Page 260 - From GMS to LTE

246 From GSM to LTE-Advanced Pro and 5G

beneficial. As a consequence, header compression is not widely used in practice today
and will only gain more traction when mobile operators introduce VoLTE.
For the LTE air interface, Robust Header Compression (RoHC) can be used if
supported by both the network and the mobile device. It is specified in RFCs 4995 and
5795 [15] and mostly used for Voice over LTE (VoLTE, cf. Chapter 5) and for NB‐IoT
(Narrow‐Band Internet of Things) applications as described further below. The basic
idea of RoHC is not to apply header compression to all IP packets in the same manner
but to group them into streams. IP packets in a stream have common properties such as
the same target IP address and TCP or UDP port number and the same destination IP
address and port number. For a detected stream of similar IP packets, one of several
RoHC profiles is then used for compression. For VoIP packets, one of the profiles
described in RFC 5225 [16] can be used to compress the IP, UDP and Real‐time
Transport Protocol (RTP for voice and other multimedia codecs) headers. For other
streams, more general profiles that only compress the IP and TCP headers can be used.
Once a stream is selected, an RoHC session is established and several sessions can be
established over the same bearer simultaneously. After the RoHC session setup phase,
static header parameters such as the IP addresses, UDP port numbers and so on are
completely omitted and variable parameters such as counters are compressed. A check-
sum protects against transmission errors.
In addition, the PDCP layer ensures that during a handover, the user and control
plane contexts are properly transferred to the new eNode‐B. For signaling and RLC
unacknowledged data streams (e.g. for VoLTE), a seamless handover procedure is
performed. Here, PDCP helps to transfer the contexts to the new eNode‐B, but
PDCP packets that are transferred just before the handover and not received cor-
rectly might be lost. For VoIP packets such a loss is even preferred to later recovery
due to the delay sensitivity of the application. For RLC acknowledged mode (AM)
user data streams, a lossless handover is performed. Here, PDCP sequence numbers
are used to detect packets that were lost in transit during the handover; these are
then repeated in the new cell.

4.3.12 Protocol Layer Overview

In the previous sections, some of the most important functions of the different air inter-
face protocol layers such as HARQ, ARQ and ciphering have been discussed. These
functions are spread over different layers of the stack. Figure 4.15 shows how these
layers are built on each other and where the different functions are located.
On the vertical axis, the protocol stack is split into two parts. On the left side of
Figure 4.15, the control protocols are shown. The top layer is the NAS protocol that is
used for mobility management and other purposes between the mobile device and the
MME. NAS messages are tunneled through the radio network, and the eNode‐B just
forwards them transparently. NAS messages are always encapsulated in Radio Resource
Control (RRC) messages over the air interface. The other purpose of RRC messages is
to manage the air interface connection and they are used, for example, for handover or
bearer modification signaling. As a consequence, an RRC message does not necessarily
have to include an NAS message. This is different on the user data plane shown on the
right of Figure 4.15. Here, IP packets always transport user data and are sent only if an
application wants to transfer data.