Umts Transmission Network

7/29/2019 Umts Transmission Network

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UMTS TRANSMISSION NETWORK

Like see in the introduction the UMTS

core network conicaxion to the radio

access network(RAN) that asynchronous

transfer mode (ATM) is used to transport

traffic, specifically, the Iu, Iub and Iur

interfaces. The UMTS core networkconsists of two domains of operation: First

domain is the circuit switched core

network(CS-CN) which is essentially

dealing principally with voice traffic.

Secondary domain is the general packetradio service (GPRS) core, which is

essentially dealing with data packet (i.e.

video streaming or web browsing.(

The RAN must be able to connect to both

domains, through the Iu interface. ATM is

currently the only technology that can

effectively connect to both.

The demands placed on modern networks

have increased dramatically in recant years


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in terms of types of service and speed of

operation. The following are some of the

requirements:

-Handling of different types of traffic on

the same network (voice, video, and data.(

-Provision of economically priced access

to users.

-A reliable and flexible communications

link.

ATM is now awidy used technology,

which may best address all of these

requirements. The different types of trafficpose vastly differing demands on a

network; see Table 1, 1:

Table1.1Network demands of differenttraffic types


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INTRODUCTION TO ATM

ATM has developed from the operating

principles ofX-25 and Frame Relay in that

it is based on the virtual circuit concept,

where connections are established in

advance of any data transfer, either by

network management or through the

operation of a signaling protocol. Thesetechnologies allow multiple logical

connections to be multiplexed over a

single physical interface. The information

flow on each logical connection is

organized into small, fixed-size packetscalled a cell, so that ATM is also referred

to as ((cell relay.((

An ATM cell is 53 bytes in size,

consisting of a 5-byte header and 48-bytes

of user data, as shown in figure 1.2.


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Figure1.2The ATM cell structure

The different types of payload need to be

handled differently and this is done by an

ATM adaptation layer (AAL). The data

can be any thing voice or video packets,

for example the data which come from

some other protocol, split up into cells fortransportation.

ATM is a technology definition that is

independent of physical medium; however,

most ATM is carried over an optical fibersystem, most notably SDH/SONET at

155Mbps and 622Mbps.

One key advantage ofATM is that it is

scalable in that it is easy to multiplex

circuits together to provide faster circuits.

ATM is termed "asynchronous", since


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traffic can arrive at any time, and is not

required to align to any framing or

boundaries.

VIRTUAL CIRCUITS AND IRTUAL

PATHS:

The logical connections in ATM are

referred to as virtual channel (VCs). A

VCs is analogous to a virtual circuit in X-

25 or a frame relay logical connection.

ATM implements a virtual circuit typepacket switched network. The basic unit of

an ATM system is the virtual circuit, or

virtual channel (VC). A VC is setup

between two end users through the

network, and a variable-rate, full- duplexflow of fixed-size cells is exchanged over

the connection. A VCs are also used for

user-network exchange (UN) and network-

to-network exchange (NN). Between a

source and a destination, a group of virtual

channels can be grouped together into a


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virtual path (VP). A virtual path provides

the advantage that if a re-routing of

channels is required, then re-routing of thevirtual path automatically and

transparently re-routes all the virtual

channel which it encapsulates like see in

figure (1.3.(

Figure 1.3Virtual circuits andvirtual paths

The virtual path technique helps contain

the control cost by grouping connections

sharing common paths through the

network into a single unit. Network

management actions can then be applied to


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a small number of groups of connections

instead of a large number of individual

connections.Several advantages can be listed for the

use of virtual path:

-Simplified network

architecture.

-Increased network

performance and reliability.

-Reduced processing and short

connection setup time.

-Enhanced network services.

Figure 1.4 show general way that a call

establishment process using virtual

channels and virtual path.


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Figure 1.4 Call Establishment Using

Virtual Path

The process of setting up a virtual path

connection is decoupled from the process

of setting up an individual virtual channelconnection:

-The virtual path control

mechanisms include calculating routes,

allocating capacity, and storing connectionstate, information.

-For an individual virtual

channel setup, control involves checking

that there is a virtual path connection to

the required destination node with

sufficient available capacity to support the


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virtual channel with the appropriate quality

of service, and then storing the required

state information (virtual channel / virtualpath mapping.(

Several characteristics can be listed for the

use of virtual channel connection:

-Quality of service.-Switched and semi permanent

virtual channel connections.

-Cell sequence integrity.

-Traffic parameter negotiation

and usage monitoring.

