AbstractThe need for mobile broadband communications has increased rapidly in recent years placing new demands for the wireless local area networks (WLANs). These requirements include support for QoS, security, handover, and increased throughput. To answer these needs, the European Telecommunications Standards Institute (ETSI) is working on HIPERLAN (HIgh PERformance LAN) standards of various types. These standards merely describe the low-level interfaces and leave the higher level functions open. This essay will concentrate on HIPERLAN/2, a state-of-the-art WLAN technology that has been developed to provide a wireless access to fixed networks. This is intended to be a short overview of its main features and protocol reference model.
3.2 Data Link Control Layer
3.3 Convergence Layer
For several years, the technical committee RES10 (Radio Equipment and Systems) of ETSI (European Telecommunications Standards Institute) has been working on HIPERLAN (HIgh PERformance LAN), which is a family of standards on digital high speed wireless communication in the 5.15-5.3 Ghz and the 17.1-17.3 Ghz spectrum. ETSI is currently referring the family as BRAN (Broadband Radio Access Networks). Four types of HIPERLAN has been proposed: HIPERLAN types 1 and 2, HIPERACCESS and HIPERLINK. The HIPERLAN standard merely describes a common air interface and the physical layer for wireless communications equipment, thus ensuring compatible communications systems while leaving the higher level functions open to the manufacturers. Its specification consists of the two lowest layers of the OSI-model. [4,5]
Unlike the HIPERLAN type 1, the HIPERLAN type 2 has been specifically developed to mainly have a wired infrastructure providing a short range wireless access to IP, ATM and UMTS networks. The HIPERLAN type 2 operates in the 5,2 Ghz frequency band with 100 Mhz spectrum. A typical topology of a HIPERLAN/2 network is depicted in figure 1. The Mobile Terminals (MTs) communicate with one Access Point (AP) at a time over an air interface; while on the move, HIPERLAN/2 automatically performs handover to the nearest AP. Ad hoc networks, where the MTs communicate directly, can also be created, but their development is still in early phase. The HIPERLAN/2 is planned to be finalized by the end of 1999. [3,4]
The HIPERLAN/2 white paper summarizes the general features of the HIPERLAN/2 technology as follows :
HIPERLAN/2 has a very high transmission rate up to 54 Mbit/s. This is achieved by making use of a modularization method called Orthogonal Frequency Digital Multiplexing (OFDM). OFDM is particularly efficient in time-dispersive environments, i.e. where the radio signals are reflected from many points, e.g. in offices. 
HIPERLAN/2 connections are time-division multiplexed and connection-oriented, either bidirectional point-to-point or unidirectional point-to-multipoint connections. There is also a dedicated broadcast channel through which the traffic from an AP reaches all terminals. 
Unlike other radio-based systems, the traffic on a LAN is inherently random and bursty. This may cause serious problems with respect to throughput, because the performance is one of the most important factors of wireless LANs. In HIPERLAN/2, each connection can be assigned either a simple relative priority level or a specific QoS in terms of bandwidth, delay, jitter, bit error rate, etc. [3,5]
The HIPERLAN/2 Access Points have a built-in support for automatic transmission frequency allocation within the AP's coverage area. This is performed by the Dynamic Frequency Selection (DFS) function. An appropriate radio channel is selected based on both what radio channels are already in use by other AP's and to minimize interference with the environment. Thus, there is no need for manual frequency planning as in cellular networks like GSM. 
The HIPERLAN/2 network supports authentication and encryption. Both the AP and the MT can authenticate each other to ensure authorized access to the network or to a valid network operator. The encryption can be used on established connections to protect against eaves-dropping and man-in-the-middle attacks. In HIPERLAN, each communicating node is given a HIPERLAN ID (HID) and a Node ID (NID). The combination of these two IDs uniquely identifies any station, and restricts the way it can connect to other HIPERLAN nodes. All nodes with the sama HID can communicate with each other using a dynamic routing mechanism denoted Intra-HIPERLAN Forwarding. [3,6]
The support for handover enables mobility of MTs. The handover scheme is MT initiated, i.e. the MT uses the AP with the best signal as measured for instance by S/N-ratio, and as the user moves around, all established connections move to the AP with the best radio transmission performance, while the MT stays associated to the HIPERLAN/2 network. 
The HIPERLAN/2 architecture is easily adapted and integrated with a variety of fixed networks. All applications running over a fixed infrastructure can also run over a HIPERLAN/2 network. 
The power save mechanism in HIPERLAN/2 is based on MT-initiated negotiation of sleep periods. The MT requests the AP for a low power state and a specific sleep period. At the expiration of the sleep period, the MT searches for a wake up indication from the AP, and in the absence of that sleeps the next period, and so forth. The MT receives any pending data as the sleep period expires. Different sleep periods are supported depending on the requirements. 
OFDM provides flexibility considering the realization of different modulation alternatives. Seven different physical layer modes (PHY modes) are specified in table 1.
Table 1: PHY modes defined for HIPERLAN/2. 
Figure 3: HIPERLAN/2 functions .
Table 2: HIPERLAN/2 functions .
The MAC protocol is used for access to the medium (the radio link). The control is centralized to the AP which inform the MTs, when they are allowed to send data. The air interface is based on time-division duplex (TDD) and dynamic time-division multiple access (TDMA), which allows for simultaneous communication in both downlink and uplink within the same time frame, i.e. the MAC frame. 
The MAC frame format consists of four elements: Broadcast Channel (BCH), Down Link (DL), Up Link (UL), and Random Access (RA) . Except for the broadcast control, the duration of the fields is dynamically adapted to the current traffic situation.  The whole DLC is based on scheduling efficiently MAC frame . The MAC frame and the transport channels form the interface between the DLC and the PHY .
Figure 4: MAC frame structure .
The Convergence Layer (CL) adapts service request from higher layers to the service offered by the DLC and converts the higher layer packets (SDUs) into a fixed size used within the DLC. This function makes it possible to implement DLC and PHY that are independent of the fixed network to which the HIPERLAN/2 network is connected. 
There are currently two types of CLs defined: cell based and packet based. The former is intended for interconnection to ATM networks, the latter is used in a variety of configurations depending on fixed network type. 
Figure 5: The general structure of the Convergence Layer .
 Harjunpää, I.
HIPERLAN/2 DLC Layer,
14.4.1999 [referenced 1.11.1999]
 Johnsson, M.
HIPERLAN/2 -- The Broadband Radio Transmission Technology Operating in the 5 GHz Frequency Band,
14.9.1999 [referenced 1.11.1999]
 Rajaniemi, A.
10.3.1999 [referenced 1.11.1999]
 Rune, T. HIPERLAN: Closing in on the Airwaves, 6.8.1999 [referenced 1.11.1999]
 Rune, T. Wireless Local Area Networks, 30.3.1998 [referenced 1.11.1999]