LTE PDCP Layer Interview Questions Answer

Q01. What is the meaning of LSB Field indicate?

Ans. In computing, the least significant bit (LSB) is the bit position in a binary integer giving the units value, that is, determining whether the number is even or odd. The LSB is sometimes referred to as the low-order bit or right-most bit, due to the convention in positional notation of writing less significant digits further to the right.
- A PDCP PDU is a bit string that is byte aligned (i.e. multiple of 8 bits) in length. In the figures bit strings are represented by tables.

PDCP Data PDU format for SRBs

PDCP Data PDU format for DRBs using 7 bit SN

PDCP Data PDU format for DRBs using a 12 bit SN

The bit order of each parameter field within a PDCP PDU is represented with the
(i) first and most significant bit in the leftmost bit.
(ii) last and least significant bit in the rightmost bit.
Note* A compressed or uncompressed SDU is included into a PDCP PDU from the first bit onward.

Q02. What is the full form of PDCP Layer?

Ans. PDCP stands for Packet Data Convergence Protocol have 3GPP specification no. 36.323.

Q03. What are the main functions of PDCP in lte?

Ans. The Packet Data Convergence Protocol supports the following functions:
* header compression and decompression of IP data flows using the ROHC protocol;
* transfer of data (user plane or control plane);
* maintenance of PDCP SNs;
* in-sequence delivery of upper layer PDUs at re-establishment of lower layers;
* duplicate elimination of lower layer SDUs at re-establishment of lower layers for radio bearers mapped on RLC AM;
* ciphering and deciphering of user plane data and control plane data;
* integrity protection and integrity verification of control plane data;
* timer based discard;
* duplicate discarding.
Note* PDCP uses the services provided by the RLC sublayer.
Note* PDCP is used for SRBs and DRBs mapped on DCCH and DTCH type of logical channels. PDCP is not used for any other type of logical channels.

Q04. Define PDCP functional block?

Ans.

Q05. What is SN_STATUS transfer, what kind of information it carry?

Ans. - The purpose of the SN Status Transfer procedure is to transfer the PDCP SN and HFN information by source enodeB to target enodeB.
- This message is used during the handover execution phase.
- The source eNB delivers it to the target eNB to indicate from which packet it should receive or send.
In lte 3 types SN length is use by pdcp layer.

Length	Description
5 Bit	Use by SRBs in Control Plane only
7 Bit	Use by DRBs, if configured by upper layers (pdcp-SN-Size)
12 Bit	Use by DRBs, if configured by upper layers (pdcp-SN-Size)

Q06. Why we need Packet Compression in lte?

Ans. - Packet compression is the process of reducing protocol header overhead in order to improve link efficiency while maintaining the end-to-end transparency.
- Reduce the Signalling and User Data size to save the bandwidth and overcome the load on network.

Q07. What is name of packet compression algorithm?

Ans. - The header compression protocol is based on the Robust Header Compression (ROHC) framework. There are multiple header compression algorithms, defined for the ROHC framework. Each profile is specific to the particular network layer, transport layer or upper layer protocol combination e.g. TCP/IP and RTP/UDP/IP.
- The detailed definition of the ROHC channel is specified as part of the ROHC framework in RFC 4995.

Q08. Which PDCP entity responsible to provide security to signaling messages?

Ans. Integrity provide security to signalling message.

Q09. Why we do integrity 1st and ciphering 2nd in pdcp?

Ans. If we do ciphering 1st in lte the information of message is totally change so when we apply integrity after is should not works properly. So we apply 1st integrity to signalling messages after we apply ciphering to user data.

Q10. What is timer based discard?

Ans. - A PDCP layer sets a timer and discards a PDCP SDU upon expiration of the timer.
- The timer may be set upon receiving the PDCP SDU from an upper layer or upon submitting the PDCP SDU to a lower layer for transmission.
- The PDCP discard timer and a RLC discard timer may be coordinated.
- Alternatively, the PDCP layer may discard the PDCP SDU based on a notification from an RLC layer or based on a PDCP status report