01 basic functionality

8/13/2019 01 Basic Functionality

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Basic Functionality Siemens

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Contents

1 What is Frame Relay? 3

2 Reasons for Frame Relay 7

3 Virtual Circuits in Frame Relay 11

4 Frame Relay Frames 15

5 Service Parameters 21

6 Error Detection and Handling 27

7 Congestion Management 318 PVC Status Management 35

9 Switched Virtual Circuits (SVCs) 43

9.1 UNI signaling 44

9.2 NNI Signaling 47

Basic Functionality


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1 What is Frame Relay?

FrameRelay?

Fig. 1


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Definition

Frame Relay is an extremely fast communications technology used globally in

numerous communications networks. It is principally used for LAN, SNA and Internetconnections as well as for voice applications.

Information flow

Fundamentally, in Frame Relay applications the information sent through a wide areanetwork is divided into frames or packets. Each frame is given an address used bythe network to find the path to the frame's destination. The frames are transmittedthrough the switches in the Frame Relay Network to reach their destination.

Fast packet switching

Frame Relay uses a simple, but fast, form of packet switching. In this way, it isparticularly suitable for intelligent terminal equipment such as PCs, workstationsand servers, which in turn communicate with each other by means of intelligentprotocols like SNA or TCP/IP. In this way, error correction can be transposed to theterminal equipment.

The Frame Relay switches themselves only perform error detection and discardfaulty frames. The Frame Relay nodes should be connected to each other with lineswith a high quality of service in order to keep down the error rate.

Summary

Viewed overall Frame Relay provides a high rate of data throughput and greatoperational reliability with low delay times, and is therefore particularly suitable for awide variety of contemporary business applications.


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Video

PBX

Controller

PC

Router

Bridge

FRAD

MUXSwitch

CPEUNI

ISDN nailed-up connection

or

ISDN dial-up connection

ordirect connection

(V.35, E1, RS232)

Desktop & LAN Network access Frame RelayNetwork

Formatspackets

in frames

Statistical

multiplexing

Port

PVC

PVC

PVC

SVC

SVC

UNI User Network InterfaceCPE Customer Premises Equipment

PVC Permanent Virtual ConnectionSVC Switched Virtual ConnectionFRAD Frame Relay Access DevicePBX Private Branch Exchange

Fig. 2


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2 Reasons for Frame Relay

Frame

Relay

~ 1990

New trends

in data networks

Fig. 3


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General

Frame Relay met with great acceptance from the very beginning because the market

exhibited a distinct demand for a fast data transmission technology with a high rate ofthroughput. The Frame Relay technology uses powerful end devices and digitalnetworks with high grades of service that were already available relativelyinexpensively at the time of the introduction of the technology. Consequently, FrameRelay was the right technology at the right time.

At the end of the 1980s there were a number of new trends, which when viewed as awhole led to the increase in demand for fast data transmission across wide areanetworks.

Transition from pure text applications to graphical applications

Increase in the number of applications producing bursty traffic

Intelligent terminal equipment (PCs, workstations, X-Windows terminals) withimproved computing performance are already available at low cost

More widespread implementation of LAN and client/server applications

Broad availability of digital networks.

Greater demand for bandwidth

Storage and retrieval of graphics for interactive applications is as common today as

the swapping of entire pages of text was in the 1970s and 1980s. Users used toworking from an early stage with graphical applications in local networks also ofcourse expect similarly fast response times from wide area networks.

As the bandwidth requirements for transfer of graphics are considerably greater thanfor text transfers, increased bandwidth and data throughput became urgently needed,particularly where fast response times were demanded.

Dynamic bandwidth requirements

Applications running via LAN typically require large amounts of bandwidth for veryshort periods. This type of traffic known as bursty traffic is particularly suitable for

statistical multiplexing on common physical lines, and is also characteristic of FrameRelay.


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Transition from pure text applications to

graphical applications

Increase in the number of applications

producing bursty traffic

Intelligent terminal equipment (PCs, workstations,

X-Windows terminals) with improved computing

performance are already available at low cost

More widespread implementation of LANand client/server applications

Broad availability of digital networks

Fig. 4

Kb

T1 T2 T3 T4 T5

SDLC

LAN

Voicet (sec)

Fig. 9


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Intelligent terminal equipment

The performance of terminal equipment developed along with the changes in network

requirements. Falling costs of computing performance led to widespread installationof powerful PCs, workstations and servers that in turn were interconnected by LAN.

