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Content for  TS 44.065  Word version:  16.0.0

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1  ScopeWord‑p. 7

The present document provides the description of the Subnetwork Dependent Convergence Protocol (SNDCP) for the General Packet Radio Service (GPRS).
The user of the services provided by SNDCP is a packet data protocol (PDP) at the mobile Station (MS) or the Relay at the Serving GPRS Support Node (SGSN). Additionally, a control entity, e.g. AT command interpreter, may be an SNDCP user. SNDCP uses the services provided by the Logical Link Control (LLC) layer [4] and the Session Management (SM) sub-layer [2].
The main functions of SNDCP are:
  • Multiplexing of several PDPs.
  • Compression / decompression of user data.
  • Compression / decompression of protocol control information.
  • Segmentation of a network protocol data unit (N PDU) into Logical Link Control Protocol Data Units (LL PDUs) and re-assembly of LL PDUs into an N PDU.
3GPP TS 44.065 is applicable to GPRS MS and SGSN.
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2  References

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.
  • References are either specific (identified by date of publication, edition number, version number, etc.) or non specific.
  • For a specific reference, subsequent revisions do not apply.
  • For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.
[1]
TR 21.905: "Vocabulary for 3GPP Specifications".
[2]
TS 22.060: "General Packet Radio Service (GPRS); Service Description; Stage 1".
[3]
TS 23.060: "General Packet Radio Service (GPRS); Service Description; Stage 2".
[4]
TS 24.007: "Mobile radio interface signalling layer 3; General aspects".
[5]
TS 44.018: "Mobile radio interface; Layer 3 specification; Radio Resource Control Protocol".
[5a]
TS 24.008: "Mobile radio interface; Layer 3 specification; Core Network Protocols; Stage 3".
[6]
TS 44.064: "General Packet Radio Service (GPRS); Mobile Station - Serving GPRS Support Node (MS-SGSN) Logical Link Control (LLC) layer specification".
[7]
TS 29.060: "General Packet Radio Service (GPRS); GPRS Tunnelling Protocol (GTP) across the Gn and Gp Interface".
[8]
ITU-T Recommendation V.42 bis: "Data compression procedures for data circuit-terminating equipment (DCE) using error correcting procedures".
[9]
RFC 1144:  "Compressing TCP/IP headers for low-speed serial links".
[10]
RFC 2507:  "IP Header Compression".
[11]
ITU-T Recommendation V.44: "Data compression procedures".
[12]
RFC 5795:  "RObust Header Compression (ROHC): Framework"..
[13]
RFC 3241:  "Robust Header Compression (ROHC) over PPP"..
[14]
"RObust Header Compression (ROHC) Profile Identifiers". IANA registry at: http://www.iana.org/assignments/rohc-pro-ids
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3  Definitions and abbreviationsWord‑p. 8

3.1  Definitions

For the purposes of the present document, the terms and definitions given in TR 21.905 and TS 22.060 and the following apply:
Feedback N PDU:
an N-PDU which is generated by an SNDCP protocol control information compression entity or data compression entity and contains only control information for its peer compression entity, but no payload from any SNDCP user.
N201:
LLC layer parameter (see TS 44.064 for clarity). Defines maximum number of octets in the information field of LL PDU. Separate values are applicable for I. (see N201-I), U and UI (see N201-U) LL PDUs.
N201-I:
LLC layer parameter (see TS 44.064 for clarity). Defines maximum number of octets available to a SN DATA PDU for a specific SAPI.
N201-U:
LLC layer parameter (see TS 44.064 for clarity). Defines maximum number of octets available to a SN UNITDATA PDU for a specific SAPI. If integrity protection is used, (N201-U - 4) defines maximum number of octets available to an SN-UNITDATA PDU for a specific SAPI.
N PDU number:
a sequence number assigned to N PDUs per NSAPI.
NSAPI:
for each SN PDU the NSAPI is an index to the PDP context of the PDP that is using the services provided by the SNDCP layer.
Receive N PDU number:
the value of the N PDU number expected in the next N PDU received by an NSAPI using acknowledged peer-to-peer LLC operation.
Recovery state:
a state for an NSAPI in which duplicated received N PDUs shall be detected and discarded. The recovery state only applies to NSAPIs using acknowledged peer-to-peer LLC operation.
SAPI:
identifies the Service Access Point that the SN PDU is using at the LLC layer.
Segment number:
a sequence number assigned to SN UNITDATA PDUs carrying segments of an N PDU.
Send N PDU number:
the value to be assigned as the N PDU number to the next N PDU received from the SNDCP user by an NSAPI using acknowledged peer-to-peer LLC operation.
Send N PDU number (unacknowledged):
the value to be assigned as the N PDU number to the next N PDU received from the SNDCP user by an NSAPI using unacknowledged peer-to-peer LLC operation.
SNDCP entity:
handles the service functions provided by the SNDCP layer. The SNDCP entity is temporary logical link identity specific.
SNDCP management entity:
handles communication with SM sub-layer and controls the operation of the SNDCP entity.
SNDCP user:
protocol entity that is using the services provided by the SNDCP layer. PDP entities and control entities, e.g. AT command interpreter, are the SNDCP users at the MS. Relay entity is the SNDCP user at the SGSN.
SNDCP XID block:
the collection of SNDCP XID parameters being negotiated. It is transferred by the LL XID and LL ESTABLISH primitives between SNDCP and LLC.
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3.2  AbbreviationsWord‑p. 9

