Page 259 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 245

location in the resource grid via a scheduling grant on the PDCCH. The second option
is to send only a NACK without any further information via the PDCCH. In this case,
the mobile device repeats the transmission on the same resources that were assigned for
the original transmission.

ARQ on the RLC Layer
Packets lost despite the HARQ mechanism can be recovered via the Automatic
Retransmission Request (ARQ) feature on the next‐highest protocol layer, the radio link
control (RLC) layer, which is specified in 3GPP TS 36.322 [14]. While used for most
bearers, its use is optional and might not be activated, for example, for radio bearers
used by VoIP applications such as the IMS. ARQ is split into two main functions:
As soon as the receiver detects a transmission failure, it sends a report to the trans-
●
mitting side which then repeats the missing RLC frame. A sliding window approach
is used so that the transfer of frames is not interrupted if a frame is not received. Only
if the missing frame has not been received when the window size is met is the overall
transmission stopped until the missing RLC frame has been received. This ensures
that the RLC buffer on the receiver side, that is, either in the eNode‐B or in the UE,
does not overrun.
During normal operation, the sender periodically requests an ARQ status report by
●
setting the polling indicator bit in the RLC header of a data frame. This way, unneces-
sary status reports do not have to be sent while also ensuring that no RLC error mes-
sage is missed.
While HARQ and ARQ ensure the correct delivery of data over the air interface,
higher‐layer protocols also have functionality built in to ensure the correct delivery
from an end‐to‐end perspective. The IP protocol, for example, contains a CRC field so
that each router in the path between sender and receiver can check the integrity of the
packet. And finally, TCP uses a sliding window mechanism similar to that of ARQ to
detect missing IP packets and to request a retransmission.

4.3.11 PDCP Compression and Ciphering
Above the MAC and RLC layers, discussed in the previous sections, is the Packet Data
Convergence Protocol (PDCP) layer. Its main task is to cipher user and signaling traffic
over the air interface. Further, it ensures the integrity of signaling messages by protect-
ing against various man‐in‐the‐middle attack scenarios. Several ciphering algorithms
have been defined for LTE and the network decides which one to use during the bearer‐
establishment procedure.
Another important but optional task of the PDCP layer is IP header compression.
Depending on the size of the user data in a packet, a more or less significant percentage
of the overall air interface capacity is taken up by the various headers in the IP stack.
This is especially the case for VoIP packets, which are usually sent every 20 milliseconds
to minimize the speech delay, and are hence very short. With efficient codecs such as
AMR (see Chapter 1), each IP packet has a size of around 90 bytes and two‐thirds of the
packet is taken up by the various headers (IP, UDP and RTP). For other applications
such as web browsing, for which large chunks of data are transferred, IP packets typi-
cally have a size of over 1400 bytes. Here, header compression is less important but still