These new terminal devices also allowed complex data communication protocols tobe processed. In other words, the resource-consuming processes for error detectionand correction were able to be relocated from data communication networks to theterminal devices. This was a further development in the application of fast packet-switching methods such as Frame Relay.

By relieving networks of their error handling processes, Frame Relay providesconsiderably greater data throughput rates than with conventional packet-switchingmethods such as X.25.

Greater performance

The increased use of local networks and the Internet protocol produced a greaterdemand for the interconnection of the LANs by wide area networks.

Initially, attempts were made to intermesh LAN bridges and routers directly with eachother with dedicated lines. This could be done provided the networks were small andsimply designed. However, with increasing size and complexity of the networks, thedisadvantages of this connection methods became evident, namely:

Greater transmission costs

Low level of operating reliability

Limited network management options

Restricted diagnostic options in the event of errors.

It quickly became clear that the alternative approach of interconnecting LANs in areliable, manageable wide area network offered distinct advantages:

Less overhead than IP

Frame relay switching is easy to implement

IP switching is rarely available in public WANs

IP routing produces delays and requires additional bandwidth.

Digital transmission networks

The conversion of the public telecommunications infrastructure from analog to digitallines meant greater availability of bandwidth and lower error rates. The errorcorrection mechanisms used by X.25 and SNA (which are particularly suited tohandling fault-susceptible analog line) were no longer necessary in wide areanetworks.


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3 Virtual Circuits in Frame Relay

Virtual Circuits

Fig. 5


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General

The Frame Relay technology is based on the concept of virtual circuits (VCs). VCs

which are usually bi-directional are software-defined paths between two ports andare used as substitutes for nailed-up connections in networks. In contrast to formerapplications in which only so-called Permanent Virtual Circuits (PVCs) were used, theconcept of Switched Virtual Circuits (SVCs) is also used nowadays.

Permanent Virtual Circuits (PVCs)

PVCs are configured by the network operator by means of a network managementsystem. The two connection end points and the service parameters for theconnection must be defined for the configuration. If necessary, a network operatorcan modify existing PVCs.

PVCs are permanently connected paths that are not immediately available whenrequired or on a call-by-call basis. However, in the event of faults, paths can beautomatically rerouted in the network; the connection end points remain the same. Inthis regard, a PVC behaves like a permanently configured point-to-point connection.

PVCs are very popular because they represent an inexpensive alternative to nailed-up connections. The provisioning of PVCs demands careful planning, knowledge ofthe traffic profile and the bandwidth utilization. Startup times are therefore neededbefore the service can be provided and the flexibility is somewhat restricted if only abrief connection duration is required.

Switched Virtual Circuits (SVCs)Unlike PVCs, SVCs are available on a call-by-call basis. Analogously to a normaltelephone call, an SVC is switched through the network using a signaling protocol.The calling party has to specify a destination address, similar to a telephone number.SVC switching through the network is considerably more complex than is the case forPVCs, but the procedure is transparent for the end user.

Simultaneous handling of large numbers of call requests in the network

Fast switching of calls with parallel allocation of necessary bandwidths

Call monitoring, traffic statistics and billing

Although SVCs were already specified on the introduction of Frame Relay, the optionwas not implemented and supported in the frame relay networks in their initial years.The situation has changed in the meantime and network operators now offer theservice. Whereas PVCs use the possible statistical multiplex gains obtained byFrame Relay, SVCs offer dynamic connection options for saving costs and providingmore flexibility.


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FR end device

supports PVCs

FR network

Frame Relay Switch

PVC

PVC

PVC

PVC

PVC

Fig. 6

FR end devicesupports PVCs

and SVCs

FR network

Frame Relay Switch

PVC

PVC

SVC

SVC

SVC

E.164 (ISDN/Telephone Numbering Plan)

or

X.121 (Data Numbering Plan)

Fig. 7


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4 Frame Relay Frames

Flag

DLCI CR EA

FCS

Flag

DLCI DE EAFC

Payload

BC

Fig. 8


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General

A Frame Relay frame consists of a header, an information field, and a checksum

(frame check sequence) field and is enclosed by flags.

