Home > Articles > Wireless Networks

Wireless Networks

May 12th, 2002

This is the second tutorial from the series of tutorials about Wireless technology. The first tutorial introduced the reader to the Wireless world and discussed Wireless devices (mobile phone, PDAs etc.), Wireless Application Protocol (WAP), WML and more. This tutorial deals with Wireless Networks (WLANs), IEEE standards for WLANs, topology of WLANs and more. Once again, part of the terms I used for writing this tutorial appear in various articles/books about networking, so I expect the reader to be familiar with them.

IEEE Standards and Wireless Networks

Before going on with Wireless Networks, it is important to understand what are the IEEE standards for Wireless Networks, and what is the common one that used nowadays in Wireless Networks.

The Institute of Electrical and Electronics Engineers (IEEE) has produced the series of standards referred to as 802.X, which encompassed LANs (Local Area Network), MANs (Metropoline Area Network) and now it includes PANs (Personal Area Network) such as Bluetooth, which will be discussed later in this article. The IEEE 802 is confined to standardizing processes and procedures, that take place in the bottom two layers of the OSI (Open System Interconnection) Reference Model – The Media Access Control (MAC) or link layer and the Physical layer.

The committee of IEEE 802 LAN/MAN/PAN standards is currently divided up into Working Groups (WGs) numbered 802.1 through 802.17. The division of the committee into smaller groups helps to set each group to deal with different issues and develop

standards that are then given the same number as the working group which produced them.

The following are the most important Working Groups and their descriptions:

– 802.1 – Security and other issues

– 802.2 – Logical Link Control (LLC)

– 802.11 – Creates standards for WLANs (Wireless LAN)

– 802.15 – Creates standards for WPANs (Wireless PAN)

Both 802.1 and 802.2 are standards that apply to Wireless LANs. Every Working Group is divided up into smaller Task Groups (TGs) that work on various supplements and enhancements to the standard.

The original standard, which is currently used to set up Wireless Networks is the IEEE 802.11 standard. The IEEE 802.11 standard was published first in 1999 and it was designed to provide data rates up to 2Mbps (like DSL connection) at 2.4Ghz. It was designed to use either Frequency Hopping Spread Spectrum (FHSS) or Direct Sequence Spread Spectrum (DSSS).

DSSS means that data is sent in small pieces over a number of discrete frequencies available for use at any time in the specified range.

FHSS sends a short chunk of data shift frequencies (hop) and then sends another chunk. FHSS networks can exist in the same physical area without interfering with each other.

At present, the task groups numbered “a” through “i” are working on various methods to standardize improvements to the 802.11 standard. The standard for WLANs has become the 802.11b (Wi-Fi), which was a supplement created by the task group 802.11b. This standard uses the DSSS and provides data rates up to 11Mbps at

2.4Ghz (more than DSL connection (6Mbps)). Of course, this standard won´t stay forever as the standard of WLANs, but rather it will eventually be replaced by products, which support higher data rates, better quality and security (an issue, which

will be discussed in the third part of ´Wireless´).

The same evolution that took place and increased the speed of Wired Ethernet is now being played out in the Wireless realm. The current speed of Wireless networks stands on 11Mbps, but it can reach much higher data rates in the nearest future. For now there are two options to get higher data rates for Wireless network – one is already defined in a published standard, and the other is being developed. Within the 802.11b standard, another three standards were developed as Wireless Networks standards:

802.11a – The 802.11a standard was published in 1999, and it uses Orthogonal Frequency Division Multiplexing (OFDM) to provide data rates up to 54Mbps in 5Ghz bands. The problem with this standard is that at 5Ghz, more path loss occurs due to increased absorption of the RF (Radio Frequency) energy by walls and other solid objects. In order to solve this problem to cover an area comparable to that of Wi-Fi, more access points should be installed because of the higher data rates.

802.11g – The 802.11g standard defines a technology for operation at 2.4Ghz (like the 802.11b Wi-Fi) that offers higher data rates, which can reach up to 22Mbps using OFDM, while remaining compatible to Wi-Fi. In comparison to 802.11a, 802.11g offers compatibility to existing 802.11b equipment (lower cost), and less path loss than 802.11a.

802.11e – For one looking for QoS (Quality Of Service) the right standard is the newly developed 802.11e standard. The 802.11e is working to enhance the current 802.11 to expand support for application with high QoS requirements. Wireless networks fit both business and home environments. They both require he support of multimedia (home environment requires this even more), the 802.11e standard brings the solution for this need. In both Wired and Wireless networks, data transmission is immune to interruptions caused when packets are resent or lost during

the transmission process (I guess that most of us have seen it more than once). These interruptions can cause problems for data to be streamed in a contiguous fashion. The 802.11e has created QoS Baseline Document that proposes methods for handling time-sensitive traffic.

After this short introduction to IEEE standards for Wireless Networks, I´m entering the major part of this tutorial – Wireless Networks. Some terms used in the above intro were taken from the Wireless Network “glossary” and will be discussed in depth in the next chapter.

The Wireless Network

Introduction

The Internet technology, in the last few years has made astounding strides. Where only a few short years ago 56kbps modems were all the rage, many people now find themselves complaining about how slow their company´s T1 connection seems compared to their 6Mbps DLS connection at home.

Never before have so many had free and fast access to so much information. As Internet connections getting improved and people have megabit download speeds, they seem only to hunger for more. The most common service that one can find is DSL, or Digital

Subscriber Line. It provides bandwidth ranging from 384kbps to 6Mbps, over standard telephone lines, which everyone has at home. This service is not cheap, and it usually ranging from 50$ – 300$ per month, plus ISP equipment. The question that was asked is ´What is the point for a company to have a high speed Internet connection, but not the ability for network elements to roam?’

