6 Elements of procedures and formats of fields

04.643GPPGeneral Packet Radio Service (GPRS)Mobile Station - Serving GPRS Support Node (MS-SGSN) Logical Link Control (LLC) layer specificationTS

6.1 General

The elements of procedures define the commands and responses that are used on the logical link connections between the MS and SGSN.

Procedures are derived from these elements of procedures and are described in clause 8.

If a bit position is marked as "spare", it shall be coded as 0. A spare bit is indicated with an ‘X’ in the format figures in this clause. For future compatibility reasons, an entity receiving frames, where spare bit positions are coded otherwise, shall ignore those values without notification of any error.

6.2 Address field format and variables

The address field consists of

– the Protocol Discriminator bit PD;

– the Command/Response bit C/R; and

– the SAPI.

The format of the address field is shown in Figure 7.

Figure 7: Address field format

6.2.1 Protocol Discriminator bit (PD)

The PD bit indicates whether a frame is an LLC frame or belongs to a different protocol. LLC frames shall have the PD bit set to 0. If a frame with the PD bit set to 1 is received, then it shall be treated as an invalid frame, see subclause 5.8.

6.2.2 Command/Response bit (C/R)

The C/R bit identifies a frame as either a command or a response. The MS side shall send commands with the C/R bit set to 0, and responses with the C/R bit set to 1. The SGSN side shall do the opposite; i.e., commands are sent with C/R set to 1, and responses are sent with C/R set to 0. The combinations for the SGSN side and MS side are shown in Table 1.

Table 1: C/R field bit usage

Type

Direction

C/R value

Command

SGSN side to MS side

1

Command

MS side to SGSN side

0

Response

SGSN side to MS side

0

Response

MS side to SGSN side

1

6.2.3 Service Access Point Identifier (SAPI)

SAPI identifies a point at which LLC services are provided by an LLE to a layer‑3 entity. Consequently, SAPI identifies an LLE that should process an LLC frame and also a layer‑3 entity that is to receive information carried by the LLC frame.

SAPI allows 16 service access points to be specified. The SAPI values are allocated as shown in Table 2.

Table 2: Allocation of SAPI values

SAPI

Related Service

SAP Name

0000

Reserved

0001

GPRS Mobility Management

LLGMM

0010

Reserved

0011

User data 3

LL3

0100

Reserved

0101

User data 5

LL5

0110

Reserved

0111

SMS

LLSMS

1000

Reserved

1001

User data 9

LL9

1010

Reserved

1011

User data 11

LL11

1100

Reserved

1101

Reserved

1110

Reserved

1111

Reserved

6.3 Control field formats, parameters, and variables

The control field identifies the type of frame. Four types of control field formats are specified:

– confirmed information transfer (I format);

– supervisory functions (S format);

– unconfirmed information transfer (UI format); and

– control functions (U format).

The control field formats for LLC are shown in Figure 8 and Figure 9. For definition of values for supervisory function bits and unnumbered function bits, see Table 4.

Figure 8: Control field format

Figure 9: SACK I and S frame control field format

6.3.1 Information transfer format ‑ I

The I format shall be used to perform an information transfer between layer‑3 entities. The functions of N(S), N(R), and A are independent; that is, each I frame has an N(S) sequence number, an N(R) sequence number that may or may not acknowledge additional I frames received by the LLE, and an A bit that may be set to 0 or 1. The use of N(S), N(R), and A is defined in clause 8.

Each I frame also contains supervisory information, in effect "piggy-backing" an S frame with each I frame, so that it may be considered to be an I+S frame.

6.3.2 Supervisory format ‑ S

The S format shall be used to perform logical link supervisory control functions such as acknowledge I frames and request a temporary suspension of I‑frame transmission. The functions of N(R) and the A bit are independent; that is, each supervisory frame has an N(R) sequence number that may or may not acknowledge additional I frames received by the LLE, and an A bit that may be set to 0 or 1. The use of N(R) and the A bit is described in clause 8.

6.3.3 Unconfirmed Information format ‑ UI

The UI format shall be used to perform an information transfer between layer‑3 entities without acknowledgement. No verification of sequence numbers is performed for UI frames. Therefore, a UI frame may be lost without notification to the layer‑3 entity if a logical link exception occurs during transmission of the frame. The information field may be encrypted or not as indicated by the E bit (see subclause 6.3.5.5.1). The frame also includes an PM bit that allows the transfer of unprotected information (see subclause 6.3.5.5.2).

