risk of a multiplebit error being interpreted as a singlebit

error, mistakenly corrected, and sent someplace. So the error detection capabilities

drop when the correction mode is used.

Notice that the HEC is recalculated link by link because it covers the VPI/VCI value,

and the VPI/VCI values changes as cells go through the network.

ATM SwitchingATM uses Virtual Paths (VPs) and Virtual Channels (VCs) to accomplish the endto

end routing. The ATM process has no internal method of cell sequencing, and so

unlike X.25, the cells must be sent and received in the correct order. This is achieved

using the Virtual Circuit principle. A virtual circuit can be thought of as a dedicated

pipe between communicating devices and down this pipe all data between those

devices will be sent. This connection is achieved using a virtual circuit, and because

all data between these two specific points uses the same route, the problem of cell

sequencing is solved.The concept virtual circuits, which are known as Virtual Channel Connections

(VCCs), can be described in the following way :

A VCC is set up between any source and any destination in the ATM network,

regardless of the way it is being routed across the network. Fundamentally, ATM is a

connectionoriented technology. The way the network sets up the connection is,

therefore, by signaling, i.e. by transmitting a setup request that passes across the

network to the destination. If the destination agrees to form a connection, the VCC is

set up between the two endsystems. A mapping is defined between the Virtual

Channel Identifiers (VCIs)/Virtual Path Identifiers (VPIs) of both UNIs, and between

the appropriate input link and the corresponding output link of all intermediate



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switches. A VCC is a connection between two communicating ATM endentities. It

may consist of a concatenation of several ATM VC links. All communication

proceeds along this same VCC which preserves cell sequence and provides a certain

quality of service. Note that the Virtual Channel Identifier (VCI) in the ATM cell

header is assigned per network entitytoentity link, i.e. it may change across thenetwork within the same VCC. A Virtual Path (VP) group VCs carried between two

ATM entities and may also involve many ATM VP links. The VCs associated with a

VP are globally switched without unbundling or processing the individual VC in any

way or changing their VCI numbers. Thus, the cell sequence of each VC is still

preserved and the quality of service of the VP depends on that of its most demanding

VC. As the cell address mechanism uses both the VCI and the VPI, different VPs may

also use the same VCI without conflict. A cell may also not be associated with any

VP. In this case, it would have a null VPI and only a unique VCI. By means of VCs

and VPs, virtual circuits can be set up either permanently (by using socalled

Permanent Virtual Channels, PVCs) or on demand (Switched Virtual Channels,

SVCs). It is likely that VPs will be used mostly between switches (i.e., across NNIs)to carry across large number of virtual circuits. In any case, all the ATM switch has to

do is to identify, on the basis of the cells VPI, VCI or both, which output a received

cell needs to be routed to and what the new VPI/VCI on this output link is. The

operation of an ATM network is, therefore, very simple and inherently can scale to

very high speeds. Fig. 7A and 7B illustrate the concept of virtual path (VP) and

virtual channel (VC).

Fig. 7A

Virtual Path & Virtual Channel



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Fig. 7B

Virtual Path & Virtual Channel

Implementation of VP and VC simplifies the switching process. VP becomes aconvenient way to bundle traffic, and, therefore, the ATM switching equipment has

only to check the VPI of each cell before it can be relayed to the next network node.

Fig.7C shows virtual paths and virtual channels switching concept.

Fig. 7CVirtual Path & Virtual Channel Switching

A user can get ATM services in two ways by setting up either a Permanent Virtual

Circuit (PVC) or a Switched Virtual Circuit (SVC).

PVC

In setting up a PVC, usually following procedure is followed.

User calls the service provider with a request for PVC.

User provides the destination address, average bandwidth requirements or committed

information rate, and duration of PVC circuit.



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Service provider enters the information on the control terminal to setup circuit path at

the subscription time and circuit is established for desired duration on permanent

basis.

User pays a monthly fee for the circuit and pays only for usage of that circuit. If that

circuit is not used, the user pays only the monthly circuit fee like rental. This is justlike monthly telephone bills.

SVC

SVC operation is similar to making a directdialed telephone call. the connection

across the network using a virtual path and virtual circuit is established using

signaling and network switching.

