by Torben Rune, Netplan ApS. Denmark
The new information society requires means to provide broadband communications with mobility functionality. During 1996, Europe will see the establishments of wireless data networks based on standards such as DECT and the 2.45 GHz ETSI standard. While these systems replace cables in office environments efficiently, they are not suitable for supporting a large number of mobile users utilizing multi-media applications. New standards such as ETSIs HIPERLAN (High Performance Radio Local Area Network) are needed. The purpose of this paper is to present research and development activities in the RLAN area. The IEEE 802.11 standard, the DECT Data Services Profiles as well as the HIPERLAN standard and the relationship with ATM are described. Finally the performance and security aspects are discussed.
Wireless Local Area Networks have been around for a number of years. However, the wireless LAN market is expected to take off during 1996. There are several reasons for this, but the most obvious one might be the rapid evolution in mobile computing. In Europe, regulatory considerations have also prevented the introduction of wireless LANs.
When wireless LANs were introduced on the market in the end of last decade, most products were designed to be used together with stationary desktop computers. Their main objective was to offer flexible and cost-effective alternatives to the LAN cable. Aside from saving the often high costs of installing and maintaining the cable, another benefit of wireless LANs was the speed and ease with which new LANs could be created or new computers added.
These benefits are of course still valid today, but office computing is changing at an amazing pace. Advances in low power microprocessors, display quality and battery technology are resulting in extremely powerful notebook PCs. Consumer electronics and personal computers are merging to create an even smaller class of computing products, the Personal Digital Assistants (PDAs). Through the integration of telephone and multi-media technology in these devices, intelligent terminals for telecommunication services are created.
Users need the freedom to move about the workplace unencumbered by network cable and power cords. Yet, they require timely access to all of the business information. This type of mobile computing has placed an entirely new set of demands on office LANs, among those: higher data-rates, more reliable services, shorter wavelength i.e. shorter antennae etc.
A number of application areas for wireless LANs have been envisaged. Except from the office environment, wireless LANs are today used in hospitals to provide personnel with the possibility to gain access to information such as patient records on the move. Similar needs are expressed by personnel working at airports, construction areas, universities etc. Wireless LANs are also used together with bar-code readers in supermarkets or vehicular mounted on mobile cranes and forklift trucks. Another application area for wireless LANs are to bridge between cabled LANs in different buildings. Ad-hoc networking is another application, which is expected to take advantage of the wireless connection.
Ad-hoc networking is what happens when two or more workstations join the same network, forming the network at the time it is needed. That network exists only as long as there are stations in it.
Today, two major technologies can be used to convey data by wireless LAN means. The first technology is the use of the Industrial, Medical & Scientific (ISM) band between 2400 and 2483.5 MHz. To use this frequency band, equipment must be compliant to the European Telecommunication Standard ETS 300 328. This standard defines the technical requirements on equipment using the 2400-2483.5 MHz frequency band.
Since the 2400-2483.5 MHz frequency band is used by other types of equipment, such as microwave ovens, techniques to avoid interference have to be used. The ETS 300 328 standard stipulates that frequency spreading must be used.
The spreading techniques normally used in wireless LAN products can be divided into two families: Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS). The first approach resists interference by jumping rapidly from frequency to frequency in a pseudorandom way. The receiving system has the same pseudorandom algorithm as the sender, and jumps simultaneously. The second approach resists interference by mixing in a series of pseudorandom bits with the actual data. The receiver, using the same pseudorandom algorithms, strips out the extra bits.
In a spread spectrum system, there is a possibility to multiplex users by assigning them different spreading keys. Such a system is called a Code Division Multiple Access (CDMA) system. However, most wireless LAN products are not CDMA systems since users belonging to the same wireless LAN utilize the same spreading key. Instead users are multiplexed in time using nearly the same Carrier Sense Multiple Access (CSMA) protocol as in Ethernet.
The raw bit rate of equipment using the 2400-2483.5 MHz frequency band is normally 2 Mbit/s giving a net throughput of typically 600-800 kbit/s. In Europe, the effective radiated power from the antenna must not exceed 100 mW which is significantly lower than allowed by the Federal Communications Commission (FCC) in USA. With 100 mW effective radiated power from the antenna, the radio range is normally 20-50 meters indoor and a few hundred meter outdoor.
The other technology available in Europe is the Digital European Cordless Telecommunications (DECT) standard, ETS 300 175. DECT was developed by ETSI for a wide range of high-density wireless applications, both private and public, throughout Europe. The standard is supported by a range of pan-European regulatory instruments, including a pan-European frequency allocation in the 1880-1900 MHz-band now available in all member states, and Common Technical Regulations (CTR), covering type approvals in the European Union.
