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Chapter: Mobile Networks : Wireless Networks

HIPERLAN(high performance local area network)

HIPERLAN stands for high performance local area network. It is a wireless standard derived from traditional LAN environments and can support multimedia and asynchronous data effectively at high data rates of 23.5 Mbps.





HIPERLAN stands for high performance local area network. It is a wireless standard derived from traditional LAN environments and can support multimedia and asynchronous data effectively at high data rates of 23.5 Mbps. It is primarily a European standard alternative for the IEEE 802.11 standards and was published in 1996. It is defined by the European Telecommunications Standards Institute (ETSI). It does not necessarily require any type of access point infrastructure for its operation, although a LAN extension via access points can be implemented.


Radio waves are used instead of a cable as a transmission medium to connect stations. Either, the radio transceiver is mounted to the movable station as an add-on and no base station has to be installed separately, or a base station is needed in addition per room. The stations may be moved during operation-pauses or even become mobile. The maximum data rate for the user depends on the distance of the communicating stations. With short distance(<50 m) and asynchronous transmission a data rate of 20 Mbit/s is achieved, with up to 800 m distance a data rate of 1 Mbit/s are provided. For connection-oriented services, e.g. video-telephony, at least 64 kbit/s are offered.


HIPERLAN uses cellular-based data networks to connect to an ATM backbone. The main idea behind HIPERLAN is to provide an infrastructure or ad-hoc wireless with low mobility and a small radius. HIPERLAN supports isochronous traffic with low latency. The HiperLAN standard family has four different versions.

The key feature of all four networks is their integration of time-sensitive data transfer services. Over time, names have changed and the former HIPERLANs 2,3, 1nd 4 are now called HiperLAN2, HIPERACCESS, and HIPERLINK.



Planning for the first version of the standard, called HiperLAN/1, started 1991, when planning of 802.11 was already going on. The goal of the HiperLAN was the high data rate, higher than 802.11. The standard was approved in 1996. The functional specification is EN300652, the rest is in ETS300836.


The standard covers the Physical layer and the Media Access Control part of the Data link layer like 802.11. There is a new sub layer called Channel Access and Control sub layer (CAC). This sub layer deals with the access requests to the channels. The accomplishing of the request is dependent on the usage of the channel and the priority of the request.


CAC layer provides hierarchical independence with Elimination-Yield Non-Preemptive Multiple Access mechanism (EY-NPMA). EY-NPMA codes priority choices and other functions into one variable length radio pulse preceding the packet data. EY-NPMA enables the network to function with few collisions even though there would be a large number of users. Multimedia applications work in HiperLAN because of EY-NPMA priority mechanism. MAC layer defines protocols for routing, security and power saving and provides naturally data transfer to the upper layers.

On the physical layer FSK and GMSK modulations are used in HiperLAN/1. HiperLAN features:

       range 50 m


       slow mobility (1.4 m/s)


       supports asynchronous and synchronous traffic


       sound 32 kbit/s, 10 ns latency


       video 2 Mbit/s, 100 ns latency


       data 10 Mbit/s


HiperLAN does not conflict with microwave and other kitchen appliances, which are on 2.4 GHz.


Elimination-yield non-preemptive priority multiple access (EY-NPMA)


EY-NPMA is a contention based protocol that has been standardized under ETSI‘s


HIPERLAN, a standard for wireless LANs. Unlike other contention based protocols, EY-NPMA provides excellent support for different classes of traffic regarding quality of service and demonstrates very low collision rates. EY-NPMA is the medium access mechanism used by HIPERLAN Type 1. It uses active signaling.


Active signaling takes advantage of the fact that the current wireless technology enables us to have a slot time very much smaller than the average packet size. Each node that wants to access the medium transmits a non-data preamble pattern consisting of slots. This pattern is made up of alternating idle and busy periods of different lengths (measured in slots). Conflict resolution and collision detection is done during this preamble. The main rule is that if a node detects a signal during one of its listening periods in its pattern, it aborts and defers until the next cycle. Otherwise, the node transmits its packet at the end of the pattern transmission.


With EYNPMA, each station may attempt to access the channel when a condition out of a group of three is met. The three conditions are:

 Channel free condition

 Synchronized channel conditio

 Hidden elimination condition

The channel free condition occurs when the channel remains idle for at least a predefined time interval. A station willing to transmit senses the channel for this time interval, the station extends its period of sensing by a random number of slots (backoff). If the channel is still sensed as idle during the backoff period, the station commences transmitting. In both modes of operation unicast transmissions must get positively acknowledged or else the transmission is declared erroneous. Multicast and broadcast packets are not acknowledged.


