Media Access Control, CDMA Spreading Codes, AARNet, And Scanning WiFi Access Points Essay.

Part –A

Q1. In relation to access to wireless network (e.g, WiFi), answer the following questions:

1. What is media access control and why is it important?
2. Give one example each of the controlled access methods and contention based media access methods?
3. When might one access method be preferred over another in a network, and
4. Under what conditions? do contention?based media access control techniques outperform controlled access techniques (i.e., have lower response time)? Explain.

Q2. Four users (U1, U2, U3 and U4) are using CDMA access techniques to transmit data to their respective receivers (U1->R1, U2->R2, U3->R3 and U4->R4). The followings are the spreading codes and the data that each user wants to send.

Users	Spreading code	Data to send
U1	1111	0 0 0
U2	0011	1 0 0
U3	1001	0 0 1
U4	1010	1 1 1

Using a spreadsheet similar to the one shown in Week 7 tutorial,

(a) show the composite signal that each receiver will receive;

(b) show how each receiver can retrieve its own data using the corresponding spreading code.

Note: encode ‘1’ in data as ‘+1’ and ‘0’ in data as ‘-1’ for this work.

Q3. Use internet resources and describe the Australian Academic and Research Network’s (AARNet) national and international network with diagram and information about the speed and the coverage.

Describe how AARNet is connected to the Internet in Australia and overseas.

Part B – Scanning Wireless Access Points

Q4. The software inSSIDer from metageek lets you scan surrounding WiFi networks. Download inSSIDer 3.1.2.1 from the following link: http://www.techspot.com/downloads/5936-inssider.html

1. Investigate wireless networks available in a shopping centre. Use the inSSIDer package installed on your laptop and walk through a shopping centre. Record the wireless networks available in at least three spots (you may consider going close to a McDonnalds). Capture the inSSIDer screen shots.

Give the list of available networks/access points including their technical characteristics like channel number, signal (SNR, signal to noise ratio in dB), security, etc. Include your screenshots from the software. Make some observations based on the data you collected, like, how do you relate SNR with the distance (from your own location to access points), overcrowded channels (i.e., multiple networks are transmitting using the same set of channels), etc.

1. Use inSSIDer at you home (or place of residence where you have other WiFi networks) and observe the signal strength and channel number of your own network and other networks. Can you suggest any way to change your access point’s (i.e., home router’s) transmission setting to get better reception within your home?

Part A: Media Access Control and CDMA Spreading Codes

A). Media Access Control

Medium Access Control is a data link sub-layer of OSI reference model that controls transmission of packets of data to a network’s interface card or from one network to another via some protocols (Wu & Pan, 2010). Media Access control is an important mechanism that provides addressing mechanisms and access to a channel to enable efficient communication to occur between wireless devices and the network. MAC is also helpful in that it decides when a communication node has to access the shared medium, while resolving any conflict among communicating nodes. Besides, without MAC mechanism, errors occurring at the physical layer become hard to correct.

B) Example of controlled-based and contention based techniques

Contention based media access control involves nodes transmitting at any time, making it possible for collisions to occur. An example of contention based technique is Carrier Sense Multiple Access. With this technique, a network device has to first detect if the network medium is transmitting or idle before attempting to transmit its own data. To avoid collision, this technique is implemented together with a mechanism to resolve medium contention. This mechanism is called collision detection. Therefore, the overall mechanism is referred to as Carrier Sense Multiple Access with Collision Detection CSMA/CD.

Control based media access control techniques ensure a collision-free data transfer. Communicating nodes may send data simultaneously without causing collisions in the medium. An example of this technique is Frequency Division Multiple Access. This technique involves dividing the frequency into several frequency bands from where data transfer between two pair of nodes occurs.

The choice of media access control technique over the other depends on a number of factors. First, efficiency in use of computing resources is called for. Reliability and latency are other factors that affect the choice of media access control method.

Since controlled access methods allocate resources to every communication node, it incurs overheads compared to contention based methods. Controlled based methods are often desirable for larger networks where there are many nodes. However, they are not suitable for small networks since they lead to waste of resources. Contention based methods therefore have low latency for smaller networks, making them to outperform controlled access methods.

a) and b)

Encode ‘1’ as ‘+1’ and ‘0’ as ‘-1’

User 1> R1 1111 000

	1	1	1	1		1	1	1	1		1	1	1	1
	0	0	0	0		0	0	0	0		0	0	0	0
XOR	1	1	1	1		1	1	1	1		1	1	1	1

 

User 2> R2 0011 100

	0	0	1	1		0	0	1	1		0	0	1	1
	1	1	1	1		0	0	0	0		0	0	0	0
XOR	1	1	0	0		0	0	1	1		0	0	1	1

 

User 3> R3 1001 001

	1	0	0	1		1	0	0	1		1	0	0	1
	0	0	0	0		0	0	0	0		1	1	1	1
XOR	1	0	0	1		1	0	0	1		0	1	1	0

 

