A Comparison Of WLAN Hardware Computer Science Essay

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In this section of the project we will explore different WLAN hardware there feature and configuration option, having this experience we will be able to make a better decision when we implement WLAN to chose the appropriate hardware of the specific scenarios.

7.1 Access points:

Access points is a point of access to the WLAN and derive their name from there functionality. AP are the most frequently installed infrastructure device. There are two main categories of AP , in other words we can say that an access will have the following man functionality.

(a)Autonomous APs, and (b) Light weight APs. (a)Autonomous APs are those APs that contain the software for the complete management of the WLAN. (b)While lightweight APs are those APs that contain limited software and depend on centralized WLAN switches or controllers. Some APs can act as either an autonomous or lightweight depending on their configurations. APs, both autonomous and lightweight, come in many shapes and sizes. Some have antennas built in, and others use external antennas. Different access comes with different features. the figure below shows the Linksys access point.

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Fig 7.1

Let we explore the common features that are found in most APs.

7.2 Common Features.

The following are the features that are seen commonly in all access points. (Operational Moods) The first mode offered by most APs is root mode. An AP operating in root mode is providing wireless clients with access to the WLAN and possibly a wired network. Root mode is the default mode of operation for all WLAN devices sold as APs. The IEEE 802.11 standard defines an AP only as a STA that provides access to the distribution services via the wireless medium for associated STAs. Instead some access points can be configured as Bridge or repeater. Bridge mode is used to create a link between two or more APs. When only two APs are used, a point-to-point link is created. When more than two APs are involved, a set of point-to-multipoint links are created. In a bridge mode implementation, the APs involved usually associate only with each other and do not accept client STA associations. Repeater mode is used to extend the range of a WLAN beyond its normal usable boundaries. The repeater AP acts as the AP for clients that would otherwise be out of range of the distant AP operating in root mode. Where a root AP is the connection point for many clients and is a client to no other APs, the AP in repeater mode is a client to the AP in root mode while also accepting connections from client stations itself. Some other features of APs are.

7.2.1 Fixed & Detachable Antennas.

The ability to move the physical location of the antenna to a different location from that of the AP is a valuable one. we can use RF cabling to move the antenna to a location that is more practical for the transmission and reception of RF signals and locate the AP itself closer to power outlets. This can be advantageous when we do not have power outlets closer to the RF signal transmission and reception location.

7.2.2 Filtering.

Most APs offer two kinds of filtering at a minimum. The first kind is MAC address filtering, and the second is protocol filtering. Filtering functionality provides the WLAN administrator with the capability to limit which STA frames can pass through the AP in terms of the hardware configuration of the STA (MAC address) or the protocol being used, such as HTTP.

7.2.3 Variable Output Power.

Variable output power provides the WLAN administrator with the capability of sizing cells more accurately. Some APs provide variable output power management based feature. For example, an AP may allow us to specify that the output power be 25, 50, or 100 mW. So that we configure the output power based on our organization needs and cell sizing.

7.2.4 Management Capabilities.

APs will provide different methods for configuration and management of the devices. These methods will vary from vendor to vendor and from model to model within vendor's product lines. However, there are common methods utilized. These common methods include Console (serial), Telnet, SSH, SNMP, Custom software applications Web-based interfaces. When choosing AP for WLAN we must look at configuration, and management interfaces so that the better one can be choose. Console or serial interfaces are usually only provided on enterpriseclass Hardware. When using a console interface to configure an AP, we will usually connect a serial cable from our computer to the AP. Once connected, we will use a terminal program such as HyperTerminal, in Windows, to connect to the device. The telnet and SSH or SSH2 interfaces will be similar to the console management method in that the CLI will be utilized. The difference is that the CLI is being utilized across the network rather than through the console port and a serial cable. Finally, web-based configuration interfaces take advantage of built-in web server software in APs to allow for remote configuration through the Ethernet interface.

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7.3 WLAN Bridges.

WLAN bridges are used for connection of remote areas that are out from range of an AP. As we discussed that an AP could be could also be configured in a bridge mode. The reason to use a special dedicated bridge is. These devises have a very little memory and processing capabilities. because of there limited amount of RAM and processing power in these devices and features or capabilities must be sacrificed to provide the best support for the intended use.

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Bridges

Fig 7.2

Generally the bridges are used for PtP and PtMP connections.

There are two fundamental modes of operation for wireless bridges:

ROOT and NON-ROOT. There are multiple usage scenarios that include various mixtures and configurations of these two modes. So only one wireless bridge can be in root mode and any number of wireless bridges can be in non-root mode; however, modern bridge devices also allow for creative combinations of non-root-mode and standard AP functionality. When one bridge is in root mode and only one other bridge is associated with it, that other bridge must be in non-root mode. This type of link is a point-to-point (PtP) link and is common between buildings.

When one bridge is in root mode and multiple other bridges associate with it, those other bridges must be in non-root mode. This type of link configuration is a point-to-multipoint (PtMP) configuration.

Fig 7.3 Fig 7.4

While connecting bridges alignment is the crucial issue. And bridges should be properly aligned so that the maximum RF signal will received.

7.4 WLAN Switches/Controllers.

This device is used in combination with Lightweight Access Point Protocol (LWAPP) to manage light weight access points in large quantities by the network administrator. The WLAN controller is part of the Data Plane within the Cisco Wireless Model. The WLAN controller automatically handles the configuration of anywhere from 6 to 300 wireless AP depending on the model. It so easy to configure a few APs one by one in a small organization or home. But as the no of APs grows it become slightly difficult to configure each AP at every edge. So this kind of configuration was no longer feasible. Vendors rushed to create their own solutions to this enterprise-class dilemma. The result was the creation of WLAN switches or controllers. The only major difference between a WLAN switch and a WLAN controller is that the WLAN switch has the controller functionality built into it and the WLAN controller may be in a switch, a router, or some other device. For this reason, the term WLAN switch to refer to the features and capabilities that are commonly found in WLAN switches with WLAN controller capabilities built in as well as those found in WLAN controller components.