The synchronized channel condition occurs when the channel is idle in the channel synchronization interval, which starts immediately after the end of the previous channel access cycle. The synchronized channel access cycle consists of three distinct phases:




Contention(Elimination and Yield) Transmission


Important features of the EY-NPMA


5.     No preemption by frames with higher priority after the priority resolution possible.


6.  Hierarchical independence of performance.

7.  Fair contention resolution of frames with the same priority


In prioritization, EY-NPMA recognizes five distinct priorities from 0 to 4, with 0 being the highest priority. The cycle begins with each station having data to transmit sensing the channel for as many slots as the priority of the packet in its buffer. All stations that successfully sense the channel as idle for the whole interval proceed to the next phase, the elimination phase.


During the elimination phase, each station transmits an energy burst of random length. These bursts ensure that only the stations having the highest priority data at a time proceed to the elimination phase. The length of the energy burst is a multiple of slots up to a predefined maximum. As soon as a station finishes bursting, it immediately senses the channel. If the channel is sensed as idle, the station proceeds to the next phase. Otherwise, it leaves the cycle.


During the yield phase, the station that survived the two previous ones, back off for a random number of slots. The station that backs off for the shortest interval eventually gets access of the channel for data transmission. All other station sense the beginning of the transmission and refrain from transmitting.


a) Prioritization Phase


Prioritization Phase is the first attempt at reducing the number of contenders for the channel. Every contender calculates the number of idle slots according to the priority of its data, and senses the channel during those slots. Contenders with highest priority data will have no idle slots, while those with lowest priority data will have all idle slots If it detects a signal during those idle slots, it defers until the next cycle. This means that only the higher priority contenders survive. If it does not detect a signal during these idle slots, it sends the priority pulse and enters the elimination phase. In the first phase of the synchronized channel access cycle, known as the Prioritization Phase, every node allows a number of idle slots, where the default slot length is 168 high rate bit-periods.


The number of the idle slots is equal to the arithmetic value of the CAM priority of the packet. Every contending node senses the channel, while it allows the idle slots. If it detects a signal transmission, it defers, that is it quits the effort to gain access to the channel and waits for the next channel access cycle to try to transmit. When a node detects no transmission during the Prioritization Phase, it transmits a pulse right after the idle slots, and proceeds to the next phase. This pulse is the one listened by every ―defeated‖ node. The nodes that proceed to the next contention phase have a packet to send of the same highest CAM priority.


b) Elimination Phase


Elimination Phase is the second attempt at reducing the contenders. This phase consists of extending the priority pulse with a randomly calculated number of busy slots. The number of slots is independently calculated for each node. The


probability of a larger than k-slot pulse is 1/2k. Therefore, the probability of a larger than 1-slot pulse is 1/2, larger than a 2-slot pulse is 1/4 and so on. Immediately after this pulse, the node senses the channel. If the channel is busy, it defers transmission until the next cycle. If the channel is idle, it enters the yield phase.


The nodes that ―survive‖ the Prioritization Phase keep on trying to gain access to the channel. The objective of this medium access mechanism is to eliminate as more contending nodes as possible, but of course not all of them. During the Elimination Phase, a great percentage of the contending nodes is eliminated, but at least one of them survives. Every node that has not been defeated during the Prioritization Phase transmits an elimination pulse which is actually the lengthening of the priority pulse. Right after the end of this pulse, the nodes allow an idle slot, which is called survival verification slot, during which they sense the channel.


If a node detects a transmission during this time interval, this means that the specific node is ―defeated‖, so it defers. Thus, the nodes that survive the Elimination Phase carry the packets of the highest priority and they have transmitted the longest elimination pulse.


Yield Phase


Yield Phase is the last phase of EY-NPMA, and is the last try to reduce collisions. Only the nodes that have survived elimination phase start the yield phase. The node selects a random number of idle slots uniformly distributed between 0 and 9. At the end of the yield phase, the node again senses the channel. If the channel is idle, it starts its transmission.


Yield Phase is the last phase before the transmission of a data packet and it is the last effort to reduce the number of the contending nodes. The nodes that have survived the Elimination Phase enter the Yield Phase allowing a number of idle slots. Every node that detects transmission during these slots quits the current effort to gain access to the channel and waits till the next channel access cycle. If a node detects no transmission, it eventually transmits its data packet. Thus, a node

―loses‖ in Yield Phase, when it listens some other node transmitting a data packet.

The number of the idle slots is random and uniformly distributed between 0 and 9.



