The network processor manages the three interface levels (layers) between synchronous equipment and the public network. X.25 configuration is governed by the software license XPLS.

The levels/layers are defined in the ITU-T X.25 recommendations and in the OSI (Open System Interconnection) standard, issued by the ISO (International Standardization Organization).

These three levels/layers, managed by the FastPad equipment, are:

Table 6-1: Interface Levels (Layers)

Levels Layers according to the ISO standard X.25 ITU-T recommendations

1

Physical

Physical

2

Data-Link

Frame LAPB

2

Multi-Link

Multi-Link MLP

3

Network

Packet X.25

Physical level

This level transmits series of bits over the physical interconnection medium.

Frame level

This level is responsible for the error-free routing of data blocks over the physical line.

The operating principle of the frame level is in conformity with the LAP-B (Link Access Procedure-Balanced) modulo 8 and 128 procedure defined in the ITU-T. This procedure is equivalent to the "balanced" mode of the HDLC standard issued by the ISO, and like HDLC and supervisory, Information and Unnumbered Frame.

On this level, the network processor might be configured C12RxP1 in the following modes:

DTE (Data Terminal Equipment),
DSE (Data Switching Equipment).
DCE (Data Communication Equipment).

The frame level is established when the DTE-configured equipment sends an SABM frame and the DCE-configured equipment replies with a UA frame (SABM = Set Asynchronous Response Mode; UA = Unnumbered Acknowledge).

DTE mode:

The FastPad equipment takes the initiative to

connect the frame level by sending an SABM frame,
disconnect the frame level by sending a DISC (= Disconnect) frame.

DSE mode:

The local as well as the remote party of the FastPad equipment could take the initiative to connect or disconnect the frame level (as is the case in the DTE mode) by sending SABM or DISC.

DCE mode:

The FastPad equipment does not take the initiative to connect or disconnect the frame level. It issues a DM (= Disconnect mode) to indicate that it requests a mode setting command. The response could be:

an SABM frame for a LAP-B frame connection (SABM = Set Asynchronous Balanced mode),
an SABM frame for a LAP-frame connection (SARM = Set Asynchronous Response Mode),
a DISC (= Disconnect) frame.

SABM contention:

Whatever the type of connection (DTE, DCE or DSE), the FastPad equipment manages contention e.g. in case two SABM frames are sent, one by the DTE and one by the DCE equipment.

The Multi-link (MLP) level

The multi-link procedure is defined in ITU-T norm X.25-84. Its function is to distribute the packets among the available lines, each line operating according to the single line procedure, and restore the sequence of the packets on the remote side for further transfer to the packet layer.

To enable an MLP line to be managed, the line must be configured as belonging to an MLP bundle of the processor of the FastPad network. Configuration of the MLP bundle takes place in class 25, recurrences 0 to 8. MLP configuration is governed by the software license XPLS, XMLP.

The lines that may be configured in a bundle are of the dedicated or switched type (PSTN or ISDN). A switched line may be assigned dynamically in a bundle on the initiative of the Network Management System, a telemaintenance center or on the basis of certain load or overflow criteria.

In the case of the ISDN, a line may be shared by several bundles. In fact, the checking of the caller is possible on integrated ISDN, contrary to the case of the PSTN.

Certain foreign ISDN networks do not send the original calling number. In this case, the ISDN line cannot be shared.

The multi-link level is established after the following procedure has been executed:

transmission of a multi-link frame with bit R = 1 by the local equipment,
transmission of a multi-link frame with bit R = 1 by the remote equipment,
transmission, by local and remote equipment, of a frame with bit C set (= 1) to acknowledge the reception of the bits R = 1,
reception, by local and remote equipment, of these level-2 acknowledgment frames (with C = 1).

For more details on MLP, refer to the section "Backup/Overflow/Dynamic Line Management (DLM)" in Chapter 7.

The Packet Level

This level assures the routing of the data packets across the network and flow control mechanism.

As was on LAPB, i sup the packet level., establish the restart phase (exchange of restart packet)
In this state the packet level is ready to send a rewire and call.

Logical Channel Organization (C12RxP4-15)

Normally logical channel 0 is reserved for the restart exchange to establish the restart phase. However, is used by most of the mps X.25 profiles.
Lc number for PVC must be lower that the Lc number used for the five SVCs.

Signalling

The signalling of the FastPad equipment is based on X.25 standards issued in 1984. It changes with national implementation. C12RxP2 is used for doing the adaptation.

Facilities in X.25 Applications

Available facilities of the FastPad equipment are the following:

negotiation of flow control (throughput class, packet size, window size), C12RxP56-58, C12RxP61-67, C12RxP68-72
charging of caller and reverse charging, C12RxP49
CUG (Closed User Group), C12RxP48
fast select, C12RxP54
called line address modified notification (CLAMN), C12RxP84
transit times. C12RxP51

Facility markers

The response of the FastPad equipment to the facility markers mentioned in the X.25 standards can be configured (C12RxP83).

Permanent Virtual Circuit (PVC)

The FastPad equipment supports the PVC function, allowing data transmission between two subscribers at any time, without transmitting call request or clear packets (from a subscriber point of view). Data may be transmitted in full duplex.

Principle

Figure 6-1:
Diagram

On one FastPad node the PVC is configured as a calling PVC and on the other node as a called PVC.

A PVC is set up between a FastPad node and one or more subscribers.

At network level, the virtual circuits established are switched virtual circuits (SVCs).

A PVC may have two states:

PVC utilizable
PVC not utilizable

These states are communicated to each device by means of reset packets.

Set-up of a PVC

Viewed from the FastPad equipment, set-up of a PVC is carried out in three phases:

Phase 1:

: A call request packet is sent by the equipment that has the "calling" PVC configured. The PVC can not be used yet.

Phase 2:

: The equipment that has the "called" PVC configured, responds with a call confirmation packet. The PVC on the called side changes state; it can be used now.

Phase 3:

: The calling side receives the call confirmation packet. The PVC on the calling side can be used now; the data transmission is full duplex.

These three phases are illustrated below

Figure 6-2:
Diagram

FastPad X.25 Interfaces

The FastPad meets the requirements of the ITU-T X.25 recommendation. It offers three types of X.25 interfaces (see Figure 6-3, Figure 6-4 and Figure 6-5):

1. an interface with an X.25 subscriber,

2. an interface with a PSPDN public switch packet data network.

3. an interface to another FastPad.

X.25 Interface of the FastPad

Figure 6-3: X.25 Interface of the FastPad

The behavior of the FastPad in case of a protocol error depends on the type of interface. The selection of the type of interface is made in the configuration.

X.25 Subscriber Interface

This interface is intended to connect X.25 subscribers to the FastPad. There are two profiles available:

Profile 1:	X.25 subscriber profile without additional services.
Profile 2:	X.25 subscriber profile with additional services.

