Chapter 8 Wireless Networking

Chapter 8 Wireless Networking

As a Network Technician wired technologies are already a high-wire act without a net, with expectations of having all systems fully available 99.999%. As though that is not tough enough now we add in the Wireless Spectrum. Now we don’t even have a high-wire to walk on and we need to be able to deliver it just about anywhere anytime. Yikes! With this in mind it is especial essential that standards are developed and followed so all the wireless devices are able to work just about anywhere around the globe.

Similar to wired the Wireless Spectrum needs to follow the Layer 3 and higher protocols, however it has no fixed path to guide the signals from one node to the next. Instead of a wire, Wireless uses an Antenna on the same frequency (Channel) to transmit and receive information. An Antennas service specifications determine the power, frequency and radiation pattern. The two types of antennas are directional which are used more for point to point wireless communications. Satellites use direction antennas for downlinks such as cable TV, where omnidirectional signals give equal strength in all directions and used in wireless networks or wireless phones.

Signal Propagation LOS (line-of-sight) or in straight line, directly from transmitter to receiver is going to give you the clearest signal possible. As you know obstacles are going to get in the way affecting the signal. The signal needs to pass through them, get absorbed by them, bounce off them… These phenomena are Reflection, which it bounces back to the source, Diffraction, which splits it into secondary waves or Scattering, which diffuses it into multiple different directions.
Because of the three phenomena Multipath signals are created which can both enhance or cause delays in the wireless connection. Remember this is an extreme high-wire act and we are working without a net and wire (no wireless conduit/shielding).

Frequency Ranges are the area we are able to do our high-wire act. The oldest range is 2.4–2.4835 GHz contains 11 communication channels which include wireless phones which can interfere with your wireless network devices. The newer 5.1 GHz, 5.3 GHz, 5.4 GHz, 5.8 GHz contains 24 communication channels and because these are also used by weather, military radar communications your wireless gear must be able to sense this and switch to a different cannel automatically.

Narrowband , Broadband, and Spread Spectrum Signals
In Narrowband the Transmitter concentrates signal energy at single frequency, very small(narrow) frequency range where Broadband uses a relatively wide(broad) wireless spectrum band giving it higher throughputs than narrowband. The last one is Spread-spectrum used in WWII, which uses multiple frequencies to split them and transmit signals. The split signals are only known by transmitters and receivers so it is difficult for unauthorized receivers to capture the information because they need to know both frequencies to capture and decode the signals. This offering more security when send transmissions. There are two types of Spread-spectrum FHSS (frequency hopping spread spectrum) which the signal jumps between several different frequencies within a band. The other is DSSS (direct-sequence spread spectrum) where signal’s bits distributed over entire frequency band at once. Each bit then coded and the receiver reassembles original signal upon receiving.

WLAN (Wireless LAN) Architecture can be as simple as an Ad hoc WLAN where wireless nodes transmit directly to each other (laptop to laptop) or complex to where you would have one or many Access point (AP) throughout your location or locations to have multiple nodes authenticate to them to use the network. The AP is strategically placed for easy access and wired to the network to retransmit signals to the network and network operating system. Depending upon number of nodes you may need multiple APs (maximum number of nodes varies: 10-100).

IEEE’s Wireless technology standard is 802.11(802.11b, 802.11a, 802.11g, 802.11n (draft)) work at the Physical and Data Link Layer of the OSI model and work in the Half-duplex, access method, frame format.

Access Method for 802.11 same physical addressing schemes as 802.3 however, wireless devices are not designed to simultaneously transmit and receive so they cannot quickly detect collisions. Therefore instead of using CSMA/CD (Carrier Sense Multiple Access with Collision Detection) they use a method called CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) to minimize potential collisions. It uses ACK packets to verify every transmission which requires more overhead than 802.3 therefore is slower than 802.3. Another method is RTS/CTS (Request to Send/Clear to Send) protocol it ensures packets are not inhibited by other transmissions and is more used for large transmission packets however; it further decreases overall 802.11 efficiency.

Association is the process of scanning and gaining access to an AP. The wireless NIC surveys its surroundings for access point using two types: Active scanning which transmits a special frame known as a probe to the AP waiting for it to issue a probe response and Passive scanning where it is listens for special signal known as a Beacon fame issued by the AP. The Beacon frame contains the transmission rate and SSID (service set identifier) the name of the AP. The node must then enter an access code, if the security is setup, then once authenticated they can begin to communicate.

There multiple frames in 802.11 are divided into three groups: control, management and data. The management frames are those involved in association and reassociation probe, beacon frames. Control frames are those related to medium access, data delivery such as ACK and RTS/CTS frames. Where Data frames carry data sent between stations.

802.11b is the oldest standard and uses DSSS (direct-sequence spread spectrum) signaling. It operates on the 2.4-GHz band and separated into 22-MHz channels. Throughput in theory is 11-Mbps but actual throughput is at 5-Mbps and has a 100 meters node limit.

802.11a although the standards task group started before 802.1b it was released after 802.11b. It operates on the 5-GHz band which is not as congested (like 2.4-GHz band) and has lower interference from other devices (microwaves, cordless phones) however it requires more power to transmit and travel shorter distances than the lower-frequency signals. Throughput in theory is 54 Mbps but actual throughput is between 11 and 18 Mbps and has a 20 meter node limit. It is also more expensive hence least popular.

802.11g is just as affordable and compatible with 802.11b networks. It operates on the 2.4-GHz band. Throughput in theory is 54-Mbps but actual throughput is between 20 to 25 Mbps and has a 100 meters node limit.

802.11n appears to have been in September 2009 and will be backward compatible with 802.11a, b, g standards. Therefore it operates on both 2.4-GHz or 5-GHz frequency ranges. Throughput in theory is a maximum of 600 Mbps.

Bluetooth Networking is a wireless technology compatible with multiple devices requiring little power and short range coverage. It is designed for PANs (personal area networks) cellular telephones, phone headsets, computer peripherals, PDAs. The standard uses FHSS (frequency hopping spread spectrum) RF signaling in 2.4-GHz band. Version 1.1 has a throughput in theory is 1 Mbps but actual throughput is 723 –Kbps and is recommended nodes be no more than 10 meters apart. Version 2.2 has an actual throughput is 2.1 Mbps and is recommended nodes be no more than 30 meters apart. If you ever catch me walking in store talking with a blue tooth head set please shoot me!

In designing a wireless network you need to first determine the needs of your organization. Are you a coffee shop having hot spots for your users, a large warehouse needing access to databases to update information via handheld devices and/or an office that uses it for convenience for laptop users to easily logon to the network when they are in the office? In your design you will need to determine the placement of APs for best coverage along with using the latest technologies 802.11g or 802.11n if available. You will also what to use hardware all one standard and along with one manufacturer, this will ensure the ability to document your wireless network easier.

In 2001 IEEE standardized a new wireless technology 802.16 WiMAX (Worldwide Interoperability for Microwave Access). The current version is 802.16e (2005) which has improved mobility and QoS characteristics making it better suited for digital voice signals and mobile phones. It functions in 2 and 66 GHz range on either Licensed or nonlicensed frequencies giving it two distinct advantages over Wi-Fi of a much greater throughput (70 Mbps) and a of range (30 miles). It is an alternative to DSL or broadband cable, well suited to rural users, internet access to mobile computerized devices, residential homes… One of the drawbacks are that it is expensive.

Satellite Internet Access, from my experience, is not a good option if you have any mission critical applications because weather can be big factor in flakey connections. However, if this is all that is available other than dialup it may be your best option.