ATM NetworksA simplified example for the structure of an ATM network is shown in Fig.8. It is

important to understand that the various UNI and NNI connections could be carried

via different physical media, such as the existing Plesiochronous Digital Hierarchy

(PDH) layers or the new Synchronous Digital Hierarchy (SDH). Several standards

have been defined on how to interface the physical layers and work is continuing to

specify additional physical layers to be used to transport ATM cells.

Fig. 8

ATM Networks

Adaptation Layer ConceptATM is a packet technology that directs traffic using a label contained in the packets

header. Unlike other packet technologies, such as X.25 or frame relay, ATM uses

short fixedlength packets called cells. As we already know the ATM cell structure

consisting of 53 bytes long : 48 bytes for the information field and 5 bytes for the

preceding header. The header field contains information about the virtual channel

(VCI : Virtual Channel Identifier) and Virtual Path (VPI : Virtual Path Identifier) in

use, Payload type (PT) and cell loss priority (CLP). Inserting payload data into the

48byte information field of the ATM cell is accomplished by the ATM Adaptation

Layer (AAL). The AAL is what gives ATM the flexibility to carry entirely different

types of services within the same format. It is important to understand that the AAL is



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not a network process but instead is performed by the network terminating equipment,

i.e. end systems/stations. Thus, the networks task is only to route the cell from one

point to another, depending on its header information. It should be noted that up to

four bytes might be used by the adaptation process itself with some AAL types,

leaving 44 bytes for payload information. Several adaptation layers have already beenstandardized. And these are :

Type 1

Constant Bit Rate (CBR) services. AAL1 handles traffic where there is a strong

timing relation between the source and the destination. Examples include PCM

encoded voice traffic, constant bit rate video and the emulation of public network

circuits (e.g. the transport for E1 links).

Type 2

Variable Bit rate (VBR) timing sensitive services. AAL2 handles traffic where a

strong timing relation between the source and the destination is required, but the bitrate may vary. Examples include variable bit rate voice and compressed, for instance

MPEGcoded, video.

Type 3/4

Connectionoriented and connectionless VBR data transfer. AAL3/4 is a fairly

complex layer that can handle VBR (i.e. bursty) data both with and without pre

establishing an ATM link. Examples for the connectionoriented type include large

file transfers like CAD files or data back up. The connectionless type is intended for

short, highly bursty transfers as might be generated by LANs.

Type 5

Simple and Efficient Adaptation layer (SEAL). AAL5 may be looked upon as a

simplified version of AAL3/4 that is designed to meet the requirements of local, high

speed LAN implementations. AAL5 is intended for connectionless or connection

oriented VBR services.

ATM Service CategoriesThe introduction of new ATM service categories increased the benefits of ATM,

making the technology suitable for a virtually unlimited range of applications. An

ATM network can provide Virtual Path (VP) or Virtual Channel (VC) connections

with different levels of service. The concept of negotiating the behaviour expectedfrom the ATM layer in terms of traffic and performance for each connection allows

users to optimise network capabilities to suit the applications requirements.

The first ATM implementation offered limited options. A typical network behaviour,

common to most of the first generation ATM Networks, is to reserve a fixed amount

of bandwidth for each connection for the duration of the call on the basis of the

maximum emission rate of the source (i.e., the peak cell rate, PCR) and to provide a

single level of quality of service. The ATM service categories represent new service

building blocks that make it possible for users to select specific combinations of

traffic and performance parameters.



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Why New ATM Service Categories ?

ATM is a multiservice technology. Actually, most of the requirements that are

specific to a given application may be resolved at the edges of an ATM network by

choosing an appropriate ATM Adaptation Layer (AAL). However, in accordance with

the standards definitions the ATMlayer behaviour should not rely on the AALprotocols since these are service specific (and are in many cases supported by the user

terminal, i.e. outside the core network visibility), nor on higher layer protocols which

are application specific.

Given the presence of a heterogeneous traffic mix and the need to adequately control

the allocation of network resources for each traffic component, a much greater degree

of flexibility, fairness and utilisation of the network can be achieved by providing a

selectable set of capabilities within the ATMlayer itself. The Service Categories

have been defined with this goal in mind. Both users and network operators can

benefit from the availability of a selectable set of ATMlayer services. These services

are, in effect, the tools, which will allow the promise of ATM to be fully met :

Customer perspective

ATM customers (e.g. endusers, IT and telecommunications managers) aim to save

on network usage costs, provided that their substantial efficiency and quality

requirements are matched. Requirements vary in nature depending on what

application (e.g. data, voice, video, multimedia) is running. As a matter of fact, users

that produce variable traffic patterns would like to be able to get bandwidth just when

actually needed and, in case of elastic sources, to have fast access to as much

available bandwidth as possible, achieving a satisfactory compromise between

performance and cost.

