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Inside Networking
March 2000


Tony Rybczynski

Wireless LANs: Infrastructure Counts


Go Right To: A Wireless LAN Enhances Health Education

Just as PCs were the initial driving force behind wired Ethernet innovations in the 80s, the wide acceptance of laptops, palmtops, PDAs, and (of course) wireless telephony is creating an environment that is ripe for innovation in the form of wireless Ethernet connectivity. To find evidence of this trend, we need look no further than wireless LANs, which are becoming more and more refined. Already, products exist that conform to standards while offering high performance, security, and ease of use.

Who might use these products? Businesses considering an add-on network strategy, whether they are large, multi-site corporations or small/medium businesses. Such a strategy, once implemented, could help these businesses meet the need for continuous connectivity to critical network resources (such as financial and inventory databases) and for intranet and Internet access.

More generally, wireless networking offers a new dimension in productivity for PC users, enabling true portability through continuous, non-stop network access. Continuous connectivity makes for continuous productivity.

Until a couple of years ago, the world of wireless data networking had been dominated by proprietary niche products that targeted vertical markets. But with most vendors adopting the IEEE 802.11 standards, wireless LANs are reaching a new level of acceptability with mainstream corporate users.

But what are the IEEE 802.11 standards? Let�s take a close look. Such an examination will help us weigh the implications of adding telephony. In addition, a review of the standards will put us in a better position to relate these developments to events in the public wireless space. (Wireless LANs are well positioned to fulfill the wireless Internet�s inherent infrastructure requirements. These requirements include both the unification of untethered data and telephony systems, as well as the deployment of next-generation public wireless systems.)

The Original Wireless Ethernet Standard
Wireless Ethernet standards are the responsibility of the IEEE 802.11 committee. The original 802.11 standard delivered throughputs in the 1 to 2-Mbps range shared across a number of users. With technology advances, the new standard supports throughputs of 11 Mbps.

Analogous to wired Ethernet, the IEEE 802.11 standard specifies the operation of the Physical Layer and the Media Access Control or MAC Layer. It uses the 83.5-MHz-wide, 2.4-GHz ISM (Industry, Science, and Medical) band, which is available for unlicensed operation worldwide, using low power spread spectrum radio frequency modulation technology to minimize interference.

Physical Layer: Two Physical Layer modes of operation are supported: namely, Direct Sequence and Frequency Hopping. A third mode is Diffuse Infrared, which, while cost effective, has very limited reach.

Direct Sequence (DS) operation takes the original data stream and multiplies it by a spreading factor or �chipping code.� Depending on the coding method, this approach can result in 2 or 11-Mbit/s throughputs to a particular user. Limited simultaneous transmissions are possible, but they have to be substantially non-overlapping spectrum.

In contrast, Frequency Hopping (FH) consists of grabbing one of 78 separate 1-MHz channels for short intervals (usually on the order of 100 ms) and changing frequencies in some algorithmic fashion. Scalability is greater with FH because it allows many multiple simultaneous transmissions. Interference from other ISM sources (for example, microwave ovens) is low because the frequency is changing 10 times per second. At the same time, FH is limited to 1 Mbps throughputs per user. Either spread spectrum technique is relatively immune to tapping.

MAC Layer: The MAC layer specified rules of access, providing mechanisms to provide contention mechanisms independent of which Physical Layer mode is used. Like the Ethernet IEEE 802.3 standard, IEEE 802.11 uses Carrier Sense Multiple Access (CSMA). However, unlike IEEE 802.3, it uses collision avoidance rather than collision detection. Simply, rather than detecting collisions on the shared media LAN and then backing off if collisions occur, it uses a request to send/clear-to-send handshake to minimize possibilities of collisions. During this handshake, the length of the data burst is also communicated. Collision avoidance is used instead of carrier sensing as other transmitters may be �hidden� and therefore not detected. This approach makes 802.11 more bandwidth-efficient than traditional 802.3 systems.

Wireless Ethernet Refinements
While the first-generation wireless LAN standards and products operated in the 1�2-Mbps range, two new standards have been developed which deliver much higher bandwidths. These standards are called IEEE 802.11a and IEEE 802.11b.
IEEE 802.11a: Based on a variant of Frequency Division Multiplexing, IEEE 802.11a can accomplish data modulation, the main benefit of which is its immunity to multipath echoes, which are typical to the indoor and mobile environments. While other bit rates are allowed, all implementations are required to support 6, 12, and 24 Mbps. The multi-rate mechanism of the MAC protocol ensures that all devices communicate with each other at the best data rate.

IEEE 802.11a is specified for use at 5 GHz. Currently, this band is open to unlicensed devices only in the United States. Another downside is that 802.11a�s low penetration power may limit its value in many office environments.

IEEE802.11b: This variant uses the 2.4 GHz band, but achieves 11 Mbps rates (with fallback to 5.5 Mbps) through a coding scheme derived from the Direct Sequence technology. The industry is focusing on 802.11b as the volume market wireless LAN technology. The first PC product supporting 11 Mbps operation is the Apple iMac.