6.3.4 Unnumbered format ‑ U

The U format shall be used to provide additional logical link control functions. This format contains no sequence number. The format includes a P/F bit that may be set to 0 or 1.

6.3.5 Control field parameters and associated state variables

The various parameters associated with the control field formats are described in this subclause.

6.3.5.1 Poll/Final bit (P/F)

All U frames contain the Poll/Final (P/F) bit. The P/F bit serves a function in both command frames and response frames. In command frames the P/F bit is referred to as the P bit. In response frames it is referred to as the F bit.

The P bit set to 1 is used by an LLE to solicit (poll) a response frame from the peer LLE. The F bit set to 1 is used by an LLE to indicate the response frame transmitted as a result of a soliciting (poll) command.

The use of the P/F bit is described in clause 8.

6.3.5.2 Acknowledgement request bit (A)

All I and S frames contain the Acknowledgement Request (A) bit.

The A bit set to 1 is used by an LLE to solicit an acknowledgement (i.e., an I+S or S frame) from the peer LLE. The A bit set to 0 is used by an LLE to indicate that the peer LLE is not requested to send an acknowledgement.

The use of the A bit is described in clause 8.

6.3.5.3 Modulus

Each I and UI frame is sequentially numbered by a sequence number that may have the value 0 through 511.

Arithmetic acting on parameters and variables that are related to such sequence numbers operates modulo 512 (i.e., N(S), N(R), N(U), V(S), V(R), V(A), V(U), V(UR); see the following subclauses).

NOTE: Modulo 512 operation on negative numbers is performed by adding multiples of 512 to the negative number until the result becomes non-negative. Then common modulo 512 operation is applied.

6.3.5.4 ABM variables and sequence numbers

6.3.5.4.1 Send state variable V(S)

In Asynchronous Balanced Mode, each LLE peer shall have an associated send state variable V(S) when using I frames. V(S) denotes the sequence number of the next in-sequence I frame to be transmitted. V(S) can take on the value 0 through 511. The value of V(S) shall be incremented by 1 with each successive I frame transmission, and shall not exceed V(A) by more than the maximum number of outstanding I frames k. The value of k may be in the range 1  k  255, as defined in subclause 8.9.8. V(S) shall not be incremented when an I frame is retransmitted.

6.3.5.4.2 Acknowledge state variable V(A)

In Asynchronous Balanced Mode, each LLE peer shall have an associated acknowledge state variable V(A) when using I frame and supervisory frame commands and responses. V(A) identifies the first I frame in the transmit window, so that V(A) ‑ 1 equals N(S) of the last in-sequence acknowledged I frame. V(A) can take on the value 0 through 511. The value of V(A) shall be updated by the valid N(R) values received from its peer (see subclause 6.3.5.4.5). A valid N(R) value is one that is in the range V(A)  N(R)  V(S).

These inequalities shall be interpreted in the following way:

N(R) is valid if, and only if, ( N(R) ‑ V(A) ) mod 512  ( V(S) ‑ V(A) ) mod 512.

Furthermore, from subclause 6.3.5.4.1, ( V(S) ‑ V(A) ) mod 512  k.

6.3.5.4.3 Send sequence number N(S)

In Asynchronous Balanced Mode, only I frames contain N(S), the send sequence number of transmitted I frames. At the time that an in-sequence I frame is designated for transmission, the value of N(S) is set equal to the value of the send state variable V(S).

6.3.5.4.4 Receive state variable V(R)

In Asynchronous Balanced Mode, each LLE peer shall have an associated receive state variable V(R) when using I frame and supervisory frame commands and responses. V(R) denotes the sequence number of the next in-sequence I frame expected to be received. V(R) can take on the value 0 through 511. The value of V(R) shall be incremented by one with the receipt of an error-free, in-sequence I frame whose send sequence number N(S) equals V(R).

6.3.5.4.5 Receive sequence number N(R)

In Asynchronous Balanced Mode, all I frames and supervisory frames contain N(R), the expected send sequence number of the next in-sequence received I frame. At the time that a frame of the above types is designated for transmission, the value of N(R) is set equal to the value of the receive state variable V(R). N(R) indicates that the LLE transmitting the N(R) has correctly received all I frames numbered up to and including N(R) ‑ 1.