Network and service operators perspective

All types of operators investing in ATM infrastructures and services aim to achieve

maximum use of the deployed resources, avoiding congestion while being able to

share network resources among a large number of customers and fulfilling the

different user needs in a costeffective way. This allows for appropriate tariff

strategies to be deployed. The ability to offer a range of network services, with

selectable cost/performance levels, is a key issue for network operators, particularly in

a competitive market.

A unified approach to the definition of ATMlayer services in the ATM Forum and in

ITUT is presented in Table 2.An ATM Service Category (ATM Forum name) or ATMlayer Transfer Capability

(ITUT name) is intended to represent a class of ATM connections that have

homogeneous characteristics in terms of traffic pattern, QoS requirements and

possible use of control mechanisms, making it suitable for a given type of resource

allocation.



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Table 2

Correlation of ATM Forum and ITUT ATM Services

ATM Forum TM 4.0

ATM Service

Category

ITUT 1.371

ATM Transfer

Capability

Typical use

Constant Bit Rate (CBR)Deterministic Bit Rate

(DBR)

Realtime,

QoS guarantees

RealTime Variable Bit

Rate (rtVBR)For further study

Statistical mux,

realtime

NonRealTime Variable

Bit Rate (nrtVBR)Statistical Bit Rate (SBR) Statistical mux

Available Bit Rate (ABR) Available Bit Rate (ABR)Resource feedback

control

Unspecified Bit Rate

(UBR)(No equivalent)

Best effort,

no guarantees

Guaranteed Frame Rate

(GFR)

Non real time

application minimum

rate guaranteed.

(no equivalent) ATM Block Transfer (ABT)Burst level feedback

control.

A first classification of these services/capabilities may be seen from a network

resource allocation viewpoint. We can identify :

A category based on a constant (maximum) bandwidth allocation. This is called

Constant Bit Rate (CBR) in the ATM Forum and Deterministic Bit Rate (DBR) in

ITUT;A category based on a statistical (average) bandwidth allocation. This corresponds to

the ATM Forum Variable Bit Rate (VBR) and ITUT Statistical Bit Rate (SBR). The

ATM Forum further divides VBR into realtime (rtVBR) and nonrealtime (nrt

VBR), depending on the QoS requirements. A further partitioning, commonly

adopted, defines three VBR subclasses depending on the conformance criteria

adopted;

A category based on elastic bandwidth allocation, where the amount of reserved

resources varies with time, depending on network availability. This is the Available

Bit Rate (ABR). The same name is used both in the ATM Forum and ITUT;

A category considered only in the ATM Forum is the Unspecified Bit Rate (UBR).

No explicit resource allocation is performed; neither bandwidth nor QoS objectivesare specified;

A further category is considered in ITUT only, and is based on block (or burst)

allocation. This is called ATM Block Transfer (ABT). The feature of this class is the

idea that network resources can be negotiated and allocated on a per block basis rather

than on a per connection basis.

The GFR Service category is intended to support nonreal time applications. It is

designed for applications that may require a minimum rate guarantee and can benefit

from accessing additional bandwidth dynamically available in the network. The

service guarantee is based on AAL 5 PDUs (Frames) and under congestion



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conditions, the network attempts to discard complete PDU instead of discarding cells

without reference to frame boundaries.

The ATM Service Architecture

The ATM Service Architecture makes use of procedures and parameters for trafficcontrol and congestion control whose primary role is to protect the network and the

endsystem in order to achieve network performance objectives. An additional role is

to optimize the use of network resources. The design of these functions is also aimed

at reducing network and endsystem complexity while maximizing network

utilization. To meet these objectives, the set of functions forming the framework for

managing and controlling traffic and congestion can be used in appropriate

combinations.