Flags

All Frame Relay frames must be separated from each other by one or more flags inorder to be able to detect the start of a new frame. The flags are one byte long andare set to the binary value 01111110.

Frame Relay header

The standard Frame Relay header comprises a 10-bit DLCI address and six specialfunction bits. The DLCI address field can be optionally extended.

Data Link Connection Identifier (DLCI)

In order to distinguish between the frames of different virtual circuits on a link, aconnection identifier is needed in the frames themselves. The DLCI field in eachFrame Relay header is used for this purpose. Different DLCI values must bespecified for different connections for each interface. A DLCI must be provided foreach data link connection, but it is only locally significant.

DLCI Function (ITU-T and ANSI)

0 ITU-T Q.933 Annex A and ANSI T1.617 Annex D LinkManagement

1-15 Reserved

16-991 User Data Link Connections

992-1007 Layer 2 Management for Frame Mode Bearer Service

1008-1022 Reserved

1023 In Channel Layer 2 Management

DLCI Function (LMI)

0 Reserved for Call Control (in-channel)

1-15 Reserved

16-1007 User Data Link Connections

1008-1022 Reserved

1023 Local Management Interface


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FLA

G

Frame Relayheader Information field FCS FLA

G

DLCI (most significant bits) EAC/R

DLCI (least significant bits) EADEBECNFECN

Fig. 9

B

R

FRAD

R

Virtual circuit

Router

Bridge

Frame Relay network node

R

B

FR-networkDLCI=16

DLCI=32

DLCI=16 DLCI=16

DLCI=21

DLCI=17

DLCI=17DLCI=32

Fig. 10


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Command /Response (C/R)

The Frame Relay protocol does not use the C/R bit. It can be used by end users for

command/response functionality.

Discard Eligibility (DE)

If this bit is set, the frame is first discarded on occurrence of congestion in thenetwork.

Backward Explicit Congestion Notification (BECN)

If this bit is set by a network node, congestion is indicated in the backward directionto the transmitting user.

Forward Explicit Congestion Notification (FECN)

If this bit is set by a network node, congestion is indicated in the forward direction tothe receiving user.

Extended Address (EA)

This bit is used to limit the address field. In the standard frame format, the EA bit isset to 0 in the first octet and to 1 in the second octet.

DLCI/Control Indicator (D/C)

The D/C bit indicates whether the remaining 6 bits in the octet are to be interpretedas the least significant DLCI bits (D/C = 0) or as DL-CORE (D/C = 1) control bits.

Information field

The length of the information field is variable. The field has an integral number ofoctets. The Frame Relay protocol itself does not interpret the field. The maximumlength of the frame is limited.

Frame Check Sequence (FCS)

The FCS field has two octets. The frame check sequence includes the Frame Relay

header and the information field.

Transparency

To prevent accidental occurrence of a flag or frame abort bit sequence, a zero (0) isinserted after 5 successive "1" bits during transmission. The zero is automaticallyfiltered out by the receiver.

Frame Abort

A frame can be aborted by transferring at least 7 consecutive "1" bits. The receiverinterprets this bit sequence as "frame abort" and ignores the frame currentlyreceived.


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DLCI (most significant bits) EA=0C/R

DLCI EA=0DEBECNFECN

DLCI (least significant bits) EA=1D/C

DLCI (most significant bits) EA=0C/R

DLCI EA=0DEBECNFECN

DLCI EA=0

DLCI (least significant bits) EA=1D/C

Fig. 11


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5 Service Parameters

Fig. 12


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Committed information rates and burstiness

Bandwidth guarantees (committed information rates) can be given to users for their

connections in Frame Relay networks. Moreover, users may be allowed to exceedthe guaranteed bandwidth to a certain extent for example, if strongly bursty trafficoccurs.

Service parameters

There are three configurable parameters for defining the guaranteed bandwidth andpossible excess bandwidth. The parameters can be set for each Data LinkConnection (DLC):

CIR(Committed Information Rate)

Bc(Committed Burst Size)

Be(Excess Burst Size).

Two further variables are derived from CIR, Bc and Be:

EIR(Excess Information Rate)

Tc(Committed Measurement Interval).

These service parameters apply to incoming frames in a Frame Relay switch.

