WEEK 10

FACTOR EFFECT OF VOICE QUALITY

Delay is caused when packets of data / voice take more time than expected to reach their destination. This causes some disruption in the voice quality. However, if it is dealt with properly, its effects can be minimized . 
Delay mainly comes from three components :

1.  delay caused by voice codec algorithms 
2.  delay caused by queuing algorithms of communications equipment
3.  variable delay caused by various factors(i.e. network conditions, VoIP equipments, weathers etc).

It is very important to minimise the voice traffic delay. Thus, a codec algorithm and queuing algorithm needs to be carefully considered. High QoS should be assured by control delay so that one-way communication delay should be less than 150ms. (ITU states that one-way, end-to-end telephony applications should have less than 150ms delay in echo-free environments to ensure user satisfaction). Although traditionally think the end-to-end delay of 150 ms was considered as acceptable for most applications.
However, in reference [James, 2004], the authors state that a delay of up to 200ms is considered as acceptable. Moreover, a one way end-to-end delay between 150ms to 400ms is considered as acceptable for planning purposes. In this study, 200ms will be considered as the maximum acceptable one way end-to-end delay, high end-to-end delay can cause bad voice quality perceived by the end user.

WEEK 9

TYPE OF WIRELESS NETWORK

• IEEE802.11b

802.11b is a WLAN communication standard. 802.11b has a maximum raw data rate of 11 Mbps and uses CSMA/CD as media access method [Wiki, 2011]. 802.11b is using Direct Sequence Spread Spectrum (DSSS) modulation technique defined in the original standard. Compared to alternatives like 802.11a and 802.11g, 802.11b network equipment costs less. Its relatively low cost naturally resulted in many home and small business networks adopting 802.11b. 802.11b transmits in the 2.4 GHz frequency range. This range is unregulated, which means that radio transmitters built into other products may use the same frequency and interfere with the 802.11b network. These products include some cordless telephones, microwave ovens, garage door openers, and baby monitors [Compnetworking, 2011].

• IEEE802.11n

802.11g is another standard in 802.11 groups. 802.11g supports a maximum           data rate of 54Mbps compared to the 11Mbps of 802.11b. To achieve backward compatibility, 802.11g uses the same communication frequency range 2.4GHz as 802.11b, but uses the Orthogonal Frequency Division Multiplexing (OFDM) based transmission scheme as 802.11a [Compnetworking, 2011]. Similar to 802.11b, 802.11g devices suffer interference from other products operating in the 2.4 GHz band, for example wireless keyboards [Wiki, 2011].

• IEEE802.11n 

802.11n is the newest standard for high-speed Wi-Fi networking. 802.11n is designed to replace the 802.11a/b/g Wi-Fi standards for local area networking. 802.11n operates by utilizing multiple wireless antennas in tandem to transmit and receive data. The associated term MIMO (Multiple Input, Multiple Output) refers to the ability of 802.11n and similar technologies to coordinate multiple simultaneous radio signals. MIMO increases both the range and throughput of a wireless network [Compnetworking, 2011].

802.11n operates on both the 2.4GHz and the 5 GHz frequency bands. An additional technique employed by 802.11n involves increasing the channel bandwidth. As in 802.11a/b/g networking, each device uses a preset Wi-Fi channel on which to transmit. Each 802.11n channel will use a larger frequency range than these earlier standards, which will increase the data throughput. Channels operating at 40 MHz are incorporated into 802.11n which doubles the channel width from 20 MHz in transmitting data. These allow data rates up to 600Mbps and can only be achieved with the maximum of four spatial streams using a 40 MHz-wide channel. Channel operating at 20MHz has a data rates up to 248Mbps [Wiki, 2011]. For this study, only 802.11n operating at 20MHz and 2.4GHz frequency band will be used.