Within the frequency allocation for DECT there are 10 channels, each of which uses Time Division Multiple Access (TDMA) and Time Division Duplex (TDD) to support 12 duplex speech channels or 24 simplex data channels. This gives a total of 120 duplex speech channels or 240 simplex data channels that can be accessed by any DECT system. A DECT system automatically chooses which of the channels to use utilizing a Dynamic Channel Allocation (DCA) algorithm. This allows multiple, independent DECT systems to operate in the same radio environment without interfering with each other.
DECT works with a raw bit rate of 1152 kbit/s. When DECT is used to convey data it gives a net throughput of 552 kbit/s at a bit error rate not exceeding 10-8 and when using a single radio. Higher system capacities can be achieved by adding several base stations within the same cell. As DECT was designed to operate in residential, business and public environments, it also supports full authentication and encryption, thus ensuring that it is a suitable medium for confidential information.
One important difference between DECT and wireless LAN products using the 2400-2483.5 MHz frequency band is that higher output power and directive antennae are allowed in DECT. The DECT standard allows 250 mW plus the gain offered by directive antennae. This can give more cost-effective solutions when building larger wireless LAN infrastructures where premises can be covered from the outside by a single base station. The radio range of a DECT system is normally 30-100 meters indoors and up to 3 kilometers outdoors.
The addition of data services to a wireless PABX can also use the same installed set of base stations that already provide speech services. In particular, services such as fax, e-mail and access to files stored on a LAN can be provided. An infrastructure that can offer both wireless speech and data services over the same infrastructure such as DECT will offer extremely flexible and cost effective solutions.
Today there is no wireless LAN standard that caters for interoperability of different manufacturers' equipment. However, standardization efforts are under way both within ETSI and IEEE.
In recognition of the importance and potential of using DECT for wireless LAN access, the DECT standards committee established a working party in August 1993, RES-03/DATA, to develop the DECT Data Services Profiles. The first standard in this family of profiles contains interworking conventions to Ethernet and Token Ring. This standard will be the first pan-European wireless LAN standard that caters for interoperability of different manufacturers equipment. It supports net throughput data rates of up to 552 kbit/s using a single radio. The standard was finalized in February 1994 and is expected to be approved in mid 1995. Other standards are expected to cover interworking with services such as Fax, Modem, RS-232 and File Transfer.
Another standardization effort within ETSI is HIPERLAN (High Performance Radio Local Area Network). The committee responsible for HIPERLAN is RES-10 which has been working on the standard since November 1991. CEPT has allocated two frequency bands for HIPERLAN, one band between 5.15 and 5.30 GHz and one band between 17.1 and 17.3 GHz. The standard for the 5 GHz band was finalized in January 1995 and is expected to be approved in the beginning of 1996. Work on the 17 GHz standard has recently started.
The 5 GHz HIPERLAN standard has specified a raw bit rate of approximately 24 Mbit/s using a MAC protocol supporting multi-media communication. The 17 GHz version is expected to offer an ATM 155 Mbit/s compatible bit rate. Both standards will cater for interoperability of different manufacturers' equipment and are aimed to work efficiently with ATM cell transfer. Products compliant to the HIPERLAN 5 GHz standard shall be possible to implement on a PCMCIA Type III card and should be available on the market 1997.
Within IEEE the 802.11 committee is working on a standard for wireless LAN products using the 2400-2483.5 MHz frequency band. This standard may be used in Europe since its European version applies to the ETS 300 328 standard. The IEEE 802.11 standard also caters for interoperability of different manufacturers' equipment offering a raw bit rate of 1 or 2 Mbit/s. A first draft og the standard was finalized in March 1995 and is expected to be approved in the end of 1995. Products compliant to the IEEE 802.11 standard will probably be available on the market shortly after the standard has been approved.