Consequently, a check can be made for each incoming frame to determine if itcomplies with the traffic agreement or not.

There are two additional service parameters:

AR(Access Rate)

N203(Maximum Octet Length of the Frame Relay Information Field).


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CIR Bc Be

AR N203

Tc EIR

Fig. 13


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Committed Information Rate (CIR)

The CIR is the guaranteed data rate in a virtual circuit. The maximum value of the

CIR can be the same as the rate available at the access channel (i.e., the accessrate). However, the committed rate is usually much less. The CIR is generally set tothe average required rate.

Committed Burst Size (Bc)

Bc is defined as the maximum quantity of data that the network can switch undernormal conditions. These frames are not marked with the DE bit.

Excess Burst Size (Be)

Be is defined as the quantity of data additional to the committed burst size that the

network can attempt to switch under normal conditions. The excess burst frames areeither marked with the DE bit from the user or at the user interface.

Committed Rate Measurement Interval (Tc)

The system monitors the number of bits received in a data link connection and cantherefore ensure that the CIR is not exceeded. Tc is the duration in which the bitstream is monitored in the DLC.

If CIR>0: Tc = Bc (kb)/CIR (kb/s)or

if CIR=0: Tcis administratively defined for a specified interval

Excess Information Rate (EIR)

The EIR is the non-guaranteed rate additional to the CIR that can be used in a virtualcircuit. The availability of this rate depends mainly on the usage of the accesschannel by other virtual circuits. If no excess information rate (additional bandwidth)is currently available, the frames marked with the DE bit are discarded. The excessinformation rate is calculated as follows:

EIR = Be/Tc

Access Rate (AR)

The access rate is the maximum data rate at which an end device can send orreceive in the access channel.

Maximum frame length (N203)

The length of the Frame Relay information field (N203) is defined by the number ofoctets between the address field and the frame check sequence field. All networksshould support a maximum frame length of at least 1600 bytes.


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TI2430EU01AL-0127

6 Error Detection and Handling

Flag

DLCI CR EA

FCS

Flag

DLCI DE EAFC

Payload

BC

Fig. 14


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Discarding frames

There is a simple rule for keeping Frame Relay as uncomplicated as possible:

If there is a problem with a frame, discard it

There are two basic causes for discarding a frame:

Invalid frame

Congestion

Invalid frame

An invalid frame is a frame that

a) is not correctly positioned between two flags

b) has less than three octets between the address field and end flag

c) does not consist of an integral quantity of octets before insertion of the zero(0) bit or after the zero bit has been filtered out

d) indicates a checksum error

e) only has a single octet in the frame header

f) has a DLCI value that is not supported by the receiver

g) has 7 or more consecutive bits set to 1 after insertion of the zero bit or afterthe zero bit has been filtered out (violation of the transparency or frame

abort)h) has an information field that is too long.

Invalid frames are discarded without notifying the sender. The end devices areresponsible for the error correction.

Congestion

There are two causes of congestion in a network:

Receiver congestion: A network node receives more frames than it can handle.

Line congestion: A network node wants to send more frames in a transmission

channel than the allocated bandwidth allows.

In both cases the buffer areas in the network node are overloaded and frames needto be discarded until sufficient space becomes available in the receive or sendbuffers again.

Since LAN traffic is extremely bursty, the probability of congestion occurring isrelatively great. A superior congestion management function is particularly importantto minimize the occurrence and effects of congestion.

The end devices are again responsible for error correction in these cases.


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TI2430EU01AL-0129

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

PVC

PVC

PVC

SVC

SVC

FR-Knoten

Invalid frame?

Congestion ?

CPE

Fig. 15


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TI2430EU01AL-0131

7 Congestion Management

Fig. 16


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32

General

The chief objective of Congestion Management is to maintain the specified grade of

service (e.g., throughput, delay, frame loss) in the virtual circuits.Congestion Management contains functions for avoiding congestion (CongestionAvoidance). In other words, congestion should be prevented from ever occurring.There are also functions for recovering from existing congestion situations(Congestion Recovery) and for detecting congestion.

The congestion functions have the following objectives:

To minimize the number of frames discarded

To keep the degradation in service quality during congestion to a minimum

To be easy to implement and to produce only a minor amount of additional load

To equally distribute the resources (memory, bandwidth, processing capacity)between the virtual circuits

To prevent congestion from spreading to other network elements

To optimize the utilization of the existing network resources

To effectively function regardless of the direction the traffic is flowing.