Figure 6-4:
X.25 Subscriber Interface

Profile 1 offers the following services:

Throughput class negotiation.
Fast select service on reception.

The call confirmation packet format and the reset sent by the FastPad are reduced (no address and no additional services).

Profile 2 has the following services available:

Throughput class negotiation
Fast select service on reception
Fast select service on transmission
Packet size negotiation
Window size negotiation.

The call confirmation packet format and the reset sent by the FastPad are extended (additional services but no address service).

Public network interface

This interface gives the FastPad direct access to a public switched packet network or across a switched circuit network. Two profiles are available:

Profile 0:	Direct access connection to a public network with additional services; the throughput class and the packet window size are set to 2.
Profile 3:	Direct access connection to a public network without additional services; the packet window size set to 3.

Profiles to Connect to the PSPDN (PDN)

Figure 6-5: Connecting to the PDN

These types of interfaces with a PDN (Public Data Network) need special address processing.

The public network considers the FastPad as the CPE of a private PDN. At the subscriptions X.121 address is assigned to the CPE by the carrier.

A) Compacting/Decompacting (C12RxP52,1)

With the "Compacting/decompacting" tables in the FastPad, it is possible to translate a private network address (DNICZOAB) into a sub-address and vice versa.

To enable the FastPad to determine the position of the subscriber, the user must also supply the subscriber number on the public network.

So, when configuring the FastPad, the user must complete the compacting/decompacting tables (C11) and the PDN address table (C10).

The PDN address table gives information only about public networks where two addresses (called/calling) are used. The calling address then identifies the switch access point to the public network.

B) Address transport (C12RxP52,4)

The numeration over the public network can be done by using:

the compacting/decompacting option,
address transport option.

The private network "calling" and "called" addresses are transported across the public network in the complementary address extension service using the DTE marker (see the X.25 recommendations).

Figure 6-6: Public Network Lines

The lines with the public data network must be configured with parameter 52 = 4.

Node Zl puts the private addresses in the extension address facility field and adds the DTE marker. If the marker already exists, the addresses are inserted after the marker.

To be able to reach subscriber B, the private address of DTE B is translated into a public address using the called address inversion table for outgoing calls. This address corresponds with the public address of the node.

On the outgoing side of the public network, node Z2 re-forms the calling and called addresses, using the extension address facility. When only the facilities with the DTE marker are transmitted, the marker is suppressed.

For the operator, this procedure is transparent and compatible with the subscriber's use of the DTE marker and address extension facilities, at least when the maximum facility field size of a call packet is respected (See ITU-T X.25 recommendations).

FastPad Interface

This interface is intended to connect two FastPads to each other, directly or via a modem.

When the FastPads are connected via modems, an automatic backup via the PSTN can be made (see Figure 6-7).

The internal protocol assures the continuation of the communication in progress during the switch- over to the PSTN and back.

Backup takes place transparently for the users of the network.

The Network Management System is informed of the switch-over to the PSTN by the reception of an outstanding event, CT117 closed (CT117 = standby indicator).

Switching back is indicated by the opening of the CT117 contacts.

Figure 6-7: FastPad Interface

There are four profiles available:

Profile 4:	Inter node line, receiving clock signals (RC), circuits 114/115. The primary address = 0l and the logical channels are scanned in decreasing order.
Profile 5:	Inter node line, transmitting clock signals (TC), circuits 114/115. The primary address = 03 and the logical channels are scanned in increasing order.
Profile 20:	Inter node line with automatic backup via PSTN. The primary address = O1 and the logical channels are scanned in decreasing order.
Profile 21:	Inter node line with automatic backup via PSTN. The primary address = 03 and the logical channels are scanned in increasing order.

The X.25 protocol used between the FastPad requires that certain Parameters (primary address, scanning direction of logical channels) are in reverse on both ends of one FastPad link. Profiles 4 and 5, 20 and 21 manage these reversals.

Table 6-2: X.25 service parameters

Class 1 R1: type of line
<port #> : 1	X.25

Class 12 R <port#>
0 ?	X.25
Physical level parameters
P20,21,24,25 P26 P28	Signals requested to be present to declare the line in service. Signals forced on the interface. Access rate
Frame Level:
P1 P29 P32 P33 P34 P35 P36 P37 P38	Type of link N1, frame size T1, Time-out to receive an acknowledgment T2, Time-out to acknowledge a frame N2, retransmission counter K, frame window Modulo Primary @ Secondary @	1 DTE, 2 DCE, 3 DSE up to 8KB (1-250)* 100ms (1-127)* 100ms (2-250) (1-7) if modulo 8, (1-30) if modulo 128 8 for 8, 128 for 128 1 DTE, 3 DCE 1 DCE, 3 DTE








Packet Level:
P3 P4 - P15 P39 P40 P41 P42 P43 P44 P45 P46 P74	Logical Channel scanning direction Logical Channel organization T10, T20 restart timer T12, T22 reset timer T13, T23 clear timer Nbr of retry for P39,40,41 Diagnostic code suppression for P39,40,41 Signaling type Nbr of addresses in Call Request packet Subscriber number, significant if P44 = 1 Call Conf packet format	0 decreasing, 1 increasing (1-250)* 10s (1-250)* 10s (1-250)* 10s (1-250)* 10s 1-250 (0 X.25 NT, 1 X.25 TE, 2 X.75 1 or 2 0 F+@, 1 F, 2 nothing (1-250)* 10s 0 F+@, 1 F, 2 nothing

Facilities:
P48	CUG, Closed User Group
P49	RC, Reversed Charging
P54	FS,	Fast Select
P5	Throughput negotiation
P57	Def	Tx
P58	Def	Rx
P59	Max	Tx
P60	Max	Rx
P61	Packet negotiation
P62	Def	Tx	16B - 8KB
P63	Def	Rx
P64	Max	Tx
P65	Max	Rx
P66	Min	Tx
P67	Min	Rx
P68	W, window negotiation		1-7
P69	Def	Tx
P70	Def	Rx
P71	Def	Tx
P72	Def	Rx
P90	Nbr of PVC		Up to 250 per equipment
P91	Entry index for the First PVC in C17R0
Specific:
P2 P52 P53 P81 P82 P83 P89		Type of connection PDN link entry index in C10r0 Call return (trunk) Call return(subscriber) facility marker control @ conversion/aimed point		08 Telenet, 12 Tymnet, 52 Uninet, 64 Itapac 0 no, 1 yes, 4, yes & @ transport to define the X.121 PDN @. Only if 45 = 2

Table 6-3: LAP-B Frame Overview

General X.25 Packet Overview

Table 6-4: X.25 Packet

7

6

5

4

3

2

1

0

BIT

BYTE

1

GENERAL FORMAT ID

LOGICAL CH. GROUP NO.