THE ATM REFERENCE

MODEL:

TO explore the application ofATM to aUMTS network understanding of the

structure of the ATM protocol is required.

Like most protocols, ATM can be split

into a layered model. Each layer performs

particular function, but is self- contained


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communication with layers above and

below through primitives the point at

which layers communicate and exchangeprimitives is referred to as service access

point (SAP).The block of data exchanged

across a ''SAP" The contents of which

are not altered, is known as service data

unit (SDU). This distinguishes it from a

protocol data unit (PDU), which includes

all the header and\or trailer information

that may be added, at that layer (the data

plus Protocol control information). A

general model of this is shown in figure

(1.5.(The reference model for ATM consists of

three layers, the three layers are the

physical, ATM and AAL, as show in

figure (1.6). The ATM reference model is

a three-dimensional one, with control, userand management planes.


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Figure 1.5Layered model


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Figure 1.6The ATM referencemodel

The role of each can be summarized; the

user plane handles data transport, flow

control, error correction and other user

functions. The control plane handles

connection management; for examples the

signaling protocols operate. The

management plane handles resource

management and interlayer coordination.

The key operation and function of each the

layers is presented in figure (1.7.(

Figure 1.7 ATM layer functions


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THE PHYSICAL LAYER:

That ATM cells can be transmitted at one

of several data rates, so that we need to

specify the transmission structure that will

be used to carry this pay load. In the

control ofUMTS, Table (1.2) presents the

specified formats for the physical carriers

that may be used.

Table 1.2UMTSphysical carriers

The physical layer deals with interactions

with the physical medium. However,

ATM is designed to be independent of


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transmission medium and flexible in it is

use of the underlying infrastructure.

Figure (1.8) shows the physical layer.

Figure 1.8The physical layer

PMD SUB LAYER:

The lower sub layer is the physical

medium dependent (PMD) sub layer,which interfaces to the physical medium

it is concerned with moving bits ''on'' and

''off'' the cable or physical layer protocol

and handling timing. A different sub layer

is used for different media or carriers.

Table 1.3 lists some of the principle


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carrier system, the data rate and the

medium/media that can be used.

Table1.3ATM carrier schemes

SYNCHRONOUS DIGITALHIERARCHY (SDH:(

SDH is an optical communications

standard and was established by the

(CCITT), now the ITU-T.SDH specifiesthe communications mechanism at the

physical layer on the fiber today, most

long distance telephone traffic runs over

SDH, and because of the availability of

SDH equipment, it is straight forward for

companies to plug into the network.


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The key aims ofSDH may be summarized

as follows:-Internetworking between

different carriers.

-Unification of world digital

networks.

-Multiplexing together of

digital channel.

-Operations, administration and

maintenance support.

SDH is a TDM system where the total

bandwidth of the fiber is considered to beone channel. It is asynchronous system and

therefore the bits are transmitted at precise

interval, controlled by a master clock. An

SDH frame is a block of2430 bytes of

data, sent out every 125s, with emptyframes being transmitted if there is no data

to send. The basic transmission rate is

155.52Mbps.

This rate is referred to as STM-1

(synchronous transmission module) and all

lines are multiples of this basic building


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block. To provide higher speeds of

operation the STM-1 is multiplexed, to

provide higher-orderSTM signals, asshown in figure 1.9. The SDHSTM-1

frame containing data shown in figure

1.10.

Figure1.9SDH

signals


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Figure 1.10 SDH frame structure

The first nine columns of the frame are

overhead. This comprises three rows of

section overhead, followed by six rows of

line overhead. The first row of the line

overhead contains pointer to the location

of the first full cell. This byte is the first

In a column of path overhead, with the

data area containing for example ATM

cells. Note that as illustrated the data canbe placed anywhere with the payload area

and in fact, span more than one area,

allowing data such as ATM that is

asynchronous to be transported efficiently

for example, if an ATM cell arrives evenas an empty frame is being constructed, it

is inserted in the current frame rather than

waiting for the next. Table 1.4

summarizes the SDH signals, the SONET

equivalent, and the associated data rates.


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Note that ATM was originally designed to

travel over155mbps.

Table 1.4 SDHmultiplexed signals

TRANSMISION CONVERGENCE(TC) SUB LAYER

It is responsible for passing the cells to the

PMD as a bit stream, and also for splitting

an incoming bit stream up into cells. Therole of the TC layer can summarize in

Table 1.5.

