The Untethered LAN

The mobile machines, which are essentially client PCs, "talk" to a

server on a wired LAN using these access points. An access point simply acts as

an intermediary between the wireless PC and the wired LAN.

During the last three years, we saw a mushrooming of technologies and their

related applications designed for mobile telephony. Applications such as

mobile-commerce and virtual gaming are now becoming possible due to 2.5G

(Generation) and 3G technologies. Technologies such as General Packet Radio

Service (GPRS), a 2.5G technology that is a stepping-stone to 3G Universal

Mobile Telecommunications System (UMTS), are beginning to transform the way we

interact with our favorite voice/data transmission device called the cell-phone.

The revolution continues, with Japan and US leading in research and development

of hyper-speed content rich mobile solutions that are envisioned as 4G

deliverables.

With increased usage of wireless devices such as pagers, cell-phones,

Personal Digital Assistants (PDAs), it became necessary to explore the

application of wireless technologies in the confines of enterprise-wide

computing. In 1997, IEEE ratified the 802.11 as a standard that promised the

movement of PCs without any wires within a Local Area Network (LAN). The beauty

of this standard was that it created LAN environments running on radio waves.

802.11 was designed to provide 1 and 2 Mbps data rates. In 1999, similar

variations called 802.11a and 802.11b were confirmed. With two distinct data

transmission rates for 802.11a and 802.11b, this technology has started to draw

the attention of IT groups that have dedicated "move and install

teams". These teams are primarily focused on moving and "cabling"

their users’ PCs on a departmental LAN.

What and How?

To keep technology structured and interoperable, IEEE adopted 802 series of

protocol standards for LANs. The 802 series deals with the lowest layers 1 and 2

of the ISO Reference Model. Protocols 802.2, 802.3, 802.4 and 802.5 deal with

the physical and link layers. More specifically, 802.2 is the Logical Link

Control (LLC) protocol, whereas 802.3, 802.4 and 802.5 are Medium Access Control

(MAC) protocols. The lowest layers are network dependent and therefore must

understand low-level protocol associated with the data communications network

connecting two devices.

Technology for wireless LAN has been around for over ten years, but it has

started to gain respect and popularity with the arrival of the 802.11 family of

protocols — the specifications 802.11, 802.11a and 802.11b. This is primarily

because it has been simplified and standardized by IEEE. Strong support exists

for this technology due to reasons such as Non-Line-of-Sight (NLoS)

transmission, freedom to move PCs within a reasonable periphery and integrated

encryption. Wired Equivalency Privacy (WEP), the encryption algorithm, is

included in the 802 standard. Some vendors, such as Cisco have released products

based on 128-bit encryption (Aironet 350), thus substantially reducing the

security risk.

Even though 802.11, 802.11a and 802.11b, use Carrier Sense Multiple Access

with Collision Avoidance (CSMA/CD) "listen before you talk" scheme as

path sharing protocol, 802.11 and 802.11b specs are for wireless Ethernet LANs,

and 802.11a is suitable for wireless ATM. 802.11b transmits signals between 2.4—2.483

GHz, the unlicensed section of the spectrum targeted for Industrial, Scientific

and Medical (ISM) industry. The original 802.11 encodes signals using Direct

Sequence Spread Spectrum (DSSS). Using DSSS technology the 0s and 1s are

modulated with a second pattern called the chipping sequence, thus generating

streams called chips. The data transmission rates varies with 802.11 supporting

2 Mbps, and 802.11b peaking at 11Mbps due to a Complementary Code Keying (CCK)

modulating method that supports a higher data rate. 802.11b sends 64 chips in

one burst, achieving a data rate of 11Mbps while 802.11a specification is

capable of supporting up to 54Mbps. The table above shows both standards with

their respective distinct characteristics.

In 802.11 architecture, the mobile nodes communicate with other nodes through

fixed network access points. The mobile machines, which are essentially client

PCs, "talk" to a server on a wired LAN using these access points. An

access point simply acts as an intermediary between the wireless PC and the

wired LAN. It serves a group of users within several hundred feet by

transmitting and receiving signals over radio waves from a group of PCs.

When a PC is moved from one "cell" or place to another, its access

point also changes. In cellular terminology this is called a hand-off. A

hand-off does not involve any technical support for the PC and it is fully

transparent to the user of the PC, making a PC completely mobile.

The appeal for 802.11 exists because the standard is from IEEE, supports

higher-speed networks similar to wired LANs, and is cost effective. The access

point transceivers can be bought for $1000, along with transceivers for desktop

and laptop PCs for up to $200. The standard also supports interoperability, a

main concern for LAN administrators. To top all this, the standard is completely

wireless, thus giving complete mobility freedom to PCs without any strings

attached.

Data coming over a dedicated link out of the telecom carrier, enters the

corporate router through modem and then encryption device. Once packets arrive

into the enterprise network, the switch pushes them on to the right destination.

For example assume that PC1, server A and printer A are all connected and

accessible on a wired local area network running ethernet. Also, available on

the network are PC2 and PC3 through a radio-link via access point device that is

connected to the ethernet. A network may have multiple access points, each

communicating with several laptops, desktops or PDA devices. Each access point

acts just like a base station in the world of cellular communications. When one

PC moves from one area to the other, a migration from current base station to

the next base station occurs. The PC is "handed off" to the next base

station.

A new PC issues a request for authentication, which is acknowledged by the

network by sending a block of random text. The PC encrypts the packet and radios

it to the access point. This way authentication is established. More

illustratively, the PC sends a Ready-To-Send (RTS) packet to the access point.

In return, access point sends a Clear-To-Send (CTS) signal. On receiving it, PC

sends data that is acknowledged by the access point.

Challenges and Future

Enhanced Quality of Service (QoS) has to be guaranteed in order for 802.11 to

have a long-term sustained survival. The standard itself took seven years to

develop and some technologists already have doubts on the large-scale appeal to

the standard. There have also been issues related to 40-bit encryption that it

incorporates since it is generally accepted that 40-bit is not considered as

"industrial strength" LAN coding scheme. Some IT shops have reported

roaming to be not very reliable in wireless LANs. Some believe that the hand-off

from one to another access point is far from being perfect. Another challenge to

802.11 is High Performance Radio LAN, or HiperLAN standard as developed by

European Telecom Standards Institute (ETSI) subtechnical committee RES10.

HiperLAN operates at 5.12-5.30 GHz and 17.1 to 17.3 GHz bands.

In spite of all these reasonable concerns, the future for 802.11 looks very

strong and promising. The wireless LAN industry is estimated to grow to $3.0

billion by the year 2005. With mobile computing becoming a necessity in most

organizations, with new product development and support from vendors, with

tumbling costs of products, and with blessings from IEEE, the 802.11 is here to

stay. It is showing the way to a promised land of interoperable, transparent and

secured mobile computing in an enterprise-wide domain.

Subhash Nigam, manager, Information

Technology Services, Motorola, Inc.