6.3.5.4.6 SACK bitmap R(n)

In Asynchronous Balanced Mode, all I+S and S SACK frames contain R(n), the SACK bitmap. At the time that a SACK frame is designated for transmission, the value of each bit R(n) in the bitmap shall be set to 0 or 1 depending on whether I frame number N(R) + n has been received or not. R(n) = 1 indicates that the LLE transmitting the SACK frame has correctly received I frame number N(R) + n. R(n) = 0 indicates that the LLE transmitting the SACK frame has not correctly received I frame number N(R) + n.

The SACK bitmap contains a maximum of 255 bits, or 32 octets, as shown in Figure 9. The bitmap shall be truncated so that only bitmap octets up to and including the last bitmap octet containing at least one bit set to 1 are transmitted. The trailing bitmap octets shall not be transmitted.

The I+S SACK frame contains a bitmap length indicator K. K + 1 indicates the number of octets in the bitmap. K can take any value 0 through 31.

6.3.5.4.7 I frame buffer variable B

In Asynchronous Balanced Mode, each LLE peer shall have an associated I frame buffer variable B when using I frame and supervisory frame commands and responses. The value of B has a range of 0  B  M, where M is defined in subclause 8.9.7.

Function L(x) gives the total information field length in octets of the I frame with sequence number x. B shall be incremented with L(x) of each transmitted I frame as defined in subclause 8.6.1. B shall be decremented by L(x) of each acknowledged I frame as defined in subclause 8.6.3.2.

6.3.5.4.8 Other parameters and variables

For definition and values of additional parameters and variables, see subclause 8.9.

6.3.5.5 Unacknowledged operation variables and parameters

6.3.5.5.1 Encryption mode bit (E)

The E bit is used to indicate whether the information and FCS fields of the UI frame are encrypted (ciphered) to provide user data confidentiality. The E bit is set to 1 to indicate an encrypted frame. The E bit is set to 0 to indicate a frame sent without encryption.

6.3.5.5.2 Protected Mode bit (PM)

The PM bit is used to indicate whether the FCS field shall be calculated using both the frame header and information fields.

The PM bit is set to 1 to indicate that the FCS covers the frame header and information fields.

The PM bit is set to 0 to indicate that the FCS covers only the frame header field and the first N202 octets of the information field. If the length of the information field is less than N202 octets then the FCS shall cover the complete information field. This permits UI frames to transport "unprotected" information, such that errors beyond the first N202 octets of the information field do not result in the frame being discarded.

Table 3: UI frame content

PM

E

UI frame information field

0

0

unprotected, non-ciphered information

0

1

unprotected, ciphered information

1

0

protected, non-ciphered information

1

1

protected, ciphered information

6.3.5.5.3 Unconfirmed send state variable V(U)

Each LLE peer shall have an associated unconfirmed send state variable V(U) when using UI frame commands. V(U) denotes the sequence number of the next UI frame to be transmitted. V(U) can take on the value 0 through 511. The value of V(U) shall be incremented by 1 with each successive UI frame transmission.

6.3.5.5.4 Unconfirmed sequence number N(U)

Only UI frames contain N(U), the unconfirmed sequence number of transmitted UI frames. At the time that a UI frame is designated for transmission, the value of N(U) is set equal to the value of the unconfirmed send state variable V(U).

6.3.5.5.5 Unconfirmed receive state variable V(UR)

Each LLE peer shall have an associated unconfirmed receive state variable V(UR) when using UI frame commands. V(UR) denotes the sequence number of the next in-sequence UI frame expected to be received. V(UR) can take on the value 0 through 511.

6.3.5.5.6 Other parameters and variables

The only other parameter defined for unacknowledged operation is the number of octets (N201‑U) in the information field of the UI frame. See subclause 8.9.4.

6.4 Commands and responses

The following commands and responses are used by the MS and the SGSN LLEs and are represented in Table 4. Each logical link connection shall support the appropriate set of commands and responses for the type of operation desired (see clause 8).

Those frame types not identified in Figure 8, Figure 9, or Table 4, shall be identified as having undefined command and/or response control fields, and shall be treated as defined in subclause 8.8.2.

Table 4: Commands and responses

Encoding

Format

Commands

Responses

S1

S2

M4

M3

M2

M1

RR

RR

0

0

Information +

ACK

ACK

0

1

Supervisory

RNR

RNR

1

0

SACK

SACK

1

1

DM

0

0

0

1

DISC

0

1

0

0

Unnumbered

UA

0

1

1

0

SABM

0

1

1

1

FRMR

1

0

0

0

XID

XID

1

0

1

1

The commands and responses in Table 4 are defined in the following subclauses.