The effects of congestion on the service quality is defined by two points. If the trafficexceeds the point A (see diagrams), the delay increases overproportionally. The

network element is in a so-called mild congestion state; the quality of the servicedecreases with increasing traffic volume. From point B the network begins to managethe congestion by discarding frames in order to prevent further degradation in servicequality. The network element is in the severe congestion state.

Points A and B can usually be defined for specific networks or network elements bysetting threshold values. In this way the service quality can be manipulated.


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TI2430EU01AL-0133

Offered load

Network

Throughput

A

B

No congestion

within network

Region 1

Mild

congestion

Region 2 Region 3

Severe

congestion

Congestion

Avoidance

Congestion

Recovery

Fig. 17

Offered load

Delay

B

No congestionwithin network

Region 1

Mildcongestion

Region 2 Region 3

Severecongestion

A

Congestion

Avoidance

Congestion

Recovery

Fig. 3


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34


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TI2430EU01AL-0135

8 PVC Status Management

Annex D

Annex A

LMI

Fig. 18


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36

General

The procedures of the PVC Status Management at the user-to-network interface

(UNI) and network-to-network interface (NNI) enable the following: Link integrity verification between the user and network or between networks

Notification of the user or network node if a new PVC has been switched

Detection by the user or network node that an existing PVC has been canceled

Transmission of status information on existing PVCs to users or network nodes

There are three slightly different standards for the PVC Status Management:

ITU-T: Q.933 Annex A (uses DLCI 0; can be used at the UNI or NNI)

ANSI: T1.617 Annex D (uses DLCI 0; can be used at the UNI or NNI)

Industrial standard: LMI (uses DLCI 1023; can only be used at the UNI)

The procedures for all three standards of the PVC Status Management are based onthe replacement of the following message types between the user and network:

Status enquiry

Status

Single PVC asynchronous status

Status enquiry and status messages

"Status Enquiry" and "Status" messages are synchronous handshaking messages forensuring the connection between the two interfaces of the access channel. At theUNI, a Status Enquiry message is sent in the direction of the network from the userside. The network side replies with a Status message.

At the interfaces between networks (NNI), a Status Enquiry message is sent fromboth sides and the two sides reply with a Status message (bi-directional procedurescan also be optionally used at the UNI for example, when connecting a private FRnetwork to a public FR network).

Status Enquiry messages always have the same format.

Status messages can have two different formats:A "Keep Alive" signal that verifies the connection at the access channel

A "Full Status" signal that indicates the status of all virtual circuits in the accesschannel.

The Status Enquiry signal indicates whether a "Keep Alive" or a "Full Status" signal isrequested.


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CPE

FR network access node

or

FR customer premises equipment

UNI

Frame Relay

network

FR network node

Port

Q.933 Annex A

T1.617 Annex D

LMI

User Network

Status

Status Enquiry

Fig. 19

Frame Relay

network

FR network node

NNI

Frame Relay

network

FR network node

Port Port

Status

Status Enquiry

Status Enquiry

Status

User+

Network

User+

Network

Q.933 Annex A bidirectional

T1.617 Annex D bidirectional

Fig. 20


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38

Single PVC Asynchronous Status message

This message type is automatically sent without solicitation by the network side at the

UNI and NNI whenever the status of a PVC changes (active to inactive or vice versa)or whenever a PVC is canceled. If the status of more than one PVC changes, themessage is sent separately for each relevant PVC.

Single PVC Asynchronous Status messages have no effect on Status Enquiry andStatus messages.

TIP:The use and implementation of this message type are optional in the case of theAnnex A and Annex D protocols. Network nodes using the LMI protocol in the accesschannel at the UNI have to be set to "LMI Extended" in order to support Single PVCAsynchronous messages. This message type is known in the LMI protocol as an

Update Status message.

System parameters

The accepted value ranges for the configurable parameters are shown in the tablesbelow. The parameters themselves are explained later.

Counter Description Range Default

N391 Full Status Polling Counter 1-255 6

N392 Error Threshold 1-10 3

N393 Monitored Events Count 1-10 4

N392 must be less than or equal to N393

N391 is always assigned to the user side. N391 is assigned to the user and networksides if the bi-directional procedures are used.