Q

D

N

N

2

LOGICAL CHANNEL NUMBER

3

PACKET TYPE IDENTIFIER

4

OTHER FIELDS

n

Table 6-5: X.25 Packet

FROM DTE TO DCE

FROM DCE TO DTE

PACKET TYPE ID. (hexadecimal)

Call set-up and clearing

Call Request

Call accepted

Clear request

DTE Clear confirmation

Incoming call

Call connected

Clear indication

DCE Clear confirmation

/0B

/0F

/13

/17

Data and interrupt

DTE data

DTE interrupt

DTE interrupt confirmation

DCE data

DCE interrupt

DCE interrupt confirmation

/even

/23

/27

Flow control and reset

DTE RR (mod 8)

DTE RR (mod 128)

DTE RNR (mod 8)

DTE RNR (mod 128)

DTE REJ (mod 8)

DTE REJ (mod 128)

Reset request

DTE reset confirmation

DCE RR (mod 8)

DCE RR (mod 128)

DCE RNR (mod 8)

DCE RNR (mod 128)

Reset indication

DCE reset confirmation

/x1

/01

/x5

/05

/x9

/09

/1B

/1F

Restart

Restart request

DTE restart confirmation

Restart indication

DCE restart confirmation

/FB

/FF

Diagnostic

Diagnostic

/F1

Registration

Registration request

Registration confirmation

/F3

/F7

Examples

X.25 SVC Configuration

Figure 6-8:
X.25 SVC Configuration

Table 6-6:

C1R1 ( type of the link)

C1R1

1 1

<trunk #> 1

1 1

<trunk #> 1

C1R1 ( type of the link)

C1R1

0 900000

0 800000

C12R1 (service parameters)

C12R1

0 1 ( X.25 DCE user profile)

1 2,0 (X.25 network)

2 5,11 (1st incoming Lcn)

3 9,1 (1st both ways Lcn)

4 11,10 (Nbr of bothways)

5 13,1 (1st outgoing Lcn)

6 28,10 (access rate 9600b/s)

7 32,30 (T1 = 3s)

8 33,10 (T2 = 1s)

9 35,7 (k = 7)

10 45,2 (mode 2 addresses)

11 48,0 (no CUG)

12 56,0 (no throughput neg)

13 74,0 (short call conf format)

default value

29,2 (frame size = 256B)

62,7 (Tx-packet size = 128)

63,7 (Rx-packet size = 128)

69,2 (Tx-window size = 2)

70,2 ( Rx-window size = 2)

0 1 ( X.25 DCE user profile)

1 2,0

2 5,3

3 9,1

4 11,2

5 13,1

6 28,10

7 32,30

8 33,10

9 35,7

10 45,2

11 48,0

12 56,0

13 74,0

default value

29,2 (frame size = 256B)

62,7 (Tx-packet size = 128)

63,7 (Rx-packet size = 128)

69,2 (Tx-window size = 2)

70,2 ( Rx-window size = 2

C12R <trunk #>

C12R <trunk #>

0 5 X.25 DCE trunk profile

default value

11,20 (Nbr of bothways)

29,2

62,7

63,7

69,3

70,3

0 4 X.25 DTE trunk profile

default value

11,20 (Nbr of bothways)

29,2

62,7

63,7

69,3

70,3

X.25 Configuration

X.25 PVC Configuration

Figure 6-9: X.25 PVC Configuration

Table 6-7: X.25 PVC Configuration

C1R1 ( type of the link)

C1R1

1 1

<trunk #> 1

1 1

<trunk #>

C1R2

C1R2

0 900000

0 800000

C12R1 (service parameters)

C12R1

0 1 ( X.25 DCE user profile)

1 2,0 (X.25 network)

2 5,13 (1st incoming Lcn)

3 9,3 (1st bothways Lcn)

4 11,10 (Nbr of bothways)

5 13,3 (1st outgoing Lcn)

6 28,10 (access rate 9600b/s)

7 32,30 (T1 = 3s)

8 33,10 (T2 = 1s)

9 35,7 (k = 7)

10 45,2 (mode 2 addresses)

11 48,0 (no CUG)

12 56,0 (no throughput neg)

13 74,0 (short call conf format)

14 90,2 (Nbr of PVC)

15 91,1 (1st entry in C17R0)

0 1 ( X.25 DCE user profile)

1 2,0

2 5,4

3 9,2

4 11,2

5 13,2

6 28,10

7 32,30

8 33,10

9 35,7

10 45,2

11 48,0

12 56,0

13 74,0

14 90,1

15 91,1

C12R<trunk #>

C12R<trunk #>

0 5 X.25 DCE trunk profile

0 4 X.25 DTE trunk profile

C17R0

C17R0

0 0,1,1,1

0 not significant

1 called side

1 local Lcn number

1 remote Lcn number

1 0,1,2,2

0 not significant

1 called side

2 local Lcn number

2 remote Lcn number

0 1,0,1,1

1 1st entry in C8

0 calling side

1 local Lcn number

1 remote Lcn number

C8R0

C8R0

0 90001001 Called address

C8R4

C8R4

0 1

X.25 PSPDN configuration

X.121 address of the FastPad is 196810. The PSPDN works in two addresses.

Figure 6-10:
X.25 PSPDN Configuration

Table 6-8: X.25 PSPDN Configuration

C1R1 ( type of the link)
1 1 <trunk #> 1
1 1 <trunk #> 1
C1R2	C2R2
0 800030	6 0 (insertion of the compacted Sub @)
C12R1 (service parameters)	C12R<trunk #>
0 1 ( X.25 DCE user profile) 1 2,0 (X.25 network) 2 5,1 (1st incoming Lcn) 3 9,1 (1st bothways Lcn) 4 11,4 (Nbr of bothways) 5 13,5 (1st outgoing Lcn) 6 28,10 (access rate 9600b/s) 7 32,30 (T1 = 3s) 8 33,10 (T2 = 1s) 9 35,7 (k = 7) 10 45,2 (mode 2 addresses) 11 48,0 (no CUG) 12 49,0 ( no Reverse Charging) 13 54,0 ( no Fast Select) 14 56,0 (no throughput neg) 15 74,0 (short call conf format) 16 85,2 (short clear request format) default value 29,2 (frame size = 256B) 62,7 (Tx-packet size = 128) 63,7 (Rx-packet size = 128) 69,2 (Tx-window size = 2) 70,2 ( Rx-window size = 2)	0 1 ( X.25 DCE user profile) 1 2,0 2 5,3 3 9,1 4 11,2 5 13,1 6 28,10 7 32,30 8 33,10 9 35,7 10 45,2 11 48,0 12 56,0 13 74,0 default value 29,2 (frame size = 256B) 62,7 (Tx-packet size = 128) 63,7 (Rx-packet size = 128) 69,2 (Tx-window size = 2) 70,2 ( Rx-window size = 2


















































C10R0
0 196810 (PSPDN @ for the FastPad)
C11R0	C11R1
0 2 (length of the compacted Sub@)	0 3001,01 (compacting/decompacting) 1 3002,02 2 3003,03

Leased line backed-up by modem itself

In this case, profiles allow a failure of the leased line, which is transparent to users. This is possible because of the high values used for N2 and T1 at the Data Link Level.