6.4.1 Unnumbered (U) frames

6.4.1.1 Set Asynchronous Balanced Mode (SABM) command

The SABM unnumbered command shall be used to place the addressed MS or SGSN side into ABM acknowledged operation.

An LLE shall confirm acceptance of a SABM command by the transmission at the first opportunity of a UA response. Upon acceptance of this command, the LLE’s send state variable V(S), acknowledge state variable V(A), and receive state variable V(R), shall be set to 0. The transmission of a SABM command indicates the clearance of any exception condition, and a busy condition that was reported by the earlier transmission of an RNR frame by that same LLE.

Previously transmitted I frames that are unacknowledged when this command is actioned shall be discarded. It is the responsibility of a higher layer to recover from the possible loss of the contents of such I frames.

An information field is permitted with the SABM command. If included, the information field shall contain XID parameters. This allows the LLC peers to negotiate LLC layer parameters and layer‑3 parameters with the SABM command and UA response, using the procedure (but not the XID frames) defined in subclauses 6.4.1.6 and 8.5.3.

6.4.1.2 Disconnect (DISC) command

The DISC unnumbered command shall be transmitted in order to terminate the ABM operation.

No information field is permitted with the DISC command. Prior to executing the command, the LLE receiving the DISC command shall confirm the acceptance of a DISC command by the transmission of a UA response. The LLE sending the DISC command shall terminate the ABM operation when it receives the acknowledging UA or DM response.

Previously transmitted I frames that are unacknowledged when this command is executed shall remain unacknowledged and shall be discarded. It is the responsibility of a higher layer to recover from the possible loss of the contents of such I frames.

6.4.1.3 Unnumbered Acknowledgement (UA) response

The UA unnumbered response shall be used by an LLE to acknowledge the receipt and acceptance of the mode-setting commands (SABM or DISC). Received mode-setting commands are not actioned until the UA response is transmitted.

An information field is only permitted when UA is the response to a SABM command. The UA response shall in this case contain XID parameters with negotiated XID values, using the procedure (but not the XID frames) defined in subclauses 6.4.1.6 and 8.5.3.

The transmission of the UA response indicates the clearance of any busy condition that was reported by the earlier transmission of an RNR frame by that same LLE.

6.4.1.4 Disconnected Mode (DM) response

The DM unnumbered response shall be used by an LLE to report to its peer that the LLE is in a state such that ABM operation cannot be performed. An LLE shall transmit a DM response to any valid command received that it cannot action.

No information field is permitted with the DM response.

6.4.1.5 Frame Reject (FRMR) response

The FRMR unnumbered response may be received by an LLE as a report of a frame rejection condition not recoverable by retransmission of the identical frame:

1) receipt of a command or response control field that is undefined or not implemented (see subclause 6.4, second paragraph);

2) receipt of a supervisory or unnumbered frame with incorrect length; or

3) receipt of an I frame with an information field that exceeds the maximum established length.

An undefined control field is any of the control field encodings that are not identified in Figure 8, Figure 9, or Table 4.

An information field that immediately follows the control field and that consists of 10 octets shall be returned with this response to provide the reason for the FRMR response. This information field format is given in Figure 10. Only the first 6 octets of the control field of the rejected frame shall be sent. If the control field of the rejected frame is fewer than 6 octets, then the unused octets shall be set to 0.

Figure 10: FRMR frame information field format

The information fields defined for the FRMR response are listed in Table 5Error: Reference source not found.

Table 5: FRMR frame fields

Field

Description

Rejected frame control field

The control field of the received frame that caused the frame reject.

V(S)

The current send state variable value of the LLE reporting the rejection condition.

V(R)

The current receive state variable value of the LLE reporting the rejection condition. V(R) shall not be treated as an acknowledgement of I frames.

C/R

Set to 1 if the frame rejected was a response and set to 0 if the frame rejected was a command.

W1

Set to 1 to indicate that the control field received and returned in octets 1 and 2 was considered invalid because the frame contained an information field that is not permitted within this frame or is a supervisory or unnumbered frame with incorrect length. Bit W3 shall be set to 1 in conjunction with this bit.

W2

Set to 1 to indicate that the information field received exceeded the maximum established information field length (N201) of the LLE reporting the rejection condition.