Timer Description Range Default

T391 Link Integrity Verification Polling Timer 5-30 10

T392 Polling Verification Timer 5-30 15

T392 should be longer than T391

T391 is always assigned to the user side. T391 is assigned to the user and networksides if the bi-directional procedures are used.

T392 is always assigned to the network side. T392 is assigned to the user andnetwork sides if the bi-directional procedures are used.


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TI2430EU01AL-0139

Status Enquiry Message

Control Field 03 Unnumbered Information Frame /P=0Protocol Discriminator 08 Annex A (09 bei LMI)

Call Reference 00

Message Type 75 Status Enquiry

Locking Shift * Nur bei Annex D untersttzt

Identifier

Length

Report Type

51 Report (01 bei LMI)

01

00=Full Status/01=Link Integrity Verification

Identifier

Length

Current Sequence Nr.

Last Received Seq. Nr.

53 Sequence Numbers (03 bei LMI)

02

01-FF

01-FF

Status Message

Control Field 03 Unnumbered Information Frame /P=0

Protocol Discriminator 08 Annex A (09 bei LMI)

Call Reference 00

Message Type 7D Status Enquiry


Identifier

Length

Report Type


01

01 Link Integrity Verification only

Identifier

Length




02

01-FF

01-FF


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40

Full Status Message


Call Reference 00

Message Type 7D Status Enquiry


Identifier

Length

Report Type


01

00 Full Status (fr alle PVC)

Identifier

Length




02

01-FF

01-FF

Identifier

Length

DLCI (msb)

DLCI (Isb)

Status

CIR (msb)

CIR

CIR (Isb)

57 PVC Status (7 bei LMI)

03-06

00XXXXXX Most 6 significant bits

1XXXX000 Least 4 significant bits

1000NDAR New/Delete/Active/RNR

RNR (Receive Not Ready) und CIR Felder werden bei

Annex A und Annex D nicht untersttzt


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TI2430EU01AL-0141

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Single PVC Asynchronous Status Message


Call Reference 00

Message Type 7D Status

Locking Shift Nur bei Annex D untersttzt

Identifier

Length *

Report Type *

51 Report (7B Update Status bei LMI ext.)

01 * nur bei Annex A und Annex D

02 Single PVC Asynchronous Status

Identifier

Length

DLCI (msb)

DLCI (Isb)

Status

CIR (msb)

CIR

CIR (Isb)

57 PVC Status (7 bei LMI ext.)

03-06

00XXXXXX Most 6 significant bits

1XXXX000 Least 4 significant bits

1000NDAR New/Delete/Active/RNR

RNR (Receive Not Ready) und CIR Felder werden bei

Annex A und Annex D nicht untersttzt


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TI2430EU01AL-0143

9 Switched Virtual Circuits (SVCs)

CPE

CPE

CPE

FRF.4/

X.36

FRF.4/

X.36

FRF.4/

X.36

FRF.10/

X.76

Fig. 21


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46

RELEAS

E

Frame Relay

NetzCPE CPE

UNI UNI

ITU-T X.36FRF.4

ITU-T X.36FRF.4

Calling

DTE

Called

DTEDCE DCE

Active

Disconnect

Indication

Active

Disconnect

Request

Null

Null

Null

Disconnect

Indication

Disconnect

Request

Active

Active

Null

DISCONNECT

RELEAS

E

RELEASECOMPLETE

RELEASECOMPLETE

DISCONNECT

Fig. 23


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48

Originating

NetworkTerminating

NetworkCPE CPE

UNI NNI UNI

ITU-T X.76

FRF.10

Calling User Called User

Calling

STE

Called

STE

Null

Call

Proceeding

Received

Call

Initiated

Null

Call

Present

Call

Proceeding

Sent

Active

Active

SETUP

CALLPR

OCEDING

CONNECT

Fig. 24


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TI2430EU01AL-0149

Originating

NetworkTerminating

NetworkCPE CPE

UNI NNI UNI

ITU-T X.76

FRF.10

Calling User Called User

Calling

STE

Called

STE

Null

Release

indication

Null

Release

Request

Active

Active

RELEASE

RELEASE

COMPLET

E

Fig. 25


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01 basic functionality

Documents