In the last decade of the twentieth century, technological improvements developed ways to achieve the objective of location and time independent communications. This objective has come into the light by the concept of personal communications networks and services. With the increasing role of multimedia and computer applications in communications, the main objective has become the extension of mobile communications and design a new generation of wireless personal communication networks, capable of supporting a variety voice, video and data traffic. The user demand for higher transmission speed and multimedia capability, as well as for mobile computing using portable computers becomes remarkable. These developments have motivated the studies on broadband wireless network technologies such as Wireless ATM (Asynchronous Transfer Mode) or WATM.


The concept of WATM was first proposed in 1992 as pointed out in and now it is actively considered as a potential framework for next-generation wireless communication networks capable of supporting integrated, quality-of-service (QoS) based multimedia services. The strength of wireless ATM technology is said to be its ability to provide support for different protocols, such as ISDN1 and Internet protocols. As the volume of wireless traffic is increasing, so is the role of QoS support, which will become very important when multiple services are multiplexed into the same radio access technology. As QoS support is a fundamental property of ATM technology, WATM promises a solution for this requirement. ATM is a very complex system and modifications for wireless communication and mobility management is going to make it more difficult.


Need for WATM


The area of wireless transmission systems has been increasing rapidly. Mobility raises a new set of questions, techniques, and solutions. This growth will occur in an environment characterized by rapid development of end-user applications and services towards the Internet and broadband multimedia delivery over the evolving fixed-wired infrastructure. Therefore, new developments of wireless networks are needed to enable wireless technologies to interwork with existing wired networks. Therefore, in order for ATM to be successful, it must offer a wireless extension. Otherwise it cannot participate in the rapidly growing field of mobile communications.

As ATM networks scale well from local area networks (LANs) to wide area networks (WANs), and there is a need for mobility in local and wide area applications, a mobile extension of ATM is required in order to have wireless access in local and wide environments. Many other wireless technologies, such as EEE 802.11, typically only offer best-effort services or to some extend time-bounded services. However, these services do not provide as many QoS parameters as ATM networks do. WATM could offer QoS for adequate support of multimedia data streams.


a. Reference Model


The WATM system reference model, proposed by ATM Forum Wireless ATM (WATM) group, specifies the signaling interfaces among the mobile terminal, wireless terminal adapter, wireless radio port, mobile ATM switch and non-mobile ATM switch. It also specifies the user and control planes protocol layering architecture. This model is commonly advocated by many communication companies, such as NEC, Motorola, NTT, Nokia, Symbionics, and ORL.


The major components of a Wireless ATM system are: a) WATM terminal, b) WATM terminal adapter, c) WATM radio port, d) mobile ATM switch, e) standard ATM network and f) ATM host. The system reference model consists of a radio access segment and a fixed network segment. The fixed network is defined by "M (mobile ATM)" UNI and NNI interfaces while the wireless segment is defined by "R (Radio)" radio access layer (RAL) interface.

The "W" UNI is concerned with handover signaling, location management, wireless link and QoS control. The "R" RAL governs the signaling exchange between the WATM terminal adapter and the mobile base station. Hence, it concerns channel access, datalink control, meta-signaling, etc. The "M" NNI governs the signaling exchange between the WATM base station and a mobile capable ATM switch. It is also concerned with mobility-related signaling between the mobile capable ATM switches



The broadband radio access networks (BRAN), which have been standardized by the European Telecommunications Standards Institute (ETSI), could have been an RAL for WATM (ETSI, 2002b). The main motivation behind BRAN is the deregulation and privatization of the telecommunication sector in Europe. The primary market for BRAN includes private customers and small to medium-sized companies with Internet applications, multi-media conferencing, and virtual private networks. The BRAN standard and IEEE 802.16 (Broadband wireless access, IEEE, 2002b) have similar goals.


BRAN standardization has a rather large scope including indoor and campus mobility, transfer rates of 25–155 Mbit/s, and a transmission range of 50 m–5 km. Standardization efforts are coordinated with the ATM Forum, the IETF, other groups from ETSI, the IEEE etc. BRAN has specified four different network types (ETSI, 1998a):


4. HIPERLAN 1: This high-speed WLAN supports mobility at data rates above 20 Mbit/s. Range is 50 m, connections are multi-point-to-multi-point using  ad-hoc or infrastructure networks  


5. HIPERLAN/2: This technology can be used for wireless access to ATM or IP networks and supports up to 25 Mbit/s user data rate in a point-to-multi- point configuration.  


