754
21 Wireless Local Area Networks
multiple access (TDD/TDMA) and uses a MAC frame of 2 ms duration. An AP
provides centralized control and informs the mobile
terminals at which point in
time in the MAC frame they are allowed to transmit their data. Time slots are
allocated dynamically depending on the need for transmission resources.
HIPERLAN/2 operates as a connection-oriented wireless link. It supports
different QoS levels required for the transmission of various traffi c types. The
convergence layer (CL) between the data link layer and network layer provides
QoS. The role of the convergence layer is two-fold — it
maps the service require-
ments of the higher layer to the service offered by the data link control layer,
and converts packets received from the core network to the format expected at
the lower layers. There are two types of convergence layer. One is cell based and
the other is packet based. We focus only on the
packet based convergence layer,
which can be further divided into a common part and a service specifi c part. The
packet based service specifi c convergence sublayer (SSCS) is for switched Ethernet
Table 21.14 A comparison of HIPERLAN/2 and IEEE 802.11.
Characteristic
IEEE 802.11
HIPERLAN/2
Spectrum
2.4 GHz
5 GHz
Max. physical rate
2 Mbps
54 Mbps
Max. data rate, layer 3
1.2 Mbps
32 Mbps
Medium access control/
Media
sharing
CSMA/CA
Central resource control, TDMA/TDD
Access scheme
DCF/PCF
Elimination yield-non preemptive
priority
multiple access
Connectivity
Connectionless
Connection oriented
Multicast
Yes
Yes
QoS support
PCF
ATM/802.1p/Rsource reSerVation Proto-
col/Differential service (full control)
Frequency
selection
Frequency hopping
or DSSS
Single carrier with dynamic frequency
selection
Authentication
No
Network access identifi er/IEEE address/
X.509
Encryption
40-bit RC4
Data Encryption Standard (DES),
triple DES
Handover support
No
No
Fixed
network support
Ethernet
Ethernet, IP, ATM, UMTS, Firewire, PPP
Management
802.11 MIB
HIPERLAN/2 MIB
Radio link quality control
No
Link adaptation
Ch21-P373580.indd 754
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and IEEE 1394 Firewire. Broadband radio access for IP-based networks (BRAIN)
focuses on the specifi cations of an innovative SSCS dedicated
to provide support
to IP traffi c in a mobile environment. The architecture of the convergence layer
makes HIPERLAN/2 suitable as a radio access for different types of fi xed net-
works, e.g., Ethernet, IP, ATM, UMTS, etc. The main function of the common
