Chap07 sndcp 03t_kh
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The document describes the Subnetwork Dependent Convergence Protocol (SNDCP) layer in GPRS networks. It discusses the functional description and services of SNDCP including compression of headers and data, segmentation and reassembly. It also describes SNDCP message flows for data transfer and XID negotiation, SNDCP header formats and fields, and provides an example SNDCP header.
Chap07 sndcp 03t_kh
- 1. Chapter 7 SNDCP Contents: 7.1 Functional description of the SNDCP layer 1. Functions of the SNDCP layer 2. SNDCP services 3. Compression of header 4. Data compression Segmentation 7.2 Messages and message flows 1. SNDCP message flows for data transfer 2. SNDCP message flows: XID negotiation 3. SNDCP header formats 4. SNDCP header fields 5. SNDCP Header example
- 2. Chapter 7 SNDCP 7.1 Functional description of the SNDCP layer 1. Functions of the SNDCP layer 2. SNDCP services 3. Compression of header 4. Data compression Segmentation
- 3. Subnetwork Dependant Convergence Protocol Transparent transfer of N-PDUs Transparent transfer of N-PDUs Network layer protocols like e.g. IP v4 or v6 (until Rel 98 for X 25) 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 must be possible without any changes to GPRS. Therefore, all functions related to transfer of Network layer Protocol Data Units (N-PDUs) are carried out in a transparent way by the GPRS network entities. A compression technique as an optional function of the Datacompression/decompression Data compression/decompressionSNDCP improves the channel efficiency. The compression / Application TCP/UDP - -of user data of user data decompression is done for the user data as well as for the protocol - -of protocol control information of protocol control information control information (e. g. TCP/IP header). Compression parameters IP v 4/6 are negotiated between the MS and the SGSN (XID parameters). SNDCP SNDCP Segmentation and Re-assembly This task comprises Segmentation and Re-assembly the segmentation of a LLC LLC Network Protocol Data Unit (N-PDU) into a Logical Link Control Relay Protocol Data Units (LL-PDUs) and the reassemble of a LL-PDUs into RLC RLC BSSGP BSSGP a N-PDU. Multiplexing of PDPs Multiplexing of PDPs NS NS MAC MAC FR FR This includes the multiplexing of N-PDUs from e.g. several network GSM RF GSM RF L1 L1 layer entities onto the appropriate LLC connection Um Gb Buffering Buffering MS BSS SGSN at the SNDCP for acknowledged service.
- 4. SNDCP services N-PDU Service primitives used for communication between header data the SNDCP layer and other layers: SN-DATA.Request Control MBMS PDP PDP SNDCP users SN-UNITDATA.Request Entity control info control info 1 5 6 15 NSAPI compression decompression SNDCP layer SNDCP entity SNDCP data data management SNDCP entity compression decompression entity SNSM segmentation reassembly Session LL3 LL5 LL9 LL11 SAPI Management entity LL-DATA.Request LL-DATA .Confirm LLC layer LL-UNITDATA.Request SN-PDU SNSM - SNDCP Session Management LLC MBMS - Multimedia Broadcast/Multicast Service
- 5. Compression of header The protocol control information compression/decompression is an optional SNDCP control info features. Up to now, only TCP/IP-header compression has been specified for protocol compression control information compression. The compression applies to both SN-DATA (acknowledged mode) and SN-UNITDATA (unacknowledged mode) primitives. Negotiation of the supported algorithms and their parameters is carried out between MS and SGSN using the XID-parameter. header data compressed header compression algorithms defined in Rel 6: TCP/IP header compression (RFC1144) TCP/IP (IPv4) header compression or header TCP/IP and UDP/IP header compression (RFC 2507) or header compression for IPv4 and IPv6 Robust Header Compression (ROHC, RFC 3095) in RFC 3095 for RTP/UDP/IP, UDP/IP, ESP/IP and uncompressed
- 6. Data compression Data compression is an optional SNDCP feature. Data compression applies to both SN ‑DATA data and SN‑UNITDATA primitives. Data compression, if used, shall be performed on the entire N ‑PDU, including the possibly compression compressed protocol control information. Several NSAPIs may use a common data compression entity, i.e. the same compression algorithm and the same dictionary. Separate data compression entities shall be used for acknowledged (SN‑DATA) and unacknowledged (SN ‑UNITDATA).The negotiation of the supported algorithms and their parameters shall be carried out using the XID parameter. header data Compressed header + data Compressed (Compressed header + data) Compression may work in UL, DL, or UL + DL or not at all compression algorithms defined in Rel 6: V.42 bis data compression or V.44 data compression V.42 bis and V.44 data compression are ITU‑T Recommendations
- 7. Segmentation A (possibly compressed) N-PDU is segmented into one or more SN-PDUs. The length of each SN-PDU is not greater than N201-I (for acknowledged mode) or N201-U (for unacknowledged segmentation mode). The F bit in the SNDCP header is set to 1 for the first segment, and 0 for all subsequent segments. For unacknowledged peer-to-peer LLC operations, DCOMP and PCOMP are included in the header when the F bit is set to 1, and are not included when the F bit is set to 0. For acknowledged peer-to-peer LLC operation, DCOMP, PCOMP and N-PDU number (#1, #2, ...) are included in the header when the F bit is set to 1, and are not included when the F bit is set to 0. The M bit in the SNDCP header is set to 0 for the last segment, and 1 for all previous segments. If only one SN-PDU is generated for an N-PDU, the F bit is set to 1 and the M bit set to 0. Compressed N-PDU: Next Compressed N-PDU: 1. Segment 2. Segment 3. Segment N201 N201 N201 N201 N201 SNDCP header F=1, M=1 F=0, M=1 F=0, M=0