A WLAN switch look like other switches but additional functionality of WLAN controller.

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Fog 7.5

7.4.1 Common features.

WLAN switch usually centralizes the "AP" processing into the switch and away from the AP. For this reason, WLAN switches often implement the features that are traditionally found in thick or autonomous APs. The following features are common, PoE injection into the Ethernet ports. Built-in firewall capabilities, Standards-based and proprietary WLAN security technologies such as WPA, WPA2, EAP, and IEEE 802.11i. Common management interfaces (web, telnet, CLI, SSH, console, etc.) and Configuration file management.

The other feature that should be take care some WLAN switcher are enterprise class and perform heavy duty, while on the other hand for small office and home (SOHO) a verity of switches are available.

7.5 Wireless LAN Accessories.

7.5.1 Amplifiers.

RF amplifiers are used to increase or amplify the RF signal strength. They are usually placed in-line between the AP or bridge and the antenna. Amplifiers may also be used when the link budget calculations reveal that the signal strength will be too weak otherwise.

As there are two types of gain: Active and Passive. Amplifiers create active gain because they increase the "amount" of RF energy being transmitted. Antennas create gain by focusing the energy in a specific direction so that the amount of energy going in that direction is higher, though the overall amount of energy is not increased.

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There are two types of amplifiers, (1) Unidirectional and (2) Bidirectional. As the names imply, (1) Unidirectional amplifiers amplify the received or transmitted signal only.

(2)Bidirectional amplifiers amplify both the sent and received signals. A bidirectional amplifier may be used with a stationary WLAN client that needs to increase both the received signal strength and the transmitted signal strength. A unidirectional amplifier could be used on each end of a bridge link. Both bridges could be configured with a unidirectional amplifier in the transmit path. Since they are both transmitting with stronger power, they should be able to hear each other well enough.

Finally, two variations are also available in amplifiers: fixed-gain and fixed-output types. Fixed-gain amplifiers add a preconfigured amount of gain to the signal. For example, such an amplifier may increase the strength by 6 dB. With this setup, whatever be the input strength, it will be quadrupled by the amplifier-usually up to a certain maximum threshold. Fixed-output amplifiers are configured so that a certain range of input will always result in the same output power. For example, as long as the input power is in the range 5-50 mW, the output power will be 100 mW. This is an example of how one might function.

Fig 7.6

Choosing the Right Amplifier.

When purchasing an amplifier, we must be sure to match frequency response. In other words, if we are using a 2.4 GHz AP, we will need to use a 2.4 GHz amplifier. If we are using a 5 GHz AP, we will need to use a 5 GHz amplifier, and so forth. It is important to match frequency, other wise system will not work. Second, we will need to ensure that the amplifier matches your system in ohms and VSWR. Otherwise, we will create an impedance mismatch

and degrade the performance of the system or even damage the equipment. we should know the input power and gain we need, given the link budget calculations that we have made.

7.5.3 Attenuators.

Attenuators do the opposite of amplifiers. They decrease the strength of the RF signal. RF attenuators may be fixed-loss or variable-loss type. Fixed-loss attenuators reduce the signal by the same amount, regardless of the input signal, assuming the input signal is strong enough to be reduced to that amount. Variable-loss attenuators can be set to varied levels of loss.

Attenuators are usually used when a system exceeds the allowed output power specified by the Government. Variable-loss attenuators are useful because you may need to add or remove cable lengths over time and these different lengths of cable will result in different loss needs. Like amplifiers, attenuators must be designed to work on the same frequency as the system being attenuated. Attenuators, unlike amplifiers, can be placed practically anywhere in-line between the AP or bridge and the antenna because strength of signal is less important than weakness of signal in this case.

7.5.4 RF Cables.

RF cables are used to connect the transceiver to the antenna. Cables have different levels of loss, and this should be considered when selecting the cabling for our system. We will have to Keep the following factors in mind when selecting RF cables for WLAN implementation.

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Fig 7.7

Different cables have different levels of loss, so not all cables are the same. Make sure the impedance of the cable matches the rest of the system. We must be assure to select a cable that is rated for the frequency we will be using. Check with the vendor to discover the loss incurred per foot or per 100 feet before selecting the cable. Higher frequencies mean greater loss in the same cable.

7.5.5 RF Connectors:

Normally we used RJ45 and RJ11 for connecting wired Ethernet .But in WLAN with the use of these connectors we also use some connector (specific to RF) that is called RF connector. Which are used for connecting antenna to the wire are two wires with each others. RF connectors come in many shapes and sizes. The following types are common:

N-type

SMA

BNC

TNC

In addition, there are common variations of these types such as reverse polarity and reverse threading. These different types exist in an effort to comply with FCC regulations for components used in a wireless system. While dongles and pigtails exist, if they are used to convert from one type to another for the purpose of transmission, they constitute a breach of FCC regulations. Thies connectors are found on the ends of cables, the backs of APs and bridges, and the ends of antennas.

7.5.6 RF Signal Splitters:

RF splitters are installed in series between the transceiver and the antennas. The splitter receives a single input and has two or more outputs. They are not recommended for common use in WLANs but may be used with Sectorized antennas. Other than these scenarios, RF splitters should be avoided in WLANs.