Seen from the perspective of the datacommunications industry, HIPERLAN is the most interesting of the coming standardized wireless technologies. HIPERLAN has the highest datarate, and its design allows for a variety of applications. The tentative design parameters for HIPERLAN were:
Parameter |
Traffic Type |
Value |
| Data rate | asynchronous | 1 to 20 Mbit/s |
| time-bounded | 64 kbit/s to 2.048 kbit/s | |
| Systems throughput | 20 Mbit/s 1000 Mbit/s per hectare per floor | |
| Mean latency | asynchronous | <1 ms. (at 30% capacity |
| Latency of service initiation | time-bounded | <3 S |
| MSDU Delay variance | asynchronous | no limit |
| time-bounded | <3.0 mS2 | |
| Range | to 50 m at 20 Mbit/s to 800 m at 1 Mbit/s |
|
| Area Coverage | 99.9% (single hop) | |
| Temporal Coverage | 99.9% (single hop) | |
| MPDU detected loss/error rate | <10-3 | |
| MPDU undetected loss/error rate | <8x10-8 per octet pf MPDU length | |
| MSDU undetected loss/error rate | <5x10-14 per octet of MPDU length | |
| Co-location tolerance | 50 cm of free space | |
| Mobility tolerance | 10 m/s linear, 360o/s angular | |
| Packet information field maximum size | 16 Kbytes | |
| Physical size target (excl. antenna system) | PC-Card (PCMCIA) type III (85x54x10.5 mm) | |
| Power consumption | few hundred mW |
The ETSI HIPERLAN Reference Model defines the components needed to build a Private Virtual Radio Sub-Network. Private in this context means user-owned and -operated. A Virtual Sub-Network is a logical subset of a set of communicating entities each of which is differentiated from the rest by some unambiguous identifier [Node Identifier or NID]. Virtual Sub-Networks are differentiated by an additional level of unambiguous identifier [HIPERLAN Identifier or HID].
Performance is one of the most important factors when dealing with wireless LANs. In contrast to other radio-based systems, data traffic on a local area network has a randomized bursty nature, which may cause serious problems with respect to throughput.
To give just a brief view of the problems we are faced with, we use HIPERLAN as an example. The following may be through for any radio-based LAN. HIPERLAN is, among other things, a MAC standard. This means that not only the "radio" operation of a HIPERLAN station, but also details on how to solve protocol-related issues like re-transmission, relaying and routing is covered.
HIPERLAN is designed to work without any infrastructure. Two stations may exchange data directly, without any interaction from a wired (or radio-based) infrastructure. The simplest HIPERLAN thus consists of two stations. Further, if two HIPERLAN stations are not in radio contact with each other, they may use a third station (i.e. the third station must relay messages between the two communicating stations). How this is done in practice we will deal with later. Figure 1 shows the relative throughput as a function of the relative network utilization, for various types of LANs.
Figure 1 Throughput S as a function of relative utilization a for different network types
It shows, that a slotted CSMA/CD scheme has the best over-all performance, while the ALOHA schemes has the poorest. In nature, a simple HIPERLAN between two stations that are in direct radio contact, will be an unslotted CSMA/CD system. In a system like that, the throughput S is given by:
where A is the probability of one station attempting to acquire the medium, and
The number of stations, or networking nodes, between two communicating parties in a HIPERLAN affects the overall throughput of the network. Not only is the bridging nodes loaded with network traffic of its own, but also with pass-through traffic that requires path and route resolving. Depending on the processing capacity of the bridging nodes, throughput is reduced primary by transition delay. To overcome this, the data packet size at the MAC layer must be as large as possible, reducing the overhead per package routed through the HIPERLAN. If we assume, that the packet size is such that the delay caused by routing is 1/10 of the transmission time for each packet, the throughput for a HIPERLAN network consisting of two or more bridging nodes can be calculated. Figure 2 shows, that as long as the utilization of the network is low, the number of bridging nodes plays no significant role to the throughput, while when the utilization grows, a degradation of throughput is expected.
Figure 2 Throughput in a HIPERLAN as function of utilization and the number of intermediate bridging nodes
This result is not surprising. The question is, if it will be possible to define a MAC protocol that allows for large packets on one hand, and on the other hand may interface to existing LAN driver interface specifications such as NDIS or ODI. Further, another question is, if the assumption, that the transition delay in the forwarding nodes can be as little as 1/10 of the transmission time. At 16 Mbit/s and with an average package size of 8 Kbytes, the total transition time must be less than 4,1 mS which corresponds to about 200.000 CPU clock-cycles at 50 MHz.
Further one should have in mind, that HIPERLAN has been defined as bearer of both asynchronous and synchronous traffic. The consequence of mixing asynchronous and synchronous traffic on the same HIPERLAN is not yet known.
When the signal path is no longer limited to be in a shielded coaxial- or UTP (unshielded twisted pair) cable, it puts a totally new set of security aspects on the usage of wireless LANs. The essential difference between a HIPERLAN and any existing standard sub-network is the intrinsic sharing of the communications medium and the mobility in the networking environment. In HIPERLAN each communicating node is given a set of identifiers: 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. Within one single HIPERLAN (that is, among nodes that have the same HID), all nodes can communicate with each other. The nodes are using a dynamic routing mechanism denoted Intra-HIPERLAN Forwarding. If a node comes within range of another HIPERLAN, it may try to establish communications to its own HIPERLAN via the new HIPERLAN, if the two HIPERLANs allow it. To establish such a connection a routing scheme denoted Inter-HIPERLAN Forwarding is used. Inter-HIPERLAN Forwarding requires a bilateral agreement between the two HIPERLANs.