W3

Set to 1 to indicate that the control field received and returned in octets 1 and 2 was undefined or not implemented.

W4

Set to 1 to indicate that the LLE was in ABM when reporting the rejection condition.

6.4.1.6 Exchange Identification (XID) command/response

This frame shall be used to negotiate and re-negotiate LLC layer parameters and layer‑3 parameters. XID frames can be transmitted in ADM and ABM.

The negotiation procedure is one-step, i.e., one side shall start the process by sending an XID command, offering a certain set of parameters from the applicable parameter repertoire (see Table 6) the sending entity wants to negotiate, proposing values within the allowed range. In return, the other side shall send an XID response, either confirming these parameter values by returning the requested values, or offering higher or lower ones in their place. As an optimisation, parameters confirming the requested values may be omitted from the XID response. See Table 6 for sense of negotiation. This shall end the negotiation process.

Parameters that are not included in neither the XID command nor in the XID response, shall retain their current values.

The responding side may respond with parameters that were not included in the XID command. A parameter that was not included in the XID command shall in this case be treated as if the current value of the parameter was included in the XID command. The responding side shall include such a parameter in every XID response until the parameter has been explicitly negotiated, either by responding to an XID command that included the parameter, or by explicitly including the parameter the next time an XID command is transmitted.

Both entities shall support the negotiated values, however under certain conditions one or more parameters may need to be re-negotiated (e.g., in the case of a change in SGSN).

XID frames shall always be used with the P/F bit set to 1.

Without any prior XID exchange, default values shall apply.

Negotiated XID parameters shall apply to the LLE identified by the DLCI of the XID frames used, except Version, Reset, and IOV‑UI that applies to an LLME (i.e., a TLLI), and except Layer‑3 Parameters that apply to the layer 3 above the LLE.

Table 6 lists the negotiable LLC layer parameters. Figure 11 shows the format of the XID parameter field.

Figure 11: XID parameter field format

A parameter item consists of one or two type/length octets followed by the value of that parameter. The XID Length (XL) bit indicates whether the Length field is 2 bits or 8 bits long. If XL is set to 0, then Length consists of 2 bits and type/length occupies one octet. If XL is set to 1 then Length consists of 8 bits and type/length occupies two octets. The length indicator gives the number of octets that the value actually occupies. Length shall be set to the value in Table 6 for XID parameters that do not have a variable length. The parameter items can be arranged in arbitrary order. The parameter items shall begin in the first octet of the XID information field and follow on contiguously.

Table 6: LLC layer parameter negotiation

Parameter Name

Type

Length

Format (87654321)

Range

Units

Sense of Negotiation

Version (LLC version number)

0

1

0000bbbb

0 through 15

down

IOV‑UI (ciphering Input offset value for UI frames), common for all SAPIs of a TLLI

1

4

bbbbbbbb
bbbbbbbb
bbbbbbbb
bbbbbbbb

0 through 232 ‑ 1

IOV‑I (ciphering Input offset value for I frames), for the SAPI under negotiation

2

4

bbbbbbbb
bbbbbbbb
bbbbbbbb
bbbbbbbb

0 through 232 ‑ 1

T200 (retransmission time-out)

3

2

0000bbbb
bbbbbbbb

1 through 4 095

0.1 seconds

up

N200 (maximum number of retransmissions)

4

1

0000bbbb

1 through 15

up

N201‑U (maximum information field length for U and UI frames)

5

2

00000bbb
bbbbbbbb

140 through 1 520

octets

down

N201‑I (maximum information field length for I frames)

6

2

00000bbb
bbbbbbbb

140 through 1 520

octets

down

mD (I frame buffer size in the downlink direction)

7

2

0bbbbbbb
bbbbbbbb

0, 9 through 24 320

16 octets

down

mU (I frame buffer size in the uplink direction)

8

2

0bbbbbbb
bbbbbbbb

0, 9 through 24 320

16 octets

down

kD (window size in the downlink direction)

9

1

bbbbbbbb

1 through 255

frames

down

kU (window size in the uplink direction)

10

1

bbbbbbbb

1 through 255

frames

down

Layer‑3 Parameters

11

Variable

See 3GPP TS 04.65

Reset

12

0

– The Range for N201‑U for SAPI 1 is 400 through 1 520 octets, and for SAPI 7 270 through 1 520 octets.

– All other Types and Ranges are reserved for future versions of the present document.

