Page 157 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 143

data arriving from higher layers to fill an air interface frame, padding is used to fill the
frame. Instead of padding the frame, it is also possible to use the remaining bits for RLC
control messages.
Depending on the kind of user data one of three different RLC modes is used:

The RLC transparent mode is used primarily for the transmission of circuit‐switched
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voice channels and for the information that is broadcast on the BCCH and the
PCCH. As the length of voice frames does not vary and as they are sent in a predefined
format every 20 milliseconds, padding is also not necessary. Therefore, no adaptation
or control functionality is required on the RLC layer, hence the use of the RLC trans-
parent mode.
The RLC non‐acknowledged mode offers segmentation and concatenation of higher‐
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layer frames as described above. Furthermore, this mode allows marking of the begin-
ning and end of layer 3 user data frames. Thus, it is possible to always completely fill
an air interface frame regardless of the higher‐layer frames. As no acknowledgement
for RLC frames is required in this mode, frames that are not received correctly or lost
cannot be recovered on this layer.
The third mode is the RLC acknowledged mode (AM), which is mostly used to transfer
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IP frames. In addition to the services offered by the non‐acknowledged mode, this
mode offers flow control and automatic retransmission of erroneous or missing blocks.
Similar to Transmission Control Protocol (TCP), a window scheme is used to acknowl-
edge the correct reception of a block. By using an acknowledgement window it is not
necessary to wait for a reply for every transmitted block. Instead, further blocks can be
transmitted up to the maximum window size. Up to this time, the receiver has the
possibility of acknowledging frames, which in turn advances the window. If a block
was lost, the acknowledgement bit in the window will not be set, which automatically
triggers a retransmission. The advantage of this method is that the data flow, in gen-
eral, is not interrupted by a transmission error. The RLC window size can be set
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between 1 and 2 frames and is negotiated between the mobile device and RNC. This
flexibility is the result of the experience gained with GPRS. There, the window size was
static; it offered only enough acknowledgement bits for 64 frames. In GPRS, this
proved to be problematic, especially for coding schemes three and four during phases
of increased block error rates (BLER), which led to interrupted data flows, as frames
cannot be retransmitted quickly enough to advance the acknowledgement window.
Once the RLC layer has segmented the frames for transmission over the air interface
and has added any necessary control information, the MAC layer performs the following
operations:

Selection of a suitable transport channel: as was shown in Figure 3.11, logical channels
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can be mapped onto different transport channels. User data of a DTCH can, for
example, be transferred either on a DCH or on the FACH. The selection of the transport
channel can be changed by the network at any time during the connection to increase
or decrease the speed of the connection.
Multiplexing of data on common and shared channels: the FACH can be used to
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transport not only RRC messages for different users but can also carry user data
frames. The MAC layer is responsible for mapping all logical channels selected on a
single transport channel and for adding a MAC header. The header describes, among