- 8. Chapter 7 SNDCP 7.2 Messages and message flows 1. SNDCP message flows for data transfer 2. SNDCP message flows: XID negotiation 3. SNDCP header formats 4. SNDCP header fields 5. SNDCP Header example
- 9. SNDCP message flows for data transfer Unacknowledged mode SNDCP SNDCP SNDCP LLC LLC SNDCP Unser User SN-UNITDATA .req LL-UNITDATA .req UI (SN-UNITDATA LL-UNITDATA PDU) SN-UNITDATA .ind .ind Acknowledged Mode SNDCP LLC LLC SNDCP SNDCP SNDCP User User SN-DATA.req LL-DATA.req I (SN-DATA PDU) SN-Data PDUs LL-DATA.ind are buffered! SN-DATA.ind Ackn LL-DATA.res LL-DATA.conf
- 10. SNDCP message flows: XID negotiation Negotiation of XID parameters between peer SNDCP entities is carried out to ensure optimized information transfer. The parameters are called SNDCP exchange identity (XID) parameters. SNDCP XID parameter negotiation can be initiated by the SNDCP entity at the MS or at the SGSN. If SNDCP XID parameters are to be changed, SNDCP XID negotiation is initiated prior to data transfer - the MS initiates SNDCP XID negotiation upon receipt of SNSM-ACTIVATE.indication; the SGSN initiates SNDCP XID negotiation upon receipt of the SNSM-MODIFY.indication primitive if an NSAPI has been created (in case of an Inter-SGSN Routing Area Update), or if the change in QoS profile to an existing NSAPI results in a change in compressor(s) used by the NSAPI. SNDCP LLC LLC SNDCP SNDCP SNDCP Unser User SN-XID.req LL-XID.req XID (SN-XID par.) Currently not LL-XID.ind applicable SN-XID.ind only one SN-XID.res LL-XID.res version!! XID (SN-XID par.) LL-XID.conf SN-XID.conf XID Parameter Parameter Sense of Length Format Range Default value Units name Type negotiation SNDCP Version 0 1 0000bbbb 0-15 0 - down number Data Compression 1 variable Depends on type of data compression: V.42 bis or V.44 Protocol Control Information 2 variable Depends on type of header compression: RFC 1144, RFC 2507 or ROHC Compression
- 11. SNDCP header formats Unacknowledged mode Acknowledged mode SN-PDU Type (T): T=0 SN-DATA PDU T=1 SN-UNITDATA PDU F= 1 first Segment of N-PDU: F= 1 first Segment of N-PDU: 7 6 5 4 3 2 1 0 7 6 5 4 3 2 1 0 X F=1 T=1 M NSAPI 1 1 X F=1 T=0 M NSAPI 2 2 DCOMP PCOMP DCOMP PCOMP 3 3 Segment number First segment First segment N PDU number N-PDU number 4 4 Indicator bit (F) Indicator bit (F) 5 … … Data segment n Data segment n : F= 0 not first Segment of N-PDU: F= 0 not first Segment of N-PDU: 1 1 X F=0 T=1 M NSAPI X F=0 T=0 M NSAPI 2 2 Seg. number=2 N PDU number 3 … Data segment n 4 … Data segment n The M -bit indicates if there are more SN PDUs to come M=0 last segment of that N-PDU M=1 more segments to follow
- 12. SNDCP header fields II X -spare bit .Is set to 0. If SN-PDU is received with the Spare bit set to 1, the field is ignored without error notification. NSAPI 0 Escape mechanism for future extensions 1 Point-to-Multipoint Multicast (PTM-M) information Network Service 2-4 Reserved for future use Access 5-15 Dynamically allocated NSAPI value Point SN-PDU with an unallocated NSAPI value are ignored by the receiving SNDCP entity without error Identifier notification. DCOMP 0 no compression Data 1-14 Points to the data compression identifier negotiated dynamically compression 15 Reserved for future extensions coding SN-PDUs with an unallocated DCOMP value are ignored by the receiving SNDCP entity without error notification. PCOMP 0 no compression Protocol control 1-14 Points to the protocol control information compression identifier negotiated dynamically information compression 15 Reserved for future extensions coding SN-PDUs with an unallocated PCOMP value are ignored by the receiving SNDCP entity without error notification.
- 13. SNDCP header fields II Segmentation and reassembly in acknowledged mode The transmitting SNDCP entity segments an N-PDU into an ordered sequence of one or more SN-PDUs. The More bit (M) is used to identify the last segment. N PDU number 0-255 N-PDU number of the N-PDU. Segmentation and reassembly in unacknowledged mode A segment number is a sequence number assigned to each SN-UNITDATA PDU. The sequence number is set to 0 in the first SN-UNITDATA PDU of an N-PDU, and incremented by 1 for each subsequent SN-UNITDATA PDU. The value range for the sequence number is from 0...15. The received segments belonging to the same N-PDU are re-ordered, if possible. If a timer (implementation dependent) elapses before all segments are received, the segments are discarded. N PDU number 0-4095 N-PDU number of the N-PDU 0-15 Sequence number for segments carrying a N-PDU Segment number N-PDU 202 N-PDU 203 N-PDU 204 Seg 0 Seg 1 Seg 2 Seg 0 Seg 1
- 14. SNDCP Header example |GPRS SNDCP, SMG#31, 04.65 V6.7.0 (TS 101 297) (SNDCP670) UDT (= SN UNITDATA) |SN UNITDATA |----0101 |NSAPI |dynamically allocated NSAPI 5 |---1---- |More bit |More segments to follow |--1----- |SN-PDU type |SN-UNITDATA PDU |-0------ |First segment indicator |Not first segment |0------- |Spare |0 |N-PDU Number |----0000 |N-PDU number |0 |0010---- |Segment number |2 |11110101 |N-PDU number continued |245