These profiles are quite similar to profiles 4 and 5. However they must be used to indicate whether the modem uses the leased line or the PSTN network (outstanding events).

Figure 6-11 illustrates this case.

Figure 6-11: Example

When the leased line (LL) fails, the modem automatically dials a stored PSTN number. When the leased line is restored, the modem hangs up the PSTN line.

During the backup/restore interval time, the virtual circuits are not cleared. The default values for parameters 22, 23 and 34 in profiles 20 and 21 configured on the mp's, are set according to the maximum backup/restore delay.

Table 6-9 shows the corresponding configuration.

Table 6-9: Configuration

900010

900020

CLASS 1 RECURRENCE 1

0 1 X.25

0 1 X.25

CLASS 12 RECURRENCE 0

0 20 X.25 DTE PSTN automatic backup by modem

Default values

22,10 signal monitoring interval

23,128 number of signal monitoring interval

34,128 frame retransmissions (N2)

0 21 X.25 DCE PSTN automatic backup by modem

Default values

22,10 signal monitoring interval

23,128 number of signal monitoring interval

34,128 frame retransmissions (N2)

Configuration of an X.25 line

The following diagram describes the steps of the configuration process of an X.25 line, using profiles.

Additional parameters can be configured according to each user's specific needs.

Often modified parameters include the following:

Number of logical channels.
Frame window K.
Number of addresses in signalling packets (Call).
Calling subscriber number.
Behavior with additional CUG service.
Default packet window.
Default packet size.

The X.25 line parameters that can be modified are described in Chapter 4.

Figure 6-12:
Configuration of an X.25 Line
Figure 6-13:
Configuration of an X.25 Line (Con't.)
Figure 6-14:
Configuration of an X.25 Line (Con't.)

HDLC-T

The HDLC-T is governed by the software license (TRAN).

Overview

The FastPad allows any HDLC-compatible device using any protocol with error recovery, usually one delimited by flags, e.g. HDLC, SDLC, LAPB, PPP synch.,...to use HDLC-T features.

Principle

HDLC-T is a point-to-point connection. When the line is in service, the subscriber port uses the automatic calling behavior (C8R0, R4) to establish a logical link (C17R0) between two subscribers.

Figure 6-15: Example

The HDLC-T function is transparent regarding the contents of the frame received from the subscriber. The frame is transported over the X.25 network within data packets.
Values of standard X.25 service parameters negotiated during the connection phase of the logical link are defined on the HDLC-T subscriber port.
Three specific parameters are used by the function:
- C12RxP32 : to check the CRC
- C12RxP33 : number of flag
- C12RxP91 : entry in C17R0
- The type of link is 20 and connection profile number is 82.

Class 17 Rec 0

For each entry, there are four fields (A, B, C, D):

: A: entry index in C8R0, R4
: B: Type of call 0 calling
1 called
2 mixed
: C: Always set to 0
: D: Subscriber No.

Figure 6-16: Example

Table 6-10: Configuration

C1

R1

C1

R1

1

20

2

20

C12

R1

C12

R2

0

1

2

82

28, 15

91, 1 (find entry in C17R0)

0

1

2

82

28, 15 64tcb/s

91, 1

C17

Rec 0

C17

Rec 0

0

1, 0, 0, 68

0

1, 1, 1, 71

C9

Rec 4

C9

Rec 4

36

68

36

71

C9

Rec 5

C9

Rec 5

36

1, 1, 0, 1

36

1, 1, 0, 2

C4

Rec 7

C4

Rec 7

0

1, 1, 0, 0

0

1, 1, 0, 0

C8

R0

C8

R0

0

80000071

0

80000068

C8

R1

C8

R1

0

01, 80

0

01, 80

C8

R4

C8

R4

0

1

0

1

C8

R5

C8

R5

0

CD*

0

CD*

* refer to Chapter 4 "Encapsulation Type".

As soon as line 1 of 9000 00 is in service, port 1 will generate a call to reach 8000 00 71.

Work sheet

Project name:
Customer @:
Contact points:
Date:
Objective:
Diagram:
Node address:
Port number:

Figure 6-17:
Example
Table 6-11:

Class 1 R1: type of line

Class 1 R1: type of line

<port #> : 20

HDLC-T port # :

<port #> : 20

HDLC-T port # :

Class 12 R <port#>:

Connection parameters

Class 12 R <port#>:

Connection parameters

0

82: HDLC profile

0

82: HDLC profile

P28

speed

P28

speed

P32

P33

P91

CRC check ( ) 0 = yes, ( ) 1 = no

Nbr. of flag: 1 up to 15

Entry R in C17R0

P32

P33

P91

CRC check ( ) 0 = yes, ( ) 1 = no

Nbr. of flag: 1 up to 15

Entry R in C17R0

Class 17 R0

Class 17 R0

<R-1>

A, B, C, D: 1,0,

A: Entry Q in C8R0, R1, R4, R5

B: 0 1 calling, 1 called
2 both-way

C: always 0

D : subscriber @, y:

<R-1>

A, B, C, D: 1,0,

A: Entry Q in C8R0, R1, R4, R5

B: 0 1 calling, 1 called
2 both-way

C: always 0

D : subscriber @, z:

C9 R4

C9 R5

C9 R4

C9 R5

? y

? 1,1,0,<port #>

? Z

? 1,1,0,<port #>

C8 R0

Remote @

C8 R0

Remote @

<Q-1>

CDZ

<Q-1>

ABy:

C8 R1

C9 R4

C8 R1

<Q-1>

01, 80

<Q-1>

01, 80

C8 R4

Slow call time

C8 R4

Slow call time

<Q-1>

( ) 0, No ( ) 1-99/*10s

<Q-1>

( ) 0, No ( ) 1-99/*10s

C8 R5

Encapsulation type

C8 R5

Encapsulation type

<Q-1>

( ) CD ( ) FD

<Q-1>

( ) CD ( ) FD

Configuration

Frame Relay

General description

Frame relay (FR) is a frame mode transfer service for long distance communication (WAN: Wide Area Network). This service is based on the modified LAP-D structure (LAP-D: Link Access Procedure on D-channel). The term LAP-F stands for: LAP-for Frame mode support services. LAP-F data is multiplexed on (OSI) level 2.

Signalling relative to this service is managed by the LMI function (Local Management Interface = ITU-T Q.933 and ANSI T1.617 (See Chapters 10 and 13 of this manual).

LAP-F = ISO standard Q.922.