For the purposes of the present document, the abbreviations given in TS 21.905, TS 22.060, and TS 23.060, and the following apply:
CID
Context Identifier
DCOMP
Identifier of the user data compression algorithm used for the N PDU
ESP
Encapsulating Security Payload
F
First segment indicator bit
GMM
GPRS Mobility Management
IP
Internet Protocol
LLC
Logical Link Control
LSB
Least Significant Bits
M
More bit used to indicate the last segment of N PDU
N PDU
Network Protocol Data Unit
MRRU
Maximum Reconstructed Reception Unit
MSB
Most Significant Bits
NSAPI
Network Layer Service Access Point Identifier
P
Propose bit
PCOMP
Identifier of the protocol control information compression algorithm used for the N PDU
PDP
Packet Data Protocol (e.g. IPv4 or IPv6)
PDU
Protocol Data Unit
PID
Protocol Identifier
PTP
Point to Point
QoS
Quality of Service
ROHC
RObust Header Compression
RTP
Real Time Protocol
SAPI
Service Access Point Identifier
SDU
Service Data Unit
SGSN
Serving GPRS Support Node
SM
Session Management
SNDCP
Subnetwork Dependent Convergence Protocol
SNSM
SNDCP-SM
TCP
Transmission Control Protocol
TLLI
Temporary Logical Link Identifier
X
Spare bit
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4  General

The present document describes the functionality of the GPRS SNDCP. The overall GPRS logical architecture is defined in TS 23.060. Location of the SNDCP in GPRS protocol stack can be seen in Figure 1.
(not reproduced yet)
Figure 1: GPRS protocol stack
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Network layer protocols are intended to be capable of operating over services derived from a wide variety of subnetworks and data links. GPRS supports several network layer protocols providing protocol transparency for the users of the service. Introduction of new network layer protocols to be transferred over GPRS shall be possible without any changes to GPRS. Therefore, all functions related to transfer of Network layer Protocol Data Units (N PDUs) shall be carried out in a transparent way by the GPRS network entities. This is one of the requirements for GPRS SNDCP.
Another requirement for the SNDCP is to provide functions that help to improve channel efficiency. This requirement is fulfilled by means of compression techniques.
The set of protocol entities above SNDCP consists of commonly used network protocols. They all use the same SNDCP entity, which then performs multiplexing of data coming from different sources to be sent using the service provided by the LLC layer (Figure 2). The Network Service Access Point Identifier (Nsapi) Is An Index To The Pdp Context (See TS 23.060) of the PDP that is using the services provided by SNDCP. One PDP may have several PDP contexts and NSAPIs. However, it is possible that each allocated NSAPI is used by separate PDP. Each active NSAPI shall use the services provided by the Service Access Point Identifier (SAPI) in the LLC layer. Several NSAPIs may be associated with the same SAPI.
Since the adaptation of different network layer protocols to SNDCP is implementation dependent, it is not defined in the present document.
(not reproduced yet)
Figure 2: Example for multiplexing of different protocols
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