It is essential to note, that HIPERLAN in itself does not support any features directly related to end-to-end security. Encryption services are planned for in HIPERLAN (the IEEE 802.11 standard has provision for MAC-layer encryption, denoted Wired Equivalent Privacy, to indicate, that the security to expect from the MAC-layer is at a comparable level to that of a wired infrastructure.), but the Intra- and Inter-HIPERLAN Forwarding are only sub-MAC layer functions that together with the HIPERLAN Management functions resolves the complex task of routing and bridging. As with other OSI standards, secure data transport is obtained in layers above the MAC layer, and is in the case of HIPERLAN the responsibility of the HIPERLAN service requester.
Figure 3 Inter- and Intra HIPERLAN Forwarding in two different HIPERLANs
Personal computer networking is undergoing a drastic transformation in response to end users' need for mobility and connectivity. During 1996, Europe will see the establishments of wireless data networks based on standards such as DECT and the 2.45 GHz ETSI standard. Forthcoming standards that cater for interoperability of different manufacturers' equipment will grow the cordless networking market to an important telecommunication market, giving the mobile computing environment the same wireless lift as has GMS to the voice market.
Among other technologies HIPERLAN will offer high data rates and relative high throughput even without any infrastructure in the network. This opens basically new applications to wireless datacommunications. Compared to the growth in for example the number of Internet nodes, which has risen from virtually nothing in 1986 to over 1.4 Millions in 1993, the potential number of HIPERLAN nodes is even higher. Radio-based LANs are doing to portables what they should be: truly movable.
John Porter, Andy Hopper, "An ATM based protocol for Wireless LANs ", Olivetti Research Ltd. OLR Technical Report 94.2
Subir Biswas, Andy Hopper, "Automatic Management Scheme for a Mobile Radio LAN", Proceedings of the IEEE Conference on Personal Wireless Communications, Bangalore, August 1994.
Alanko, T., Kojo, M., Laamanen, H., Liljeberg, M. Moilanen, M., Raatikainen, K.: "Mesured Performance of Data Transmission over Cellular Telephone Networks." University of Helsinki, Department of Computer Science, Series of Publications C, No. C-1994-53, November 1994.
Kojo, M., Raatikainen, K., Alanko, T: "Connecting Mobile Workstations to the Internet over a Digital Cellular Telephone Network". University of Helsinki, Department of Computer Science, Series of Publications C, No. C-1994-39, September 1994.
Ph.D Thesis by Mitchell Tasman, "Protocols and Caching Strategies in Support og Internetwork Mobility", University og Wisconsin, October 1994.
Jean-Paul M.G. Linnartz: "Narrowband Land-Mobile Radio Networks ", Artech House, Boston, 1993, ISBN 0-89006-645-0.
Torben Rune received his Masters degree in computer technology from the Technical University of Denmark, Copenhagen in 1983. After three years as software developer he joined Fischer & Lorenz. In 1989 he formed his own company, and has since that worked as an independent consultant in the field of tele- and datacommunications and managing director of Netplan Aps. During 1992 as an ETSI member, Torben Rune held the position as team leader of PT41, the project team under RES-10 responsible for defining the coming ETSI Hiperlan standard. Contact by internet e-mail: tr@netplan.dk
If you are interested in receiving more information on HIPERLAN, you may contact ETSI at the following address:
ETSI
Route des Lucioles
F-06921 Sofia-Antipolis
France
Phone: +33 4 92 94 42 00
Fax.: +33 4 93 65 47 16
If you want to participate actively in the HIPERLAN standardizations work, you may contact the chairman of the RES10 group, at this address:
Bernard Bourin
Dassault Automatismes et Telecommunications
9 Rue Elsa Triolet
Z.I. des Gatines
F-78373 Plaisir Cedex
France
Phone.: +33 1 30 81 20 00
Fax.: +33 1 30 55 19 31
Also read Bernard Bourin's article on HIPERLAN
If you want more general information on wireless data solutions or other related issues, you are welcome to contact me at Netplan:
Netplan ApS.
Att.: Torben Rune
Frydenborgvej 27D
DK-3400 Hillerød
Phone:+45 48 24 68 28
Fax.:+45 48 24 68 29
E-mail: tr@netplan.dk