ATM Service Category (or Transfer Capability) relates quality requirements and

traffic characteristics to network behaviour (procedures and parameters). It is intended

to specify a combination of Quality of Service (QoS) commitment and traffic

parameters that is suitable for a given set of applications (user interpretation) and that

allows for specific multiplexing schemes at the ATM layer (network interpretation).

A Service Category used on a given ATM connection, among those that are made

available by the network, has to be implicitly or explicitly declared at connection

setup. All service categories apply to both Virtual Channel Connections (VCCs) and

Virtual Path Connections (VPCs).

Functions such as Connection Admission Control (CAC), Usage Parameter Control

(UPC), Feedback Controls, Resource Allocation, etc. are made available within the

ATM node equipment and are, in general, structured differently for each ServiceCategory.

Generic Network Functions

Connection Admission Control (CAC0 is defined as the set of actions taken by the

network during the call (virtual connection) setup phase, or during call re

negotiation phase, to determine whether a connection request can be accepted or

rejected. Network resources (port bandwidth and buffer space) are reserved to the

incoming connection at each switching element traversed, if so required, by the

service category.

Usage Parameter Control (UPC) or Policing is defined as the set of actions taken bythe network to monitor and control the traffic offered and the validity of the ATM

connection at the User to Network Interface (UNI). It is an essential requirement for

any network supporting multiple services. The main purpose of UPC is to protect

network resources from malicious and unintentional misbehaviour, which can affect

the QoS of other already established connections. Procedures based on a Generic Cell

Rate Algorithm (GCRA) may be applied to each cell arrival to assess conformance

with respect to the traffic contract for the connection. Violations of negotiated

parameters are detected and appropriate actions can be taken (e.g. cell tagging,

discard).



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Feedback controls are defined as the set of actions taken by the network and by the

endsystems (possibly cooperating) to regulate the traffic submitted on ATM

connections according to the state of network elements. Specific Feedback Control

procedures may be associated with a service category.

Traffic Parameters

A source traffic parameter describes an inherent characteristic of a source. A set of

these parameters constitute a Source Traffic Descriptor which, along with Cell Delay

Variation Tolerance (CDVT) and a Conformance Definition, characterize an ATM

Connection.

The following parameters are considered for the purpose of defining the Service

Categories :

Table 3

Traffic Parameters

Peak Cell Rate (PCR)

Sustainable Cell Rate (SCR)Maximum Burst Size (MBS)

Minimum Cell Rate (MCR)

QoS Parameters

Throughput

Peak Cell Rate (PCR) can be defined as a Throughput parameter which in turn is

defined as the inverse of the minimum interarrival time T between two consecutive

basic events and T is the peak emission interval of the ATM connection. PCR applies

to both constant bit rate (CBR) and variable bit rate (VBR) services for ATM

connections. It is an upper bound of the cell rate of an ATM connection and there is

another parameter sustainable cell rate (SCR) allows the ATM network to allocate

resources more efficiently.

When an ATM end station connects to the ATM network, it is essentially making a

contract with the network based on quality of service (QoS) parameters. This contract

specifies an envelope that describes the intended traffic flow. This envelope specifies

values for peak bandwidth, average sustained bandwidth, and burst size.

It is the responsibility of the ATM device to adhere to the contract by means of traffic

shaping. Traffic shaping is the use of queues to constrain data bursts, limit peak data

rate, and smooth jitter so that the traffic will fit within the promised envelope.

ATM switches have the option of using traffic policing to enforce the contract. Theswitch can measure the actual traffic flow and compare it against the agreed upon

traffic envelope. If it finds that traffic is outside of the agreed upon parameters, the

switch can set the CLP bit of the offending cells. Setting the CLP bit makes the cell

discard eligible, which means that the switch, or any other switch handling the cell, is

allowed to drop the cell during periods of congestion.

Congestion control is a primary concern of ATM designers. For example, dropping

just one cell that is part of a FDDI frame can result in the retransmission of 93 cells.

Retransmission can lead to an exponential increase in congestion as ATM switches



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drop individual cells from different packets, resulting in retransmission of more

packets, which causes even more cells to be dropped.

Quality of Service (QoS) parameters include cell loss, the delay and the delay

variation incurred by the cells belonging to the connection in an ATM network. QoSparameters can be either specified explicitly by the user or implicitly associated with

specific service requests. The QoS parameters selected to correspond to a network

performance objective may be negotiated between the endsystems and the network,

e.g., via signaling procedures, or can be taken as default. One or more values of the

QoS parameters may be offered on a per connection basis.

