LTE Protocol Stack

In this tutorial, we will discuss on the LTE Protocol Stack. We can divide air interface protocol into two parts.

1. User plane
2. Control plane

User plane used for data flow whereas control plane is used to configure user plane layers before actual data flows.

Let’s talk about user plane first .

When data comes from application it is stored in PDCP buffered at eNB. So let’s first talk about PDCP layer.

PDCP :

PDCP layer means ‘Packet Data Convergence Protocol’

• Header Compression and header decompression of IP data packets
• Maintenance of PDCP sequence number
  • At transmitting end PDCP layer assign sequence number to each packet before sending it to next layer that is RLC. This sequence is used by PDCP layer at receiving end to align packet in a sequential order before sending them to upper layers.
• Security of the data
  • If your data is not secured at air interface somebody may tap the data.

For example, online payment for your mobile will no longer be secure if there is no security mechanism. So we need to secure this data by integrity and ciphering before transmitting on air interface .

RLC:

RLC layer means ‘Radio Link Control’

• Concatenation, segmentation and reassembly of RLC SDUs
  • All the header of IP packets has fixed size that is 40 bytes but data part is variable.
  • For example: In case of VOIP call data part is small but in case of video streaming data part of IP packet is quite big. BW available on, air interface keeps on changing very frequently because of changing radio conditions and changing number of UE under an eNB. So RLC at the transmitting end must concatenate and segment these data packets dynamically according to the current BW on air interface and RLC at the receiving end does re-ordering and reassembly of data.
• Retransmission of RLC PDUs (Only for AM data transfer)
  • If data gets corrupted on air interface then RLC layer of the transmitting end has to send the data packet again but this kind of transmission done only when the RLC is in AM mode.
  • Example of AM RLC is data download using FTP.
• Reordering of RLC data PDUs (UM and AM data transfer only)
  • Let me tell you the reason for this. Because of multiple re-transmissions order in which packets are received at the receiver could be random. So RLC has to reorder all the packets before sending them to the upper layer.

MAC:

MAC layer means ‘Medium Access Control’

• Prioritisation among various data streams for a given UE.
  • For example, packets of VOIP calls are of highest priority when available BW on air interface is not enough to carry all the data streams. This role is performed by MAC layer.
• Error correction through HARQ
  • HARQ means ‘Hybrid Automatic Repeat Request’. It is the error correction technique that has been taken from UMTS.

PHY:

• Encodes raw data before modulation.
  • Data must go with the encoding process before modulation. This processing is called physical layer processing. This processing is needed so that receiver can detect any error in the data during the transmission on air interface. If forward error correction techniques are used, then receiver can correct the data at its end if there is some corruption in the data. If data can be corrected at receiver end, then no need to retransmit the data.
• Measure the air interface to know the channel quality.
  • Let me tell you the reason behind this. Because of mobility, multiple reflection from the surroundings and noise on air interface, available BW of air interface keeps on changing very frequently. So, we need some mechanism to measure air interface quality to know the current channel BW. This is the reason behind this measurement.

RRC:

RRC layer initializes other layers such as PDCP, RLC, MAC and PHY layer. Then only actual data transfer takes place

• Broadcast of system information
  • In a cell MIB and SIB are broadcast by RRC layer. MIB and SIB contains all the cell and network related information. Whenever a UE switched ON any UE moves to a new cell in IDLE mode, it decodes its broadcast information to get all the cell parameters such as cell BW, TAC etc.
• RRC connection control
  • RRC layer of UE triggers RRC connection with RRC layer of eNB. Without RRC connection UE cannot send any data packets to network. When a UE does not have RRC connection it is set to be an IDLE mode.
• State Transition
  • RRC layer triggers state transition in UE from IDLE mode to connected mode and vice versa.
  • When there is inactivity in the data flows for a UE for long time, RRC layer of eNB triggers RRC connection release and sends such UE to IDLE state.
  • State transition is performed to save UE battery life and air interface resources.
• Paging
  • Whenever there is some MT voice calls or PS data to be send to a UE, RRC layer of eNB sends paging message to that UE.
• Initial security activation i.e. initial configuration of AS integrity protection of SRBs and AS ciphering of SRBs, DRBs.
• Measurement configuration and reporting
  • In connected mode an UE has to do many type of measurement such as intra frequency, inter frequency  and inter RAT measurement. All these measurements are configured in the UE by RRC layer of eNB. Based on these measurements RRC layer of eNB takes all the handover related decisions.

NAS:

NAS layer is composed of EMM (EPS Mobility management) and ESM (EPS session management).

EMM layer takes care of all mobility related information such as

• Tracking Area Update
• Paging
• Security Mode control
• Authentication

ESM layer takes care of bearer related functions such as

• Default EPS bearer context activation
• EPS bearer context modification
• PDN connectivity procedures
• PDN disconnect procedures

Now we have understood the LTE Protocol Stack. Let’s discuss LTE Attach Procedure in next tutorial.