Description of Relayed Frames

There are two types of frames:

Information frames
Signalling frames

Information Frame

Figure 6-18: Information Frame Structure

Legend

DLCI	=	Data Link Connection Identifier
DE	=	Discard Eligibility bit
BECN	=	Backward Explicit Congestion Notification bit
FECN	=	Forward Explicit Congestion Notification bit
C/R	=	Command/Response indication bit
msb	=	Most Significant Byte
lsb	=	Least Significant Byte
CRC	=	Cyclic Redundancy Check

Signalling Frame

Figure 6-19: Signalling Frame Structure (LMI)

Local Management Interface

A local management interface of the frame service is offered. It enables a subscriber to determine the status of the PLLs (Permanent Logical Links) of the network and prohibits him from using a PLL which is not available. It supplies the procedures making it possible to detect and modify the following events:

deletion of a PLL
availability of a configured PLL
non availability of a configured PLL
error due to protocol or sequencing of LMI messages

For this purpose, the LMI of the subscriber (subscriber LMI) regularly transmits status enquiry messages. The LMI of the network (network LMI) replies with status report messages.

Two standard protocols are used for the local management interface:

ANSI T1.-617D

ITU-T Q.933A

FRTE:	FR Interface which acts like a terminal with regard to network; it supports the FRI, FRSNA, FRIP and FRT Frame-relay stack.
FRCE:	FR interface which acts like a network with regard to a subscriber; it supports the FRA Frame-relay stack.

Figure 6-20:
FastPad Configuration as Subscriber LMI (UNI) or Network LMI (NUI)

The FastPad can be configured as a subscriber LMI (UNI) (when it is facing a Frame Relay network), or a network LMI (NUI) (when it is facing an FR subscriber).

Definitions

PLL = Permanent Logical Link

Sub-layer FR2.0 is the relay service. It manages only the bits representing the DLCI number in the heading.

Sub-layer FR2.1 is the frame switching and network congestion service. It manages the FECN, BECN, DE and C/R bits. RT2.0 and RT2.1 represent the core of Q.922.

Sub-layer FR2.2 represents the entire protocol as defined in Q.922. This protocol is generally active in the network periphery in the subscriber terminals.

Types of Interfaces

The FastPad equipment offers several types of interfaces:

A) Subscriber Interface

Local emulation at the protocols:
- Bit synchronous: SDLC, X.25,
- Character synchronous: VIP, BSC 2780, 3270,
- Asynchronous: TTY (PAD), Minitel (Videopad).
Transparent HDLC protocol, with or without multi-frame (multi-frame is the linking together of short frames when the network is congested. See note 1).
FR2.1 protocol with or without multi-frame (FRA) (FBGE); see note 1.
FR2.0 protocol managing the PLL (Permanent Logic Link) connections put into service in FR Switch (FRSW). This interface is used in particular for the connection of voice devices.

B) Network Interfaces (FRTE)

X.25 synchronous bit protocol, levels 2 (frame) and 3 (packet), (FRI).
IEEE 802.2 protocol, level 2.
Transport protocol, transparent with FR2.0: see note 2.
X.25 Protocol Levels 2 (frame) and 3 (packet) with RT2.0 on internal PLL (see note 3 below), implemented in Network FR. This protocol is also called "Internal Frame Relay" (FRI).
The RT2.0 interface (FRSW) allows the formation of a frame relay transit network; in this case, the FastPad has a fast switching function based upon the optimized frame relay technique.
This interface allows the multiplexing of voice type PLLs and Data-type PLLs. The quality of service offered by FRI is indispensable for the use of network services such as COmpRESSION and SVC.
SNA flow encapsulation protocol (LLC2) in frame relay (FRSNA) allows the connection of PV2, PV2.1 and PV4 in frame relay native IBM (RFC 1490).
IP frame encapsulation protocol in frame relay (FRIP) allows interoperability of IP router (RFC 1490).

Note As the FRIP function encapsulates only IP datagrams and is always called, the user call data must have the value CC or DC or FC depending on the desired encapsulation.

Transparent encapsulation protocol in frame relay (FRT).

Note
1) "Transparent HDLC" and "FRA" (FR2.1) protocols can be brought on-line when equipment supporting this protocol is also present on the other end of the line to guarantee end-to-end management.
2) Sub-layer FR2.0 protocol is a support service in frame relay that contributes to the establishment of a PLL between the local and the remote terminal. This protocol is put into service in FRSW. It does not allow the multiplexing of "subscriber" PLLs on the "network" PLL; this facility is offered by putting the FRA into service on the subscriber side and FRI on the network side (FR I).
3) X.25 level 2 related to the FR2.0 sub-layer are functionally identical to level FR2.2 and the related FR2.0 sub-layer. Frame relay, encapsulated in X.25 protocols, allows multiplexing of subscriber PLLs on a PLL network, but in that case the symmetrical equipment must be able to manage the same function on the remote side.

Figure 6-21: Example: Frame Relay Network

Constraints and limitations

The selection reset is offered only when the modifications are on the parameter of the classes 12 and 30.
Statistical operation is limited to the status of the physical line (ON/OFF net) and to the calculation if the number of frames relayed per second.
On FR Switch (FRSW), the DLCI couple associated with a PLL must belong to the same module. This interface manages only the DLCI.
On FR Switch (FRSW), the configuration of speeds must be done so that the sum of the incoming data rates is equal to the sum of the outgoing rates. Congestion (FECN, BECN) is not managed, But notification of these bits is transparent.
The maximum number of PLLs (Permanent Logical Links) on a machine is 140 (160-239) + (65-124).
The LMI cannot be configured on a FR line multiplexing FRSW PLLs.

Configuration of Frame-Relay Lines, Type "Switch" (FRSW) End Connection Voice Device

The following diagram gives the steps in the configuration process of a frame relay switch interface for an incoming and an outgoing line using the standard profile.

Additional parameters can be configured according to specific needs of the user.

Details of the parameters are described in Chapter 4.

Caution:

The two lines to be configured must be on the same module.

Use of Voice Device with FastPad
In this case, the FR connections reserved for voice must have priority (Class 32) without disturbing the data. For this, it is recommended that the size of data frames should be limited.The recommended size for multiplex data transfer with voice is indicated in Figure 6-22 according to the speed of the line.

Reminder.

It is necessary to correctly update Parameter 28 in Class 1 Recurrence 1, whatever the type of line (DCE or DTE). Otherwise, all frames are abortable if the flow does not have priority.
Network lines at 48 kbit/s are recommended for the use of Voice.
Whatever the speed of the network line, the bandwidth used by the priority flow (Voice) must be <30% of the total bandwidth.

Figure 6-22: Speed versus Max. Packet Size

Figure 6-23:
Configuration of Frame Relay Lines

Configuration of an HDLC or Frame Relay Subscriber (FRA) Line

The following diagram gives the steps in the configuration process of an HDLC or frame relay subscriber Interface using the standard profiles.

Additional parameters can be configured according to specific needs of the user.

