networks - LifeOverflow

802.11n - the standard

January 17, 2008 // 1 Comment

Until a few weeks ago, I was under the impression that 80.11n standard is currently in draft when a friend of mine pointed out that his MacBook supports 802.11n. This got me thinking over 802.11n and how can a device be available for supporting a standard that is not finalized. Upon researching, I found out that many companies, including apple, have launched devices supporting the latest draft of the 802.11n standard and hope to provide firmware upgrades when the final standard rolls out.

The 802.11n standard is an amazing improvement over the currently used 802.11g standard. 802.11n standard supports link rate of 600 Mbps in comparison to the current 54Mbps supported by the 802.11g standard. It also incorporates the MAC enhancements for QoS and power savings as defined in the 802.11e standard. The increase in the link speed can be attributed to the folowing enhancements:

1. Increase in subcarriers
The OFDM subcarriers were increased from 48 in 802.11 to 52 in 802.11n boosting the throughput.

2. Decrease in Guard Interval
The guard interval is decreased from 800ns to 400 ns

3. 40Mhz Channels
The other 802.11 standards have a channel bandwidth of 20 Mhz. 802.11n has an optional mode, where the channel bandwidth is increased to 40Mhz. As the channel bandwidth is doubled, the number of data subcarriers is slightly more than doubled, going from 52 to 108.

Mobile ad hoc Networks (MANETs)

January 5, 2008 // No Comments

A mobile ad-hoc network (MANET) is a self-configuring network of mobile routers (and associated hosts) connected by wireless links—the union of which form an arbitrary topology. The routers are free to move randomly and organize themselves arbitrarily; thus, the network’s wireless topology may change rapidly and unpredictably. MANETs are usually set up in situations of emergency for temporary operations or simply if there are no resources to set up elaborate networks. These types of networks operate in the absence of any fixed infrastructure, which makes them easy to deploy, at the same time however, due to the absence of any fixed infrastructure, it becomes difficult to make use of the existing routing techniques for network services, and this poses a number of challenges in ensuring the security of the communication, something that is not easily done as many of the demands of network security conflict with the demands of mobile networks, mainly due to the nature of the mobile devices (e.g. low power consumption, low processing load).

Proactive vs Reactive Routing

Proactive Schemes

Determine the routes to various nodes in the network in advance, so that the route is already present whenever needed. Route Discovery overheads are large in such schemes as one has to discover all the routes. Consumes bandwidth to keep routes up-to-date Packet forwarding is faster as the route is already present. Examples of such schemes are the conventional routing schemes, Destination Sequenced Distance Vector (DSDV).

Reactive Schemes

Determine the route when needed .Smaller Route Discovery overheads. Employs flooding(global search) A node trying to transmit a packet may have to wait for route discovery. Examples of such schemes are Dynamic Source Routing, Ad-Hoc On Demand Distance Vector Routing (AODV) etc.

Flooding

Some routing protocols will find a apart form proactive and reactive there is another mechanism called Flooding. In flooding no route is computed or discovered. A packet is broadcasted to all the nodes in the network. Flooding is the easiest routing method, however it generates heavy traffic.

Different proactive, reactive and even hybrid (reactive as well as proactive) schemes e.g. Zone Routing Protocol, have been proposed for Wireless Ad-Hoc networks