GPRS protocol stack
Introduction
This page on GPRS protocol stack describes functions of layer 1, layer 2 and layer 3 of protocol stack based on GPRS standard. As shown in figure, SM (Session Management) and GMM (GPRS Mobility Management) layers are depicted which are used for signaling. SNDCP (Sub Network dependent convergence protocol) is used for data. Above these layers there reside IP and Application layers. The data from SNDCP and GMM will go through LLC, RLC, MAC and PHY layers.
GPRS Layer 3
SM (Session management of layer 3)
It establishes data calls and negotiates QoS. Negotiated QoS parameters are reliability, delay,
throughput & precedence. It passes PDP_context_activation_request OR Modify_PDP_context_accept frame.
It adds SM header before passing the PDP to GMM layer.PDP Context consists of following fields.
-PDP Type, e.g. IPv4
-PDP Address, e.g. 192.1.1.40
-Negotiated QoS
-GGSN Address
-NSAP
GMM (GPRS Mobility Management of layer 3)
It adds GMM header to the PDP context activation request which comes from SM layer.
GPRS attach request directly fed to GMM layer which adds GMM header. GMM does following functions in general.
-Registration
-Authentication
-Attach
-Location management (Routing area update, perform cell update)
-Encryption management
-TLLI management
GMM states are IDLE, STANDBY and READY.
SNDCP (Sub network dependent convergence protocol of layer 3)
It does N-PDU compression, storage and segmentation to n201 size. Convert and segment external
network formats (N-PDUs) to sub network format (GPRS format). This format is called SN-PDUs.
It does following functions also.
-Compression of N-PDUs
-Negotiation of compression parameters between MS and sub-network.
-Management of multiple PDP context PDU transfer.
GPRS Layer 2
LLC (Logical Link Control of Layer2)
-Takes GMM PDUs as input for signaling and SN PDUs as input for data transmission.
-In addition it performs functions such as encryption, Protection and framing, storage (this feature is not available for connectionless mode), L2 data connection (connectionless mode for GMM PDUs).
-Encrypted SN PDU called LLC PDU has max. length of 1560 octets.
RLC (Radio Link Control of Layer2)
LLC PDU is divided into RLC blocks called segmentation and inserts RLC header.
RLC layer segments this LLC PDU into octets of length 23. This 23 octet RLC/MAC block has following breakup.
MAC header - 1 octet
RLC header and control octets - 2 or 3 or 4 or 6 octets
LLC PDU data - 20 or 19 or 18 or 16 octets, For CS1 operation
MAC
Inserts MAC header and take care of logical/physical channels scheduling, control and management.
RLC and MAC Control/Data Block
RLC/MAC is the block which is transmitted through Layer-1 over the air. It carries signaling/data information. BCS(Block Check Sequence) is included at the end of the block for error detection. Control block is of size 22 bytes and are encoded using CS-1 coding type.
RLC/MAC Downlink Control Block:
MAC Header- PT, RRBP, S/P, USF
Optional Bytes- RBSN, RTI, FS, AC, PR, TFI, D
Control message content
PT indicates type of data either control or data.
USF is 3 bit field and is used as multiplexing field
for more than one MSs wanted to use a single uplink time slot.
It is unique for each MS.
RBSN- Reduced Block Sequence Number, used for sequencing the RLC/MAC control blocks.
TFI(Temporary Flow Identifier)- which indicates a uplink or downlink TBF(Temporary Block Flow).
D-Direction of TBF.
RLC/MAC Uplink Control Block:
MAC Header-PT,Spare bits,R
Control message content
RLC Downlink Data Block:
MAC Header- PT, RRBP, S/P, USF
PR,TFI,FBI,BSN,E
Optional bytes- multiples of (LI,M,E)
RLC Data information
RLC Uplink Data Block:
MAC Header- PT,CV,SI,R
Spare,PI,TFI,TI,BSN,E
Optional bytes-LI, M,E,TLLI, PFI,E
RLC Data information
CV is the Countdown Value which indicates no. of RLC blocks with a particular TBF left
to be transmitted by Mobile Subscriber(UE).
TLLI will identify GPRS user. and TLLI indicator(TI) dictates presence of TLLI field.
E bit is reserved for future use.
GPRS Layer 1 (Physical layer)
It is similar to GSM physical layer used for transmission of data. To know more read our page on GSM Physical layer in Articles section.
References
1.SNDCP-GSM 44.065
2.SM- 44.008 section 6
3.LLC- GSM 44.064
GPRS useful Links
GPRS protocol stack
GPRS Modem vendors
GSM versus GPRS
EDGE/EGPRS Basics
What is GPRS
GPRS/EGPRS Modulation-Coding Schemes
Other Standard Physical Layers
• Wireless physical layer overview
• 11b physical layer
• 11a physical layer
• fixed wimax physical layer-OFDM
• mobile wimax physical layer-OFDMA
• 11n physical layer
• GSM Physical layer
• TD-SCDMA Physical layer
• GPRS physical layer
• LDACS1 Physical layer
• 10,40,100 Gigabit Ethernet Physical layer
• Zigbee Physical layer
• WCDMA Physical layer
• Bluetooth Physical layer
• WLAN 802.11ac Physical layer
• WLAN 802.11ad Physical layer
• LTE Physical layer