More IEEE 802.11 Features
Two other features of IEEE 802.11 should be noted: ad hoc mode and low latency mode. The ad hoc mode allows two laptops or PDAs to communicate directly with each other without needing to go through a base station (or Access Point) � a useful feature for exchanging data between users on an as-required basis, even in the absence of a wireless LAN infrastructure deployment. The advantage over generally available infrared links is range and the fact that IEEE 802.11 is an omni-directional technology (eliminating the need to align the communicating devices).

Latency mode introduces virtual Class of Service support over the radio spectrum and opens up wireless telephony opportunities, not just in the context of data devices but also as the next generation wireless telephony systems.

Wireless LANs have evolved driven by the needs for data connectivity, initially based on proprietary standards and more recently on IEEE 802.11. The penetration has been compatible with technologies in the early adoption stage and limited by the perception of low bandwidth (that is, 2 Mbps) and by the per user costs (that is, in the $500-plus range).

At the same time, in-building wireless telephony systems have been developed using traditional circuit establishment and switching mechanisms. Early adopters such as the healthcare and retail industries have been faced with deploying separate wireless infrastructures to support voice and data services.

Three major factors are changing this landscape significantly:

  • The introduction of 11-Mbps wireless LANs and the anticipated lowering in per user equipment costs towards the $100 range.
  • The explosion in laptop, palmtop, and PDA devices.
  • More connectivity options and new applications enabled through IP telephony.

These three factors have created the opportunity for the convergence of wireless telephony and wireless LAN systems onto IEEE 802.11b systems. Therefore, in 2000, we can expect the introduction of 802.11b devices in the form of integrated and PCMCIA solutions in lap, palm, and handheld data devices; screen-based multifunction wireless �telephones�; and specialized devices targeted at specific vertical markets such as healthcare, retail, and hospitality.

A significant technology enabler for these devices, given their limited display and storage capacity is the Wireless Application Protocol (WAP), which has been developed specifically for wireless mobility.

IEEE 802.11b systems are the preferred solution for in-building-centered wireless LAN applications, delivering 11 Mbps of bandwidth � a significant enhancement over today�s systems. However, when wide area broadband mobility is a requirement from a single device, some new opportunities emerge, opportunities enabled by new 2.5G and 3G (third generation) public wireless systems.

For example, 2.5G GSM systems allocate multiple 25 KHz channels (up to eight per carrier for approximately 100 Kbps of effective bandwidth) for IP, avoiding contention between voice and data. This contrasts with 3G systems that intermix IP and telephony across a 5 MHz band using spread-spectrum techniques for an effective peak burst data rate of 2 Mbps (when stationary), 384 Kbps (when walking), and 144 Kbps (when in a moving car). In any case, these public systems will support the same types of applications (albeit at a lower rate), as discussed above, using the WAP standard.

Customers desiring in-building and wide area broadband mobility will have several choices:

  • Use the public wireless standard in both the WAN and the building environments on a pay-as-you-go basis, and accept the lower bandwidth capacity.
  • Rely on microcellular technology. This option is similar to the option just mentioned; however, in this case, low-power 2.5G or 3G transmitters are distributed as microcells around the building (contrasted with standard multi-kilometer wide cells in the WAN). The advantage is that this approach avoids usage-based airtime charges, though there is a charge for sharing of the spectrum with the carrier.
  • Use dual-mode devices that can operate using 802.11b in the building and one of the public standards outside of the building.
  • Create �islands� of wireless LAN coverage in public areas such as airport lounges that can be connected seamlessly to the corporate LAN. Such a service is available today from Mobilestar.

The development of 802.11 wireless LAN products should make the idea of actually deploying wireless LANs more attractive. In fact, the Cahners In-Stat Group predicts dramatic growth in the number of wireless LAN users in the United States, from 2.3 million today to 23 million in the year 2003. Going forward, the introduction of IEEE 802.11b high-capacity wireless LANs will enable new telephony/data mobile applications and devices and bring us closer to Star Trek-style communications.

Tony Rybczynski is director of strategic marketing and technologies for Nortel Networks� Enterprise Solutions unit. This business unit offers a full range of enterprise terminal, workgroup, campus, and wide-area unified networks and applications, through direct and indirect channels. For more information, visit the company�s Web site at www.nortelnetworks.com. E-mail questions or comments to [email protected].

A Wireless LAN Enhances Health Education

The Johns Hopkins School of Public Health was one the first universities in the country to install a large-scale wireless laptop network, enabling students, faculty, and staff members to access the school�s computer network without being crowded into computer labs. Using this system, 500 users may surf the Internet, send and receive e-mail messages, and print documents over the school�s network, providing a more flexible collaborative environment for project research.
The implementation of 802.11 consists of a number of Nortel Networks� Baystack 600 base stations (called Access Points) deployed in 40 classrooms, two cafeterias, two libraries, three student lounges, six conference rooms, and two auditoriums. The goal is to saturate the entire campus. Each Access Point operates in one of the two Physical Layer modes and has one or more transmitters depending on the desired system capacity. User laptop PCs are equipped with an integrated or plug-in Network Interface Card (NIC).

The range of the radio can be as great as 700 meters in open space, but more typically 100 meters inside buildings. The wireless LAN vendor supplied a site survey tool, which provided instant readings of signal strength anywhere in the building from multiple Access Points, providing the data required for optimal placement of Access Points for the desired coverage.

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