Details of the parameters are described in Chapter 4.

Figure 6-24: Configuration of a Frame Relay Subscriber (FRA) Line (Con't)

Figure 6-25: Configuration of a Frame Relay Subscriber (FRA) Line (Con't)

Configuration of PLLs multiplexed on an FRTE interface, concerns the FRI, FRSNA, FRIP and FRT protocols

As these PLLs have no implicit physical output port, at least one frame relay line in class 1, recurrence 1 must be configured 18 (frame relay) enabling the routing tables to be configured in class 32.

The frame relay physical lines have an 84 profile (DTE) or 85 profile (DCE) defined in class 12. For these two profiles, only the parameters related to the physical line level are significant and possibly an 84 profile in class 13 which defines the LMI parameters (LMI is optional). LMI is not offered for transit couples (FRSW).

In class 30, the connection parameters of levels 2 and 3 of each PLL are defined by means of profiles (see available profiles).

Class 32 represents the routing tables of all PLLs of the switch. There are two recurrences: one for incoming and one for outgoing lines. In each recurrence, the line number, the DLCI type and the DLCI number must be configured.

Recurrence 0 describes the physical lines with the different DLCI numbers and their types.

Recurrence 1 describes the physical lines of the PLL: the PLL number and the recurrence of the profile defined in class 30 of the PLL are indicated.

DLCI number
This number is a local reference. Numbers are limited from 0 to 1023.
Z01 - FastPad

: Lines 1 and 2 of any protocol are multiplexed on line 3 FR. The two protocols are encapsulated in X.25 and in FR2.0 by means of the Network FR function (virtual lines must be chosen between 160 and 239 and between 65 and 124).

Z02 - FastPad

: On line 4, the two protocols P1 and P2 are received with two different DLCI numbers. For protocol Pl, the DLCI number is declared as "connection", enabling the frame relay to return the frames in the upper layers (Network FR).
: For protocol P2 the DLCI has been configured transit, enabling a fast passage of the frames through ZO1 for transmission on line 6.

ZO3 ZO3 is the destination of protocol P2.
The DLCls are of the connection type to enable the frames to be unpacked by the next software layers (FRI and X.25).

REMARK: For simplification, it is recommended that the virtual line number and the DLCI number of the PLL should be made equal on the physical line.

This rule is applicable, for adjacent nodes in which, as in this example, line 160 should have an odd profile number. This departs from the FastPad rule.

Figure 6-26: Configuration of an Internal Frame Relay (FRI) Line (Con't).

Figure 6-27: Configuration of a Frame Relay Network with Transit and PLLs.

Figure 6-28: Configuration of Z01

Figure 6-29: Configuration of Z02

Figure 6-30: Configuration of Z03

Figure 13-12: Example of FNA/FRA Configuration

C1R1

P4 = 21

line 4 = FRA

C12R4 \xde line 4

P0 = 83

FRA profile

P1 = 90,2

2 DLCIs (101 and 88)

P2 = 91,10

rest of description in C17R0 row 10

P3 = 92,2

LMI of NUI (Network to user) type and still of network type

C17R0

Figure 13-13:
C17R0

This local (AB SA) is used to format the address of the called number in the call packet as follows:

Figure 6-31: Example of FNA/FRA Configuration (Continued)

REMARK: It is possible to choose to configure only a local (AS) in C17 with all the restrictions that this implies. The calling address in the call-request packet then has the following form:

Figure 6-32: Example of FNA/FRA Configuration (Continued)

Figure 6-33: Example of FNA/FRA Configuration (Continued)

Figure 6-34:
FRSNA Example

Practical Viewpoint on Frame Relay

First Example:

Figure 6-35: Frame Relay subscriber using FRI for encapsulation

Figure 6-36: Encapsulation proposed on FR line: number of overhead bytes in parentheses.

The SEP field of the multiframe protocol is negotiated between FRA. It is thus optional.

Second example:

Figure 6-37: Any subscriber using FRI for encapsulation

Figure 6-38: Encapsulation proposed on FR line:

REMARKS

1. In these two examples, the X.25 VC encapsulated in FR is established end to end between two network elements via the FRA protocol.

2. In the following cases, the internal VC is established locally on each machine between the subscriber protocol (FRA, "P", SDLC, S.25, IP) and the protocol offered on the network interface (FRSNA, FRIP, FRT).

Third example:

Figure 6-39: Frame relay subscriber using FRT.

Figure 6-40: Encapsulation proposed on FR line:

Fourth example

Figure 6-41: Any subscriber using FRT

Figure 6-42: Encapsulation proposed on FR line:

Fifth example:

Figure 6-43: SDLC Subscriber using FRSNA

Figure 6-44: Encapsulation proposed on FR line:

Sixth example:

Figure 6-45: X.25 Subscriber using FRSNA (mpSI)

Figure 6-46: Encapsulation proposed on FR line:

Seventh Example

Figure 6-47: LAN subscriber using FRIP:

Figure 6-48: Encapsulation proposed on FR line:

REMARKS

1. The FRI, FRSNA, FRIP and FRT stacks are represented by logic lines. As there may be several types of multiplexed stacks on a physical line, routing must be via the PLL ([65, 128] and [160, 239]) initialized in Class 32 Recurrence 1.

2. ZO (DNIC ZO AB) to PLL of normal/backup line output. The supporting VC (internal or external) is established by means of the routing tables (C9...). The end of the PLL allows the remote node to be identified by means of the associated ZO number. It is thus recommended that calls be routed by configuring the ZO of the remote equipment.

Figure 6-49: Example

Example of routing table for PLL linking switch ZO = 00 with ZO = 01.

Routing table of switch 00:

C9 R2 Pi = 01 (ZO 01 known).
C9 R3 Pi = PLLv (PLL No. V to reach 01).

Routing table of switch 01:

C9 R2 Pj QO (ZO 00 known).
C9 R3 Pj = PLLv (No. V to reach 00).

When several PLLs use different protocols, it is recommended that these internal VCs be routed by using different ZOs (one ZO per PLL) or routing at the level of the AB.

FR

Two RFCs define the extension of MIB II to describe the Frame-Relay interface. RFC 1604 for DCE and RFC 1315 for DTE. Only global physical interface management is proposed and limited to the description in the MIB II of the interface group. Transmission groups is for subsequent study. One * means not used, ** means not available.