The solution for this question was the Wireless Networks (WLANs). The Wireless Network was originally employed to free network elements to roam in manufacturing. The Wireless Networks provide inexpensive and high-bandwidth network services. It costs far less to deploy a wireless network than a wired network. A major cost of installing and holding a wired network is the expense to run network and power cables. For this reason, it is better to deploy a Wireless network for non-roaming desktops. For businesses, Wireless Networks give more mobility and flexibility by allowing employees to stay connected to the Internet and to the network as they roam.

Nowadays, there are four types of Wireless networks, ranging from slow and inexpensive to fast and expensive:

– Bluetooth

– IrDA

– HomeRF

– WECA (Wi-Fi)

These four types of networks will be discussed after the next chapter.

WECA

WECA stands for Wireless Ethernet Compatibility Alliance and it certifies 802.11 products. The first family of products to be certified by WECA is that based on 802.11b standard. All the products are stamped with Wi-Fi logo and known as Wi-Fi devices. The next family is that based on 802.11a products and they are

stamped with Wi-Fi5 logo.

Wireless Networks Topology

Each Wireless Network requires a radio transceiver and antenna. Components on the Wireless network either stations (STA) or access points (AP). A Basic Service Set (BBS) is formed when two or more stations have recognized each other and established

a network. An Extended Service Set (ESS) is formed when BBBs (each one containing an AP) are connected together.

A standard Wireless network (802.11) operate in one of two modes – ad-hoc (peer-to-peer) or infrastructure mode. The ad-hoc mode is defined as Independent BBS (IBBS), and the infrastructure mode as a BBS.

In ad-hoc mode, each client communicates directly with the other clients within the network. This mode was designed such that only the clients within transmission range of each other can communicate. If a client in ad-hoc network wished to communicate outside of the range, one of the clients (members) must operate as a gateway and performing routing.

In infrastructure mode, each station sends its communications to a central station, which is the Access Point (AP). The access point acts as an Ethernet bridge and forwards the communications onto the network – either wired network, or the Wireless

Network. Before stations and access points can exchange data, they must establish a relationship, or an Association. Only and only if as Association is established can the two exchange data. The association process involving three states:

– Unauthenticated and unassociated

– Authenticated and unassociated

– Authenticated and associated

The transition between the states, the communicating parties exchange messages called management frames. The Access points were designed to transmit a beacon management frame at fixed interval. To associate with an Access point and join the BBS, a station listens for beacon messages to identify the Access Points

within the range. After the station receives a beacon frame (message) it selects the BBS to join. All the network names, or Service Set Identifiers (SSID), contained in the beacon frame, but Apple Macintosh, the user can choose the SSID he wishes

to join. A station can also send a probe request frame to find the associated Access Point with the desired SSID. After the station identifies the Access Point, they perform an authentication by exchanging several management frames. Currently, there are two standard authentication mechanisms – the Open Key Authentication and the Shared Key authentication. Usually most of the Wireless networks use the first authentication mechanism, which is set by default, and that causes a lack of security in the network.

If authentication runs well, the station moves to the second step – authenticated and unassociated. The station then sends an association request frame, and the Access Point responds with an associated response frame. If everything runs well, the station moves to the third and last step – Authenticated and associated, becomes a peer on the network and can transmit data frames on the network.

Wireless Network´s Types

Bluetooth

Bluetooth is not widely available yet, and is not expected to replace the need for high-speed data networks between computers. You can read more about the technology at http://www.howstuffworks.com/bluetooth.htm.

IrDA

IrDA stands for Infrared Direct Access, and it is a standard for devices, which use infrared light pulses to communicate. Since IrDA devices use infrared light, they depend on being in direct line of sight with each other. Wireless networks, which use IrDA capable of transmitting data at speeds up to 4Mps, but the requirement for line sight means that you would need an access point in each room.

HomeRF

HomeRF is based on the developing of the Shared Wireless Access Protocol (SWAP). Basically SWAP devices makes 50 hops per second (HPS) and transmit at 1Mbps, but some of them can step up to 2Mbps.

configure, it has no access point, it requires no additional wires, it allows up to 120 devices per network (which is pretty big number for home users), it can transmit your data securely by encrypting it, and more. You may think that this is type of network has no disadvantages, but unfortunately it does have. First, as I mentioned above, it can transmit data at 1Mbps, it has a limited range (23 to 38m), it is difficult to integrate into existing Wired network and more.

In most cases, SWAP-based networks use ad-hoc mode, but still some manufacturers do offer access points to increase the range of the Wireless network. For home users it may be better to set up a Wired network using HomePNA or traditional Ethernet,

you will get 10 up to 100 times the speed for the same amount of money that you pay for HomeRF. However, unless you plan to transmit Videos, Games etc., SWAP speed is probably adequate for most home users.

WECA (Wi-Fi)

Wi-Fi Wireless networks have gone in a completely different direction from HomeRF. Wi-Fi network targeted more on office and less on home users. The Wi-Fi is a short name for IEEE 802.11b and it stands for Wireless Fidelity. This specification

focuses on DSSS because of the higher data rate it can attain. Wi-Fi devices communicate at a speed of 11Mbps whenever possible. If signal or interference disrupting data, the devices will drop back to 5.5Mbps, then 2Mbps and finally to 1Mbps. Though it may occasionally slow down this keeps the network stable and very, very reliable.

Some of the Wi-Fi advantages:

– It is very fast and can reach up to 11Mbps

– It is reliable

– It has a long range (305m in open areas, 122m in closed areas)

– Supports all 802.11 DSSS devices

Still, even if Wi-Fi seems to be the best solution, it has its own disadvantages:

– It is expensive (from 300$ – 1,400$)

– It requires an Access Point

– It can be difficult to set up

Articles

Comments are closed.