6. HIPERACCESS: This technology could be used to cover the ‗last mile‘ to a customer via a fixed radio link, so could be an alternative to cable modems


                            or xDSL technologies (ETSI, 1998c).  


7. HIPERLINK: To connect different HIPERLAN access points or HIPERACCESS nodes with a high-speed link, HIPERLINK technology can  be chosen.  


8. As an access network, BRAN technology is independent from the protocols of the fixed network. BRAN can be used for ATM and TCP/IP networks as illustrated in Figure. Based on possibly different physical layers, the DLC layer of BRAN offers a common interface to higher layers. To cover special characteristics of wireless links and to adapt directly to different higher layer network technologies, BRAN provides a network convergence sub layer. This is the layer which can be used by a wireless ATM network, Ethernet, Fire wire, or an IP network. In the case of BRAN as the RAL for WATM, the core ATM network would use services of the BRAN network convergence sub layer.

HiperLAN2 While HIPERLAN 1 did not succeed HiperLAN2 might have a better chance. HiperLAN2 offers more features in the mandatory parts of the standard (HiperLAN2, 2002).


High-throughput transmission: Using OFDM in the physical layer and a dynamic TDMA/TDD-based MAC protocol, HiperLAN2 not only offers up to 54 Mbit/s at the physical layer but also about 35 Mbit/s at the network layer.

Connection-oriented:  Prior  to  data  transmission  HiperLAN2  networks establish logical connections between a sender and a receiver


Quality of service support: support of QoS is much simpler. Each connection has its own set of QoS parameters (bandwidth, delay, jitter, bit error rate etc.).  


Dynamic  frequency  selection:  HiperLAN2  does  not  require   frequency

Security support: Authentication as well as encryption are supported by HiperLAN2.

Mobility support: Mobile terminals can move around while transmission always takes place between the terminal and the access point with the best radio signal.


Application and network independence: HiperLAN2 was not designed with a certain group of applications or networks in mind. Access points can connect to LANs running ethernet as well as IEEE 1394 (Firewire) systems used to connect home audio/video devices.

Power saves: Mobile terminals can negotiate certain wake-up patterns to save power.




The above Figure shows the standard architecture of an infrastructure-based HiperLAN2 network. Here, two access points (AP) are attached to a core network. Core networks might be Ethernet LANs, Firewire (IEEE 1394) connections between audio and video equipment, ATM networks, UMTS 3G cellular phone networks etc. Each AP consists of an access point controller (APC) and one or more access point transceivers (APT).


An APT can comprise one or more sectors (shown as cell here). Finally, four mobile terminals (MT) are also shown. MTs can move around in the cell area as shown. No frequency planning is necessary as the APs automatically select the appropriate frequency via dynamic frequency selection. Three handover situations may occur:


                        Sector handover (Inter sector): If sector antennas are used for an AP, which is optional in the standard, the AP shall support sector handover. This type  of handover is handled inside the DLC layer  


Radio  handover  (Inter-APT/Intra-AP):  As  this  handover  type,  too,  is handled within the AP, no external interaction is needed.  


Network handover (Inter-AP/Intra-network): This is the most complex

situation: MT2 moves from one AP to another.


HiperLAN2 networks can operate in two different modes (which may be used simultaneously in the same network).


Centralized mode (CM): In infrastructure-based mode all APs are connected to a core network and MTs are associated with APs.  

· Direct mode (DM): The optional ad-hoc mode of HiperLAN2 directly exchanged between MTs if they can receive each other, but the network still  controlled.


The above figure shows the HiperLAN2 protocol stack as used in access points. Protocol stacks in mobile terminals differ with respect to the number of MAC and RLC instances (only one of each). The lowest layer, the physical layer, handles as usual all functions related to modulation, forward error correction, signal detection, synchronization etc. The data link control (DLC) layer contains the MAC functions, the RLC sub layer and error control functions. The MAC of an AP assigns each MT a certain capacity to guarantee connection quality depending on available resources.


Above the MAC DLC is divided into a control and a user part. The user part contains error control mechanisms. HiperLAN2 offers reliable data transmission using acknowledgements and retransmissions. The radio link control (RLC) sub layer comprises most control functions in the DLC layer (the CC part of an AP). The association control function (ACF) controls association and authentication of new MTs as well as synchronization of the radio cell via beacons.


The DLC user connection control (DCC or DUCC) service controls connection setup, modification, and release. Finally, the radio resource control (RRC) handles handover between APs and within an AP. On top of the DLC layer there is the convergence layer. This highest layer of HiperLAN2 standardization may comprise segmentation and reassembly functions and adaptations to fixed LANs, 3G networks etc.

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