Table 4

QoS Parameters

Cell Delay Variation (CDV)

Maximum Cell Transfer Delay (Max CTD)

Cell Loss Ratio (CLR)

A number of additional QoS parameters have been identified e.g. Cell Error Ratio

(CER), Severely Errored Cell Block Ratio (SECBR), Cell Misinsertion Rate (CMR).

Traffic Contract and Negotiation

A traffic contract specifies the negotiated characteristics of a VP/VC connection at an

ATM User Network Interface (either Private or Public UNI). The traffic contract at

the Public UNI shall consist of a connection traffic descriptor and a set of QoS

parameters for each direction of the ATM layer connection and shall include the

definition of a compliant connection. The values of the traffic contract parameters can

be specified either explicitly or implicitly. A parameter value is explicitly specified in

this initial call establishment message. This can be accomplished via signaling for

SVCs (Switched Virtual Connections) or via the Network Management System

(NMS) for PVCs (Permanent Virtual Connections) or at subscription time. A

parameter value is implicitly specified when its value is assigned by the network

using default rules.

Table 4

ATM Service Category Attributes and Guarantees

Service

Category

Traffic Description

GuaranteesUse of

feedback

control

Min.

Loss(CLR)

Cell Delay/

Variance

Band

width

CBR PCR X X X NO

rtVBR PCR, SCR, MBS X X X NO

nrtVBR PRC, SCR, MBS X NO X NO

ABRPCR, MCR+

behaviour parametersX NO X X

GFRPCR, MCR, MBS,

MFSX NO X NO

UBR PCR NO NO NO NO



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Fig. 9

Link Usage by different traffic types (CBR, VBR, ABR)

Applications SummaryFollowing tabulation is an attempt to sum up the indications outlined above. The

association and the score assignment are based on a subjective perception.

Table 6

Application Areas for ATM Service Categories

ApplicationCBR rt

VBR

nrt

VRB

ABR GFR UBR

Critical Data XX X XXX X X N/S

LAN interconnection

LAN emulationX X XX XXX XXX XX

Data Transport/interworking

(IPFRSMDS) X X XX XXX XXX XXCircuit Emulation PABX XXX XX N/S N/S N/S N/S

POTS/ISDN

Video ConferenceXXX N/S N/S N/S

Compressed Audio X XXX XX XX N/S X

Video Distribution XXX XX X N/S N/S N/S

Interactive Multimedia XXX XXX XX XX N/S X

Score to indicate the advantage :

Optimum : xxx Good : xx Fair : x N/S : Not suitable

Not quoted : Presently considered not applicable with advantage


T i m e

Bandwidth

C o n s t a n t B i t R a t e t r a f f ic

V a r i a b l e B i t R a t e t r a f f i c

U n s p e c i f i e d B i t R a t e o r A v a i l a b l e B i t R a t e T r a f f i c


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Comparison between ATM and Frame Relay Service

Cell Relay Service (ATM) Frame Relay Service

Connection Type

Virtual Path & Virtual

Connection Virtual Connections

Local Address VPI/VCI DLCI

PDU Length Fixed (48+5 = 53 Octets) Variable

Delineation Method HEC Cell Delineation Flag Delineation

Traffic DescriptorMultiparameter (PCR, SCR,

Bt)

Multiparameter (CIR, Bc,

Be, T)

Priority Indication Cell Loss Priority (CLP) Discard Eligibility (DE)

Error ProtectionHeader only

(+AAL Functions)

CRC 16 over entire

Frame

Congestion Ind.EFCI (Explicit Forward

Congestion Indication)

FECN, BECN

(NNotification)

Physical Interfaces

Following physical interfaces have been standardized for ATM :

Long Distance Media Payload (Mbps)

2.048/1.544 Mbit/s (E1/T1)

34/45 Mbit/s (E3/T3)155 Mbit/s (STM1/OC3C)

622 Mbit/s (STM4/OC12C)


UTP/coax.

Coax.SMF

SMF

SMF

1.920/1.536

33.920/40.6149.760

Campus or LAN

25.6 Mbit/s


UTP3

UTP5, MMF

25.1

149.760

*******************


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