Design  | SFC  |  NOS  |  Code |  Diary  | WWW |  Essay |  Cisco | [Home]

 

 

[Bill's Home Page]
[ Old page]

Essays

 

 

 

 

 

Wireless Networks

Wireless networks use high-frequency radio waves to transmit from node to node. The 11Mbps IEEE 802.11b standard has been designed so that nodes can connect using a peer-to-peer connection (known as an ad-hoc connection) or connect to a wireless hub (known as an infrastructure connection). The bit rate is equivalent to a base-rate Ethernet connection, and is thus able to easily integrate with existing Ethernet networks. The advantages of IEEE 802.11b include:

  • Integrates well with existing Ethernet networks.
  • Is supported by most network operating systems.
  • Provides a range of up to 800 feet, in an open environment.
  • Provides increased mobility.
  • Reduces the cost of wiring.
  • Supports 1, 2, 5.5 and 11 Mbps bit rates.
  • Supports either a point-to-point (ad-hoc) and point-to-multipoint (infrastructure) access.
  • Supports Plug and Play, and is easy to install.
  • Uses strong encryption using WEP encryption (64-bit and 128-bit)
  • Uses Direct Sequence Spread Spectrum (DSSS) which is a robust, interference-resistant and secure wireless connection.

The applications of wireless technology is likely to increase over the forthcoming year, especially with the increasing processing power of mobile devices, but typical applications include:

  • Environments which have frequently change, such as in a retail environment, or in workplaces which are continually rearranged.
  • High security networks. Ethernet has suffered from security problems, thus wireless networks with encryption can overcome this.
  • Providing remote access for a corporate network.
  • Providing temporary LANs which could be used for special projects.
  • Remote access to databases in mobile applications, such as for medical practitioners, or office staff.
  • Supporting networks in environments where cable runs are difficult, such as in old buildings, hazardous areas, and in open spaces.
  • Support for users who use SOHO (Small Office and Home Office), as it provides a quick access to networks.

Basic specification

IEEE 802.11b uses a number of channels in frequency range around 2.4 GHz to 2.45 GHz. This high frequency allows the radio wave to propagate fairly well through building and air. At 11Mbps, the maximum range is around 140 meters, but this reduces when there are ob-stacles in the way. At 1Mbps, the range increases to 400 meters. The frequencies are split into a number of channels. In Northern America, there are 11 channels, in Japan, there are 14, and in Europe, there are 13 channels (as shown in Figure 1).

Operating Channels:

11 for N. America, 14 Japan, 13 Europe (ETSI), 2 Spain, 4 France

Operating Frequency:

2.412-2.462 GHz (North America), 2.412-2.484 GHz (Japan), 2.412-2.472 GHz (Europe ETSI), 2.457-2.462 GHz (Spain), 2.457-2.472 GHz (France)

Data Rate:

1, 2, 5.5 or 11Mbps

Media Access Protocol:

CSMA/CA, 802.11 Compliant

Range:

11Mbps: 140m (460 feet)
5.5Mbps: 200m (656 feet)
2Mbps: 270m (885 feet)
1Mbps: 400m (1311 feet)

RF Technology:

Direct Sequence Spread Spectrum

Modulation:

CCK (11Mps, 5.5Mbps), DQPSK (2Mbps), DBPSK (1Mbps)

Output Power:

13 dBm

Sensitivity:

11Mbps < -83 dBm
5.5Mbps < -86dBm
2Mbps < -89dBm
1Mbps < -91dBm


Figure 1 IEEE802.11 channel setting for Europe

The wireless adapter will typically connect to a node using one of a number of ways, such as through the USB port, PCI card, PCMCIA card, and so on. The wireless protocol corresponds to a network adapter, and can thus support most higher layer protocols, such as TCP/IP, NetBEUI, and IPX/SPX, as shown in Figure 2.


Figure 2 Setting for protocols and network

Wireless network connections

IEEE 802.11b can be either connected as an infrastructure network or as an ad-hoc network. Figure 3 shows an infrastructure network where the wireless nodes connect to an ac-cess point. The access point defines the domain of the wireless network. These domains can then interconnect through an Ethernet backbone. Figure 4 shows the usage of SSID, which is a unique ID given to the access point. All the clients which connect to a certain access point must define the correct name (otherwise they may connect to another access point). If the access point ID is not known then ANY can be used (although this is not recommended for security reasons).

An ad-hoc network uses channels to define different networks, as illustrated in Figure 5. In this example, LAN 1 uses channel 3, and LAN 2 uses channel 7. In Europe, for example, it would be possible to create up to 13 different ad-hoc networks, within a certain range (between 100 and 400 meters, depending on the environment, and the bit rate). If an ad-hoc network has a range of L meters, then an infrastructure network will have a diameter range of 2L, as illustrated in Figure 6.



Figure 3 Infrastructure network

Figure 4 SSID for a wireless network


Figure 5 Ad-hoc networks

Figure 6 Span of networks


IEEE 802.11b settings

The IEEE 802.11b device must be set up properly as the communications can be picked-up by other users, who, if the connection is not setup properly, could read all the communica-tions sent and received. They may allow be able to connect to the resources of the node connected to the wireless adapter. Figure 7 shows some of the setting which must be set-up on the adapter. The settings are:

Authentication algorithm. This sets whether the adapter uses an open system (where other nodes can listen to the communications), or uses encryption (using either a WEP key, or a shared key).
Channel. If an ad-hoc network is used, then the nodes which communicate must use the same channel.
Fragmentation threshold. This can be used to split large data frames into smaller fragements. The value can range from 64 to 1500 bytes. This is used to improve the effi-ciency when there is a high amount of traffic on the wireless network, as smaller frames make more efficient usage of the network.
Network type. This can either be set to an infrastructure network (which use access points, or wireless hubs) or Ad-hoc, which allows nodes to interconnect without the need for an access point.
Preamble mode. This can either be set to Long (which is the default) or short. A long preamble allows for interoperatively with 1Mbps and 2Mbps DSSS specifications. The shorter allows for faster operations (as the preamble is kept to a minimum) and can be used where the transmission parameters must be maximized, and that there are no in-teroperatablity problems.
RTS/CTS threshold. The RTS Threshold prevents the Hidden Node problem, where two wireless nodes are within range of the same access point, but are not within range of each other. As they do not know that they both exist on the network, they may try to communicate with the access point at the same time. When they do, their data frames may collide when arriving simultaneously at the Access Point, which causes a loss of data frames from the nodes. The RTS threshold tries to overcome this by enabling the handshaking signals of Ready To Send (RTS) and Clear To Send (CTS). When a node wishes to communicate with the access point it sends a RTS signal to the access point. Once the access point defines that it can then communicate, the access point sends a CTS message. The node can then send its data.

Figure 7 Setting for IEEE 802.11b adaptor