802.11 Introduction
802.11 and 802.11x refers to a family of specifications developed by the IEEE for wireless LAN technology. 802.11 specifies an over-the-air interface between a wireless client and a base station or between two wireless clients. The IEEE accepted the specification in 1997.
There are several specifications in the 802.11 family:
802.11 — applies to wireless LANs and provides 1 or 2 Mbps transmission in the 2.4 GHz band using either frequency hopping spread spectrum (FHSS) or direct sequence spread spectrum (DSSS).
802.11a — an extension to 802.11 that applies to wireless LANs and provides up to 54 Mbps in the 5GHz band. 802.11a uses an orthogonal frequency division multiplexing encoding scheme rather than FHSS or DSSS.
802.11b (also referred to as 802.11 High Rate or Wi-Fi) — an extension to 802.11 that applies to wireless LANS and provides 11 Mbps transmission (with a fallback to 5.5, 2 and 1 Mbps) in the 2.4 GHz band. 802.11b uses only DSSS. 802.11b was a 1999 ratification to the original 802.11 standard, allowing wireless functionality comparable to Ethernet.
802.11g — applies to wireless LANs and provides 20+ Mbps in the 2.4 GHz band.
Ad Hoc Networks
Wireless Networks without access to a LAN.
In conjunction with mobile un-wired devices Mobile Ad-Hoc Networks (MANET) .
The Network is built-up spontaneous without any existing infrastructure .
Each Device is also a Router to transfer Data between devices, that are not in direct transmission-distance .
To manage the transmission between Devices, Routing-Protocols are needed (Ad-Hoc Protocols).
Some nodes may have special functions or roles.
Problems in Ad-Hoc Networks
While building up Ad-Hoc Networks, several problems can be addressed, some as a consequence of other problems:
When a node [A] inside an Ad-Hoc Network wants to send data to a second node [B], that he is not able to see, because this one is out of his transmission-rage, how can the data be transferred from [A] to [B] anyway?
How can the destination [B] be identified?
How can the way from [A] to [B] be determined?
Can a technique to determine the way be found, that is scalable for small networks (1-100 nodes) medium-large networks (100-10.000 nodes) and large networks (10.000-several million nodes)?
How can a way from [A] to [B] be determined without flooding the network with inquiries? How can the number of inquiries been kept small?
How can a way from [A] to [B] be determined without knowing each node of the network? How can the information a node has to know about the network been kept small? How much does a node realy has to know about the network?
Can a technique to determine the way be found that is scalable (under condititions) for networks with high changerate and low changrate?
How can a node [A] establish coperation relationships with the nodes that it needs to transfer data to [B]?
Is it nessessary to identify malicious or unfair nodes or is there a way to accept this conditions?
Can the network be kept resistant against malicious nodes?
How can redundance been introduced to be resistant?