Table 6-14: FR Group Objects

Object Name Meaning Access

ifIndex

Interface number

Read only

ifDescr

Interface description

Read only

ifType

Interface type

Read only

ifMtu

Maximum octets in datagram

Read only

ifSpeed

Bandwidth in bits per second

Read only

ifPhysAddress

Lowest layer physical address

Read only

ifAdminStatus

Interface status desired

Read-write

ifOperStatus

Interface status current

Read only

ifLastChange

Value of sysUpTime on interface

Read only

ifInOctets

Total octets received on interface

Read only

ifInUcastPkts

Number of packets to n + 1 layer

Read only

ifInNUcastPkts

Non-unicast packets to n + 1 layer

Read only

ifInDiscards

Inbound discarded packets/flow control

Read only

ifInErrors

Inbound packets discarded due error

Read only

ifInUnknownProtos

Inbound packets with protocol error

Read only

ifOutOctets

Total octets transmitted on interface

Read only

ifOutUcastPkts

Transmit requests from layer n + 1

Read only

ifOutNUcastPkts

Number non-unicast transmit requests

Read only

ifOutDiscards

Outbound packets discarded/flow control

Read only

ifOutErrors

Outbound packets discarded due error

Read only

ifOutQlen *

Packet size of output queue

Read only

ifSpecific

MIB-specific pointer

Read only

IfName **

not used

IfInMulticastPkts **

nbr of multicast frames received error free

Read only

IfInBroadcastPkts **

not used

IfOutMulticastPkts **

nbr of multicast frames transmitted error free

Read only

IfOutBroadcastPkts **

not used

IfHCInBytes **

nbr of bytes received useful for .DS3 interface

Read only

IfHCOutBytes **

nbr of bytes transmitted useful .DS3 interface

Read only

IfLinkUpDownTrapEnble **

trap transmission authorization

Read only

IfHighSpeed

rate in Mbps, if lower that 1Mbps then set to 0

Read only

IfPromiscuousMode

set to False

IfConnectorPresent

set to False

Configuration structure

Class 24 Rec X(6-8) 6 for M0, 7 for M1, 8 for M2			Class 9 Rec 4 Subscriber number for VR
0 1 2 3 4 5 6 7 8 9 10 11 12	112 90,x 91,o 23,1 26,x 27,o 28,x 29,p 30,q 31,r 32,1 33,s 46,z	profile for virtual router number of LLC up to 200 (0-200) 1st entry in C17 Rec 0 up to 200 (0-200) intermediate routing routing option: 0 static route; 1 static route & default one. static route type number of remote Ip @ range C31 Rec 14 1st entry in C31 Rec 14 entry in C31 Rec 15 for the default route (0-3) entry index in C31 Rec 7 for the Wan Ip @ always set to 1.(number of local range of Ip@) entry index in C31 Rec 8, 11 for the LAN Ip @ and its behavior. subscriber number.(default one is 95).	Class 9 Rec 5 SAP (virtual line) for VR
			x	1,1,0,y
				y=	60 if on M0
					61 if on M1
					62 if on M2

Class 31 Rec 7 Recurrence for the Wan Ip address (Host Id)		Class 31 Rec 19 for Community string
r		0 for read-only 1 for read/write 2 for trap messages
Class 31 Rec 8 Recurrence for the LAN Ip address.
s
Class 31 Rec 11 Recurrence for the behavior of the LAN Ip @
s	t,0,0,0,0,0
t:	entry index in C31 Rec 12,13 for the Local range of Ip @
Class 31 Rec 12 Recurrence where is define the LOWEST Ip @ of the range.		Class 31 Rec 20 for Client @(up to 6) X A, B, C, D, z A, B, C, D is the IP @ z: entry index(0-2) in C31 Rec 19
Class 31 Rec 13 Recurrence where is define the HIGHEST Ip @ of the range.
Class 31 Rec 14 Mapping between the range of Ip @ and the LLC Id.		Class 31 Rec 21 for Trap DA (up to 3)
p	A,B,0
	A:entry index in C31 Rec (12, 13) B: LLC Id(1-199) 0: field not used, always 0.


Class 31 Rec 15 up to three entries		Class 8 Rec 0 Remote X.121 @
q	B
	B: for the LLC Id of the default route	Class 8 Rec 1 Fast select
Class 17 Rec 0 Mapping between the LLC Id and the remote X.121 @.		x 01, 80 optional
o	A, B, C, 0 A: first entry in C8 B: LLC type; 0 calling, 1 called, 2 mixed, 3 datagramme C: LLC Id 0: field not used always 0.	Class 8 Rec 4 Slow call timer
		x 0 inactive
		1 steps of 10s
		Class 8 Rec 5 Encapsulation type
		x CC, 08, 00 for IP and RFC 1356 or Virtual Router.
		CC, 03, CC for IP and FRA

ISDN

Components

ISDN components include Terminals, Terminal Adapters, Network Termination devices, Line Termination equipment, and Exchange Termination equipment (see Figure 6-49).

TE1: Specialized ISDN terminals are referred to as terminal equipment type 1.
TE2: Non ISDN terminals are referred to as terminal equipment type2. TE2s connect to the ISDN network through a Terminal Adapter.
TA: The ISDN TA allows (TE2s) to operate over ISDN lines. It can either be a stand-alone device or a board inside TE2. If implemented as a stand-alone device, the TE2 connects to the TA via a standard physical-layer interface (for example,V24,V35,....).

Beyond the TE1 and TE2 devices, the connection point in the ISDN network are the Network Termination devices.

NT1: Network termination 1 performs the adaptation of the network signals on the subscriber line.
NT2: Network termination 2 is a more complicated device, typically found in digital private branch exchanges(PBXs).

Beyond the NT1 and NT2, the next connection points are:

LT: Line termination.
ET: Exchange termination.

Reference Points

A number of reference points are specified in ISDN. These reference points define logical interface between functional groupings (See Figure 6-50)

R - The reference point between non ISDN-equipment and a TA.
S - The reference point between NT2 equipment and TE1 or the TA equipment.
T - The reference point between NT1 equipment and NT2 equipment.
U - The reference point between NT1 equipment and ISDN network.
V - The reference point between LT and ET.

Figure 6-50: Communication Equipment in connectionless Mode

The ISDN exchange terminators are interconnected via communication devices using Common Channel Signaling System Nº 7 (CCSS#7). This is a connectionless mode.

Access

Two main interface structures have been defined, the Basic interface and the Primary.

ISDN's Basic Rate Interface (BRI) service offers two B-channels and one D-channel (2B + D). BRI B-channel service operates at 64 kbps and is meant to carry user data. BRI D-channel service operates at 16 kbps and is meant to carry control and signaling information. The total rate of a BRI interface is 192 kbps. The BRI physical layer specification is ITU-T I.430.
ISDN's Primary Rate Interface (PRI) service offers 23 B-channels and one D-channel in North America and Japan (T1), giving a total rate of 1, 544 kbps (the PRI D-channel runs at 64 kbps). ISDN PRI in Europe, Australia, and the other parts of the world provides 30 B plus one 64 kbps D-channel (E1) and a total interface rate of 2, 048 kbps. The PRI physical layer specification is ITU-T I.431.