Table 1.5Key TC functions


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INVERSE MULTIPLEXING FOR

ATM (IMA:(

A standard defined forATM which allows

an ATM cell-stream to be inverse

multiplexed and carried by multiple

physical links, with the original stream

retrieved at the other end of the links. The

inverse multiplexing process is performedon a cell-by-cell basis the concept is

illustrated in figure 1.11.


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Figure 1.11 Inverse multiplexingfor ATM

UTOPIA:

UTOPIA stands for universal test and

operation physical layer interface for

ATM.

It is a specification that covers the physical

layer of operation and outlines an open,

common interface for the data and control

connections between physical ATM

devices. Control primitives such as timing

and synchronization are defined within the

specifications, and are controlled by amanagement entity. In simplification,

UTOPIA from as an interface between the

ATM layer and the physical layer, as

illustrated in figure 1.12.


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Figure 1.12 UTOPIA model

The ATM cells are clocked across the bus

between devices UTOPIA currently has

four levels for different data transfer rates

and bus width. The four levels are shown

in table 1.6.

Table 1.6 UTOPIA levels

As an example, in 8-bit mode, ATM cells

are transferred across the interface as

shown in figure 1.13.


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Figure 1.13 UTOPIA cell transfer

THE ATM LAYER:

The ATM Layer provides a connection

oriented service and it is at this layer that

the virtual channel and path are

established. ATM differs from most

connection oriented protocols in one

major respect as no acknowledgment is


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given for cell receiver, but it close

guarantee that cells arrive in order

provided that they travel on the samevirtual circuits. The ATM layer provided

in ATM cells by adding 5-Octet header

for each a 48-Octet information field. The

header format for an ATM cells is slightly

different for the UNI and NNI interface, as

shown in figure 1.14.

Figure 1.14 UNI and NNI headers

VIRTUAL PATH IDENTIFIER (VPI:(


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VPI specifies the virtual path that should

be used. It is 8-bits at the user networkinterface and 12-bit at the network-

network interface.

VIRTUAL CHANNEL IDENTIFIER

(VCI:(

VCI is used for routing to and from the

end user. Thus, it functions much as a

service access point. It is 16-bits this

allows for the allocation of256 paths,

since the user can specify an 8-bit, eachcontaining up to 64k circuits. In practice

some of circuits are reserved for special

purpose such as signaling.

THE 3-BIT PAYLOAD TYPEIDENTIFIER (PTI:(

PTI field is identifier the type of

information in the information field. The

first bit indicates wither it is a data cell

(PTI msb = 0) or management information


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cell (PTI msb = 1). The middle bit is used

as a congestion indictor, when (PTI msb =

0) congestion not experienced, and when(PTI msb = 1) congestion is experienced.

The third bit, known as the service data

unit (SDU) type bit, is a one-bit field that

can be used to discriminate two types of

ATMSDU's associated with a connection.

A summary of the value for the PTI field

is presented in Table 1.7.

Table 1.7 Payload

type values

THE CELL LOSS PRIORITY (CLP:(


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It is 1-bit is used to provide guidance to

the network in the event of congestion. A

value of0 indicates a cell of relativelyhigher priority, which should none be

discarded unless no other alternative is

available. A value of1 indicates that this

cell is subject to discard with in the

network.

HEADER ERROR CONTROL (HEC:(

The HEC check sum is explained in the

previous sections, but in the case ofATM.

The input to the calculation is only 32-bits, compared with 8-bit for the code.

The fact that this input is relatively short

allows the code to be used not only for

error detection but, in some cases, for

actual error correction. This is summarizedin the flow chart offigure 1.15.

GENERIC FLOW CONTROL (GFC:(

GFC field does not appear in the cell

header internal to the network to the


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network, but only at the user-network

interface. It can be used for control of cell

flow only at the local user-networkinterface. The field could be used to assist

the customer in controlling the flow of

traffic for different qualities of service. In

any case, the GFC mechanism is used to

alleviate short-term overload condition in

the network.


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Figure 1.15 Effect of Error in Cell Header.

THE ATM ADAPTION LAYER

(AAL:(

The AAL was conceived to provide a good

interface between the different kinds of

applications (Voice, Video and Data) and

the ATM network. To minimize the

number of different AAL protocols that

must be specified to meet a variety of

needs, ITU-T has defined four classes ofservice that cover abroad range of

requirements, see figure 1.16.


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Figure 1.16 Service Classification For

AAL

Some of services which provided by AAL

are at the following:

-Header of transmission error.

-Segmentation and reassembly to enable

large blocks of data to be carried in the

information field ofATM cells.

-Handling of lost and misinserted cell

conditions.

-Flow control and timing control.