– The length for Layer‑3 Parameters shall be set equal to the number of octets received from layer 3. If an empty XID block is received from layer 3, the LLE shall include a zero-length Layer‑3 Parameters XID parameter in the XID parameter field to allow the receiving LLE to distinguish between LLC and layer‑3 initiated procedures.

Version shall not be negotiated while in ABM.

Reset shall only be negotiated with an XID frame, and only be transmitted in the downlink direction. If Reset is present in an XID frame, then it shall be the first XID parameter in the XID information field.

IOV‑UI shall only be negotiated in ADM. IOV‑I shall only be negotiated with SABM and UA frames. IOV‑UI and IOV‑I shall only be transmitted in the downlink direction.

T200, N200, and N201‑U can be negotiated in ADM and ABM. The new values of T200 shall only apply to timers set after the negotiation has been completed. If N201‑U is negotiated to a lower value than previously used, then any queued or new U and UI frames that violates the new value of N201‑U should be discarded and not transmitted.

N201‑I, mD, mU, kD, and kU can be negotiated to any value in Range in ADM. In ABM, N201‑I, mD, mU, kD, and kU can only be negotiated to the same or higher value as previously used.

6.4.2 Unconfirmed Information (UI) frame

6.4.2.1 Unconfirmed Information (UI) command

When a layer‑3 entity requests unacknowledged information transfer, the UI command shall be used to send information to its peer. No verification of sequence numbers is performed for UI frames. Therefore, the UI frame may be lost without notification to the layer‑3 entity if a logical link exception occurs during transmission of the command.

6.4.3 Combined Information (I) and Supervisory (S) frames

The function of the information (I) frame is to transfer, across a logical link connection, sequentially-numbered frames containing information fields provided by layer 3. This frame shall only be used in the ABM operation.

Numbered I frames shall also carry supervisory information, and are for this reason also called I+S frames. A separate S frame is sent when there is no information field to be transferred. Whether an I+S or S frame is transmitted as a command or as a response is insignificant in the ABM procedures.

6.4.3.1 Receive Ready (RR) command / response

The receive ready (RR) supervisory frame is used by an LLE to:

– indicate that it is ready to receive an I frame; and

– acknowledge previously received I frames numbered up to and including N(R) ‑ 1 (as defined in clause 8).

In addition to indicate the status of an LLE, the RR frame with the A bit set to 1 may be used by the LLE to request an acknowledgement from its peer LLE.

The transmission of an RR frame shall also indicate the clearance of any busy condition within the sending LLE that was reported by the earlier transmission of an RNR frame by the same LLE.

6.4.3.2 Acknowledgement (ACK) command / response

The ACK supervisory frame shall be used by an LLE to acknowledge a single or multiple I frames. Frames up to and including N(R) ‑ 1, and frame N(R) + 1, have been received correctly. The procedures associated with the ACK control field are defined in subclause 8.6.3.

In addition to indicate the status of an LLE, the ACK frame with the A bit set to 1 may be used by the LLE to request an acknowledgement from its peer LLE.

The transmission of an ACK frame shall also indicate the clearance of any busy condition within the sending LLE that was reported by the earlier transmission of an RNR frame by the same LLE.

6.4.3.3 Selective Acknowledgement (SACK) command / response

The SACK supervisory frame shall be used by an LLE to acknowledge a single or multiple I frames. Frames up to and including N(R) ‑ 1, and frames indicated by the SACK bitmap, have been received correctly. The format of the SACK control field is shown in Figure 9. The procedures associated with the SACK control field are defined in subclause 8.6.3.

In addition to indicate the status of an LLE, the SACK frame with the A bit set to 1 may be used by the LLE to request an acknowledgement from its peer LLE.

The transmission of a SACK frame shall also indicate the clearance of any busy condition within the sending LLE that was reported by the earlier transmission of an RNR frame by the same LLE.

6.4.3.4 Receive Not Ready (RNR) command / response

The receive not ready (RNR) supervisory frame shall be used by an LLE to indicate a busy condition; that is, a temporary inability to accept additional incoming I frames. The value of N(R) in the RNR frame acknowledges I frames numbered up to and including N(R) ‑ 1. Subsequent frames, if any, shall not be considered confirmed. The acceptance status of those is a matter of further status exchange.

In addition to indicate the status of an LLE, the RNR frame with the A bit set to 1 may be used by the LLE to request an acknowledgement from its peer LLE.