Table 6-15: ISDN

Access BRI PRI

Channel
2B + D

30 B + D

Data rate
2*64 + 16 = 144 kbps

30*64 + 64 = 1984 kps

Real rate
144 + 48 = 198 kbps

1984 + 64 = 2048 kbps

Access

The difference between real rate and data rate is due to the fact that in addition to these channels ISDN provides for framing control and other overhead bits.

Connector

The interface connector used for the TEs and NTs is an 8 pin so-called RJ connector.

This connector is specified in the ISO 8877 standard. The RJ connector for ISDN is denoted as RJ-45 connector. The layout of this connector is shown in Figure 6-51. The maximum number of wires in the interface is 8, but mostly only 4 wires are used.

Figure 6-51: ISDN
Connector
Table 6-16: Connector Pinout

Pin Signal

1

not used

2

not used

3

TD

4

TD

5

RD

6

RD

7

not used

8

not used

Via the balanced transmit and received lines, power is distributed from NT towards the TEs.This power distribution takes place via a so-called phantom circuit.

This power source has a nominal voltage of 40 volts and should supply a power of at least 420 milliwatts.

Figure 6-52: ISDN Recommendations for Protocols in Different Layers

Figure 6-52 illustrates the ISDN recommendations for the protocols in the different layers. Levels 2 and 3 are significant for D-channel.

Physical layer

For the physical layer, two protocols are possible:

I.430 describes the protocol for a Basic interface structure (BRI=2B+D).
I.431 describes the protocol for the Primary rate interface structure (PRI=30B+D, 23B+D).

These protocols describe how to transfer the information across the medium.The protocol of the physical layer is based on Time Division Multiplexing (TDM).

Basic rate interface structure

The bits are grouped together into frames of 48 bits each.

The nominal bit rate is 192 kbps. Every 250 µs one frame is transmitted. This results in a transmission of 4000 frames per second.

ISDN physical-layer frame format differs depending on whether the frame is outbound (from terminal to network) or is inbound (from network to terminal).

Primary rate interface structure

The Primary rate interface (E1) has a frame structure that consist of 32 time slots of 8 bits each.

The number of bits in a frame is 256. Every 125 µs one frame is transmitted. This results in a transmission rate of 8000 frames per second are transmitted, which results in a nominal bit rate of 2048 kbps.

DATA Link Layer

Layer 2 of the ISDN signaling protocol is Link Access Procedure, D-channel, also known as LAP-D. LAP-D is similar to High-level Data Link Control (HDLC) and Access Procedure Balanced (LAP-B).

As LAP-D's extended acronym indicates, it is used across the D-channel to ensure that control and signaling information flows and is received properly. LAP-D's frame format (see Figure 6-53) is very similar to that of HDLC and like HDLC, LAP-D uses Supervisory, Information and Unnumbered frames. The contention mechanism used on D-channel is the Carrier Sense Multiple Access - Collision Resolution (CSMA/CR).

The LAP-D protocol is formally specified in ITU-T I.441 (= Q921).

Figure 6-53: Data Link Layer

DLCI: Data Link Control Identifier

SAPI: Service Access Point Identifier

E/A: Address Field Expansion Bit D

C/R: Command/Response Bit

TEI: Terminal End Point Identifier

DSS1 : Digital Signaling System one (D protocol).

Figure 6-54: LAP-D
Address Field

The LAP-D address field is two bytes long. The address field identifies the intended receiver of the command frame or the transmitter. The LSB of the first byte is '0' indicating an extension address of the address field. The LSB of the second byte is a '1' indicating the end of the address field.

C/R bit indicates whether a frame is a command or a response. The user side will send commands with the C/R bit set to '0' and responses with the C/R bit set to '1'. The network side will do the opposite.

SAPI

The SAPI field identifies the Service Access Point (SAP) where the Data Link Layer services are provided to the layer 3 entities. The SAPI field enables 64 different SAPs to be addressed. Table 6-17 gives an overview of the possible SAPI values.

Table 6-17: Overview of Possible SAPI Values

SAPI RELATED ENTITY

0

Call control procedure

1

Packet communication protocol Q.931

16

Packet communication protocol X.25

63

Data Link Layer management procedures

XX

Reserved for further standardization

TEI

The TEI field identifies the network entity for which the frame is intended or from which the frame is coming. The TEI field allows the addressing of 128 different TEIs.Table 6-18 gives an overview of the possible TEI values.

Table 6-18: Overview of the Possible TEI Values

TEI RELATED ENTITY

0—>63

Non automatic TEI assignment user equipment

64—>126

Automatic TEI assignment user equipment

127

Broadcast TEI

The Control field is two bytes for Information frames and Supervisory frames and one byte for Unnumbered frames. Noted that only the Set Asynchronous Balanced Mode Extended is used.

The FCS is based on a Cyclic Redundancy Check method. It is generated over the Address field, the Control field and the Information field.

Network Layer

The network layer has been described in the I.451 (Q=931) recommendation. The protocol used is D protocol. Figure 6-55 shows the general message structure.

Figure 6-55: General Message Structure

The first three parts are common to all messages and must always be present. The last part is specific for each message type.

Protocol Discriminator

The purpose of the protocol discriminator is to distinguish messages for user-network call control from other messages within this protocol and others standards. Table 6-19 gives an overview of the possible value.

Table 6-19:

VALUE USE

09—>0F

Other messages within L451

10—>3F

50—>FE

X.25 in D-channel

Overview of Possible Values

Call Reference

Table 6-20:

0 0 0 0 Length of Call Reference Value

Flag

Call Reference Value

Call Reference

The purpose of the call reference flag is to identify the call or facility registration. The call reference flag can have the values '0' and '1'. The originating side sets the call reference flag to '0'.The destination side always sets the call reference flag to '1'.

Message type element

The purpose of the message type is to specify the function of the message being sent. The message type is the third part of every message. The message type field consists of one byte. Bit 8 is reserved for extension.

ex: 05 set-up
07 connect

Information Elements

The information elements carry the actual signaling information between the subscriber and the network. For the information elements, two categories are possible.

Single byte information element
Variable length information element

Single Byte

Table 6-21: Single Byte

1

Information
Identifier

Contents of Information
Elements

Information Element

The MSB is set to 1.This indicates a single byte information element.

Variable length

Table 6-22: Variable Length Information Element

0

Information Identifier

Length (Byte)

Contents

The MSB is set to 0. This indicates a variable length information element.

The information elements are relative to:

+ Bearer (series I.230).
+ Tele-Services (series I.240) + Supplementary Services (series I.250).

Numbering plan

Figure 6-56 shows the numbering plan. I.330 defines the dialing and addressing rules, I.331 defines the numbering plan (E.164).

Figure 6-56:
Numbering Plan

Prefix

The prefix must be used when making an international connection.

Country Code

The country code is used to select the country of destination.

National Destination

The national destination is used to select a geographical location within the selected country.

Subscriber number

The subscriber number is used to identify the user within the selected geographical place.

Table of Contents

Protocols