Wireless Future - Telenor

exchange is based on the Diffie Hellman procedure.
3 The Radio Resource Control (RRC) is responsible
for the surveillance and efficient use of
available frequency resources. Important functions
of RRC are dynamic frequency selection
(DFS), handover and power control. The DFS
ensures that HIPERLAN/2 will operate in a
plug-and-play manner and frequency planning
will not be required. The decision to initiate a
handover is primarily done by the MT (forced
handover initiated by the AP is also possible)
and three types of handover are supported.
Sector handover if a sector antenna is deployed
(Inter sector/Intra AP), radio handover
if the AP consists of several transceivers
(InterAPT/Intra AP) and network handover
(Inter AP/Intra Network). The network handover
involves higher layers, and signalling via
the backbone may be needed.
3.3 Convergence Layer
The convergence layer acts as a bridge between
higher layer protocols and the DLC Layer. This
involves adapting the service requests from the
higher layers to that used by the DLC layer, and
segmentation/re-assembly of packets to fit the
fixed length of DLC packets. Currently, convergence
layers for ATM, IEEE1394 and Ethernet
have been standardised, and work on a UMTS
CL is underway. The convergence layers can be
split into two different types, cell based (ATM)
[10] and packet based (Ethernet, IEEE1394)
[11]. The structure of the packet based convergence
layer is displayed in Figure 5, and consists
of a common part (with the segmentation/reassembly)
and a service specific part.
4 Performance of HIPERLAN/2
MAC protocol
As mentioned in section 2 the HIPERLAN/2
system is intended to provide multimedia communication
in a variety of deployment scenarios.
It is therefore of major interest to investigate the
overall performance in terms of throughput,
delay and QoS support. In order to predict the
overall performance, the characteristics of the
individual layers need to be evaluated. Already,
there is published work considering the performance
of the HIPERLAN/2 system [12], [2].
In this section the performance of the MAC protocol
for different settings will be simulated.
In essence this is done by calculating the ratio
between overhead and user data for the concerned
MAC setting. The number of OFDM
symbols in each phase of the MAC frame can
be found from the following steps. Using the
values from Table 3 the OFDM symbol interval
t OFDM , can be calculated
Telektronikk 1.2001
Service Specific Part
t OFDM = 64 ⋅ T S + T G = 4 µs (1)
where
Ethernet
Service
Specific
Common Part
IEEE1394
Service
Specific
• • •
Segmentation/Re-assembly
Common part convergence sublayer
T S = sampling rate = 20 MHz or 50 ns
T G = guard time = 800 ns
The number of OFDM per frame is then:
NOFDM = tframe
tOFDM
(2)
4.1 PDU Train Structure
It was mentioned in section 3.3 that the phases
of the MAC frame were constructed using transport
channels. These transport channels are used
together with preambles to form Protocol Data
Unit trains, or PDU trains, and represents the
interface between the DLC protocol and the PHY
layer. Three types of preambles are defined [8]:
• Broadcast control channel preamble, P BCH ,
enables frame synchronisation, automatic gain
control, frequency synchronisation and channel
estimation. The length of P BCH is 16 µs or
4 OFDM symbols.
• Downlink traffic preamble, P DL , is used for
channel estimation only, and is 8 µs long or
2 OFDM symbols.
• Uplink traffic preamble enables channel and
frequency estimation. Two types of uplink
preambles are defined P UL-S (short) = 12 µs or
3 OFDM symbols and P UL-L (long) = 16 µs or
4 OFDM symbols.
The standard specifies 6 types of PDU trains [8]:
1 Broadcast PDU train;
2 FCH and ACH PDU train (multiple antenna
elements only);
= 200 µs
4 µs
= 500
XXX
Service
Specific
Figure 5 Structure
of packet based
convergence layer
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