AAL PROTOCOLS:

The AALprotocol is split into two

sublayers, the convergence sublayer(CS)

and the segmentation and reassembly

(SAR) sublayer, as shown in figure 1.17.


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Figure 1.17 AAL sublayers

-The (CS) provides the interface to the

applications for a particular adaptation

layer, the lower part of the sublayer is

common to all applications while the

upper service specific part is application

specific. The (CS) accepts messages from

applications and splits then up into units

for transmission. A unit ranges from 44 to

48 byte payload, and is dependent on theAAL used since some of the AAL

protocols will add their own header

information at the (CS). When the (CS)

receives cells, its role is to take the cells

from the (SAR) sublayer, and reconstructthe original message or data stream.

-The (SAR) sublayer is responsible for

packaging information received from (CS)

into cells for transmission and unpacking

the information at the other end. At the


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ATM layer each cell consists of a 5-Octet

header and a 48-Octet information field.

Thus (SAR) must pack any (SAR) headersand trailer plus (CS) information into 48-

Octet blocks. As shown in figure 1.18.

Figure 1.18 AAL Protocols and PDUs

A higher layer block of data is

encapsulated in a single protocol data unit

(PDU) consisting of the higher layer data

and possibly a header and trailer


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containing protocol information at the

(CS) level. This (CS-PDU) is then passed

down to the (SAR) layer and segmented into a number of blocks. Each of these

blocks is encapsulated in a single 48-Octet

(SAR-PDU), which includes a header and

a trailer in addition to the block of data

passed down from (CS). Finally, each

(SAR-PDU) forms the payload of a single

ATM cell. The user of the different AAL's

are shown in figure 1.19.


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Figure 1.19 ATM adaptationlayers

THE AAL1 PROTOCOL:

AAL1 is designed primarily for the

transmission of traffic that requires a

constant bit rate connection-oriented

service. In this case the only responsibility

of the (SAR) protocol is to pack the bits

into cells for transmission and unpack then

at reception. The format of the (SAR-

PDU) is shown in figure 1.20.

Figure 1.20 AAL1 PDU format

AAL1 can function in one of two modes

of operation structured and unstructured.


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STRUCTURED DATA TRANSFER

(SDT:(

Structured data transfer(SDT) meaning

where boundaries between messages need

to be preserved. Where only some of the

voice channels are being extracted for

transport across the ATM network here

the first byte of the 47-byte (SAR) payload

is used as a pointer to indicate where the

next message starts within the payload, the

(CSI) bit of the (SAR) header is set to "1"

to indicate that this pointer is present. Only

cells where the (SN) is an even numbermay contain pointers. So the pointer must

be a number from (0 to 92) to cover two

payloads. The pointer occurs once and

only once in every eight cells, since the 3-

bit sequence number(SN) has wraparoundof8. Note that the payload must align to

an octet boundary. Like in figure

The sequence number protection (SNP)

field is an error code for error detection

and possibly correction on the sequence

number field. It consist of a 3-bit cyclic


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redundancy check(CRC), calculated over

the 4-bit (SN) field, and a parity bit. Parity

bit is set so that the parity of the 8-bit(SAR) header is even.

UNSTRUCTED MODE:

Unstructured meaning that there is no

requirement for the ATM to provide any

information with regard to framing. For

non-structured cells, the pointer is not used

so the full 47-byte is available for eachcell. In even (SN) cells, the (CSI) has the

default value of (0). For all cells with an

odd (SN), the (CSI) bits form a 4-bit

number over a cycle of8 cells, which

encodes the difference between the clockof the sender and that of a common

reference. This enables the receiver to

synchronize to the sender.

LIMITATION TO AAL1:


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1-The AAL supports only one user over a

virtual circuit, this requires a separate

virtual circuit for each user cellsnecessitating a large amount of signaling.

2-Since cells are sent even if there is no

traffic, bandwidth is wasted, if an

operation is paying for bandwidth this can

be of great significance.

3- The AAL is designed for64 k orn 64kvoice channels. This is not particularly

suitable for the advanced CODECs used

in cellular communications.

4- Currently, there is no mechanism for

supporting these advanced CODECswhich provide for compressed voice or

voice with silence suppression.

These limit the uses ofAAL1 in the 3G

network to those described above and

render it unsuitable for the transport of3Greal-time user data.

THE AAL2 PROTOCOL:

AAL2 is intended for analog applications

such as video and audio that requires


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timing information but do not require a

constant bit rate. In UMTS networks,

AAL2 is the main transport of user data.The AAL2 adaptation layer is used to

transport user traffic between the circuit

switched core and RANs. Figure 1.21

shows the protocol stack for this traffic

across both the Iub and Iu interfaces.

