HIPERLAN

HIPERLAN is the generic term for the standards HIPERLAN / 1 , HIPERLAN / 2 , HIPERACCESS (formerly HIPERLAN / 3 ) and HIPERLINK (formerly HIPERLAN / 4 ). The standards describe an alternative technology to the IEEE 802.11 standard for setting up radio networks between computers . They were defined by the Broadband Radio Access Network (BRAN) working group of the ETSI (European Telecommunication Standards Institute) from 1996 .

HIPERLAN is an acronym stands for Hi gh Pe rformance R adio L ocal A rea N etwork .

The standards build on each other and are each further developments.

The standards have not been able to establish themselves on the market, although they contain technically interesting concepts. In contrast to the IEEE 802.11 standard, HIPERLAN does not use any signaling in the unprotected ISM band at 2.45 GHz and used the frequency range more efficiently at the time of development. The lower layers were largely adopted in IEEE 802.11a .

HIPERLAN / 1

HIPERLAN / 1 was completed in 1996 by the BRAN working group. It provides data transmission rates of up to 23.5 Mbit / s at 5 GHz (specifically: 5120–5300 MHz), where devices based on the IEEE 802.11a WLAN standard are also operated. It achieves a range of approx. 50 meters in typical office environments, but implements a method via which nodes (network) can forward data packets in order to cover larger areas.

Power saving mechanisms have been implemented for mobile use, for which radio networks are predestined.

HIPERLAN / 1 divides its frequency range into 5 channels.

The structure is based on the ISO-OSI reference model , but the data link layer (security layer ) is divided into two sub-layers: the MAC layer ( Medium Access Control , not to be confused with the MAC address ) and the CAC layer ( Channel Access Control ).

MAC layer

In addition to service primitives for sending and receiving data, the MAC layer also provides functions for encryption and energy-saving functions. The data packets can be assigned five priorities . This priority is used to calculate the back-off time that is waited for when the package is sent.

CAC layer

The CAC layer regulates access to the radio channel. It uses the EY-NPMA procedure (Elimination-Yield Non-Preemptive Priority Multiple Access):

If the channel is free, a random time is waited. If the medium has not been used during that time, it is sent. However, if the channel is busy, the next station is selected in a three-phase process:

Prioritization phase

In the prioritization phase, a burst frame is sent after a constant waiting time that has been multiplied by the priority of the packet. All transmitters whose packets have a lower priority are switched off in this way.

Contention phase

Since several transmitters can remain, in the contention phase, which consists of two sub-phases, further transmitters are first sorted out via elimination bursting. Each remaining transmitter waits a random time and then sends out a burst. The stations that were involved in the last burst then compete in yield listening: whoever sends a burst there first is allowed to transmit data.

Transmission phase

The data transmission itself takes place in the transmission phase. If a unicast packet is sent out, the transmission phase also includes the confirmation of receipt.

Theoretically, it is conceivable that this complex process could also cause collisions to occur on the medium. However, it is very unlikely in practice.

Hidden Terminal

In radio networks, a hidden terminal is a station that is in the radio shadow for some of the transmitters . This problem is solved in the IEEE 802.11 standard by the RTS / CTS method. HIPERLAN takes a different approach: On the one hand, the headers of the HIPERLAN packets that are transmitted at a lower data rate can still be decrypted outside the actual radio range; on the other hand, stations can recognize themselves that they are a "hidden terminal" if they lose against another transmitter in the contention phase, but do not notice the later data transmission. The waiting times are then increased for the "hidden terminal" in order to prevent collisions.

Forwarding of data packets

In HIPERLAN / 1, stations can be marked as "forwarders" and "non-forwarders". Forwarders forward data packets for other stations that are in the radio shadow of the transmitter but within range of the station. The stations each manage a list of "forwarders" who are responsible for a destination. Only a small number of "forwarders", the so-called multipoint relays, are responsible for multicast or broadcast packets.

Energy saving process

There are two methods to choose from: On the one hand, a receiver can switch off its transmitter and reduce the receiving power. Since headers are sent at a reduced data rate, the recipient can still decrypt them and see whether he is meant. If so, Doze mode will exit.

Alternatively, a defined station, the "p-Supporter", can receive the packets for a deactivated receiver, called the "p-Saver". p-Supporter and p-Saver synchronize with each other with regard to their waking phases in which the p-Supporter forwards the data packets.

HIPERLAN / 2

The successor to HIPERLAN / 1 was adopted in 2000 . The main enhancements were additional functions that make it a wireless access network for wide area networks. Among other things, a coupling to UMTS and Wireless ATM (wireless ATM ) was considered. Quality of service parameters can be defined for the use of multimedia applications .

HIPERLAN / 2 can use the same frequencies as HIPERLAN / 1 plus the range from 5470 to 5725 MHz. It supports data rates of up to 54 Mbit / s (analogous to IEEE 802.11a ). The ranges are similar: 30 meters inside buildings and up to 150 meters outside.

A centralized mode , which corresponds to the infrastructure mode in the 802.11 standard, and a direct mode , which corresponds to the ad-hoc mode, have also been defined for HIPERLAN / 2 .

A different layer model is used in the HIPERLAN / 2 standard than in HIPERLAN / 1. The physical layer remains the same, but the data link layer is divided into a MAC layer over which the radio link control layer (RLC) and the error control layer (EC) are superordinate in parallel. A convergence layer is defined above these layers.

Radio Link Control

The radio link control layer defines encryption procedures , handover mechanisms, i.e. the transfer from one base station to the next, power management functions and service primitives for opening and closing connections and for sending broadcast and multicast messages.

Error control

The error control layer ensures error-free data transmission. The so-called ARQ procedure (Automatic Repeat Request) is used, in which a message is requested again in the event of an error.

MAC

The structure of the MAC layer has already been described above.

Convergence Layer

The convergence layer takes over the division of large data packets into smaller ones for the transport and represents two transmission methods: the cell-based and the packet-based transmission. The former transmits packets of fixed length in the same way as ATM , the latter packets of different sizes and is therefore compatible with Ethernet .

Interaction with other networks

HIPERLAN / 2 calls connection networks between different HIPERLAN networks core network. A core network is usually a wide area network .

Alternative: HIPERACCESS

HIPERACCESS represents an alternative to wired procedures to bridge the last mile from a wide area network to a subscriber line. It offers data rates of 23 Mbit / s and a range of up to 5 kilometers.

HIPERLINK

Fixed point-to-point connections with data rates of up to 155 Mbit / s and a range of up to 150 meters can be set up via HIPERLINK .

literature

Jörg Roth : Mobile Computing, dpunkt, 2002, ISBN 3898641651
Broadband Radio Access Network: High Performance Radio Local Area Network (HIPERLAN) Type 1. Functional Specification, 1998
Broadband Radio Access Network: High Performance Radio Local Area Network (HIPERLAN) Type 2. Requirements and architectures for wireless broadband access, 1999