Unlike the otherAALs, AAL2 has no

SARsublayer, but rather introduces a

number of sublayers at the CS. The

structure ofAAL2 is shown in Figure

1.22.

The common part of the convergencesublayer(CPS) has two components:1- A (CPS) packet which the role ofit is to allow and identify a number of

bidirectional AAL circuits, multiplexed

over a singlevirtual circuit. Thisminimizes packetization delay, critical for

voice and video applications, to reduce

problems associated with echoing. The

format of the (CPS) packet is shown in

Figure 1.23.


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Figure 1.23 CPS packet format

Figure 1.21 UMTS circuitswitched user data transport


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Figure 1.22 AAL2 structure

(CID) is the 8-bit channel ID field that is

used to identify the different AAL

channels. Since a (CID) represents a

bidirectional channel, the same (CID) isused in both directions. These are defined

in Table 1.8.

Table 1.8AAL2 CID designations


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(LI) is the length indicator gives the size,

in bytes, of the payload, which is variable

in length. The maximum length can be 45

bytes.

(UUI) is the user-to-user interface is a

means of identifying the particularSSCSlayer being used, and to pass information

to this layer. Values 027 are for different

SSCS layers, 3031 are for layer

management and 2829 are reserved for

future use. The UUI field may be null ifthe application does not define an SSCS

layer.

)HEC) is the header error control in the

forms of a (CRC) check over the reset of

the header.


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2- A CPS PDU which is fills its payload

with 48 bytes worth ofCPS packets. Note

that since the CPS packet is variable inlength, there may be more than one packet

in the CPS PDU payload, or indeed a CPS

packet may span more than one PDU

payload. The format of the (CPS-PDU) is

shown in the figure 1.24.

Figure 1.24 CPS PDU format

(OSF) is the offset field which indicates

the start of the next (CPS) packet header

within the payload. This allows packets tospan (PDUs) without wasting payload

space, or requiring alignment to the (PDU)

structure.

(SN) is the sequence number and parity

(P) bits provide some error detection on

the header. When there is no data received,


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the payload is padded out to fill the 48-

byte (ATM) payload; this is to maintain

real-time delivery.

In summary, the advantages of using

AAL2 connections are:

-AAL2 is particularly suitable for the

transport of voice packets produced by

advanced speech CODECs.

-AAL2 enable up to 248 channels to be

multiplexed on a single virtual circuit.

-The packetization delay introduced by

filling a 48-byte ATM cell can further be

reduced by using a small (CPS) payload.-The delay can be kept fixed as the

CODECs changes by allowing the size of

the AAL2 packet to vary.

The service specific part of the CS (SSCS)sublayer is not part of the AAL definition,

but rather may be specified by the

application above the AAL. There can be

multiple (SSCS) layers defined, and

indeed there need not be a (SSCS) layer

present at all. However, the most


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commonly used (SSCS) is the (SSSAR).

The (SSSAR) will accept a packet of up to

64k-byte in size (The maximum size of anIPv4 packet) from the upper layer,

segment it and reassemble it at the far end.Consider a payload of1200 bytes that

needs to be segmented. The segmentation

will consist of26 segments of45 bytes,

with the (CPS UUI) field set to 27, and

one segment of length 30, with the (CPS

UUI) field set to 26. The format of the

(SSSAR PDU) is shown in Figure 1.25.

Figure 1.25 SSSAR PDU

In addition to the (SSSAR), this (SSCS)

also provides two further optional

functions, as shown in figure 1.26.


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Figure 1.26 SAR service-specificconvergence sublayer

- The (SSTED) provides a mechanism to

detect errors in the payload. It does this byadding a trailer of 8 bytes. The format of

the trailer, shown in Figure 1.27.


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Figure 1.27 SSTED

The UU (user to user) field is passed

transparently to the user. The R(reserved)

field is currently unused and filled with

zeros. The CI (congestion indication) and

LP (loss priority) are single-bit fields and

come from the CI bit of the ATM layer(PTI) field and the (CLP) bit of the ATM

layer, respectively. The length field

provides the number of octets in the

payload, and the (CRC) field is for error

checking.

- The (SSSADT) provides for assured

delivery through acknowledgements and

flow control. This mechanism is exactly

the same as the service-specific


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connection-oriented protocol (SSCOP) as

defined in the signaling stack.

Umts Transmission Network

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