User 4> R4 1010 111

	1	0	1	0		1	0	1	0		1	0	1	0
	1	1	1	1		1	1	1	1		1	1	1	1
XOR	0	1	0	1		0	1	0	1		0	1	0	1

Part B: Scanning WiFi Access Points using inSSIDer

 

This is a ultra-high speed network that connects Australian research and education community with the public internet, the global education and research community and other Australian selected content and service provides. AARNet’s infrastructure allows for an interconnection of Australian Institutions and organizations involved in research and education with the world’s education and research network community across ultra-high speed links, usually 10 to 100 Gigabits per second. This network infrastructure provides for very high capacities to allow it to meet the needs of data intensive research, spanning across various disciplines of humanities and sciences. AARNet exists both as a national network and as an international network. The national optic fiber transmission capacity for AARNet is 80 wavelengths each at 100Gbit/s the national IP network operates at 100Gbit/s routed through diverse channels

This network spans the Australian continent from Perth to Hobart, Cairns to Darwin. It also has various geographically diverse access points in all Australian cities, delivering connectivity within these cities and also delivering substantial network capacity to rural, remote and regional blocks. This national network was built from Brisbane to Perth, with each link at 48kbit/s capacity. This link starred from a hub situated at University of Melbourne. Initially, this network was built as a multi-protocol network. The speed was later upgraded to 2Mbits/s, allowing the AARNet to connect around 40,000 computers. The national network peers with internet service providers and other external content providers to boost availability and performance of the network. The customers are served through a national backbone network capacity of between 1 and 10Gbit/s, while the end users are served with up-to 1Gbit/s Ethernet connectivity and WiFi.

This network provides a high capacity link to two continents; North America and Asia. This network allows interconnection of Australian education and research communities to more than 120 other global education and research networks. The international backbone network fiber connectivity operates at 240Gbits/s transmission capacity, through six interconnection points in North America and one in Singapore. AARNet international backbone now operates across three continents; North America, Europe and Asia. The operating capacity of this network is 120Gbit/s to North America, while it connects to Asia global connectivity of 5Gbit/s (AARNet, 2013).

The management of AARNet network is done by a national body. It oversees the management of both the National backbone network and the international backbone links. For effective internet connectivity, every Australian state is managed by a Regional Management Group. This group is responsible for providing support to the every member’s Local Area Network to the national network’s interface. The management body also monitors network operations to ensure a 24-7 365 days network connectivity. The management uses Multi Router Traffic Grapher to collect Simple Network Management Protocol interface statistics and error management. This management is active, allowing ICMP packets to be sent among Regional management sites and other Network operator regions to monitor performance. AARNet internet uses a dedicated optic fiber backbone to provide scalable network capacities according to customer needs. Research and education institutions connect to AARNet through National Broadband Network fiber optic, or through wireless and satellite connectivity.

 

International internet connectivity capacity increased from 48kbits/s to 56kbits. In 1992, the capacity of internet connectivity rapidly incremented from 56kbits/s to 1.5Mbits/s

Scanning Wireless Access Points

a) . inSSIDer software was use to scan for available networks in a nearby shopping center. Scanning was done from three spots, and screenshots taken. The available WiFi access points were recorded together with their technical characteristics including Signal to Noise Ratio SNR, security type, channel number and type of 802.11 wireless networks.

 

Figure 1: Spot 1

From the available networks, Unique Apt Chogor had the highest SNR ratio compared to the rest of the access points.  Two networks appeared to use the same network channel 6. There was no network access point without security. Signal to Noise Ratio is a measure of the comparison of signal levels to the background noise levels. SNR levels differ from one point to another depending on the position of access point.

 

Figure 2: Spot 2

 

Figure 3: Spot 3

Access Point	Channel no.	SNR	Security Type	802.11 type
Best Tech	1	-67	WPA2- Enterprise	G
TP-LINK	11+7	-70	WPA2- Personal	N
Unique Apt Chagor	6	-70	WPA2- Personal	N
Deltacream connect	1	-70	WPA2- Enterprise	G
Faiba4G_D3A69C	6	-64	WPA2- Personal	N
Walter	11+7	-69	WPA2- Personal	N
Nokia 5	11	-53	WPA2- Personal	N

From the observations above, the SNRs were different at different spots for every network access point. The more the distance from the access point, the less the SNR recorded in the software. For the overcrowded channels, the SNR was observed to be a value closer to every pair of the multichannel.

b). To improve the signal for the reception in router, the router’s antenna can be adjusted from one point to another or by raising the position of the router from the initial position it was earlier on.

 

Figure 4: Home networks

References

AARNet. (2013). AARNet Annual Reports. 

Doudou, M., Djenouri, D., & adache, N. (2014). Synchronous contention-based MAC protocols for delay-sensitive wireless sensor networks: A review and taxonomy. Journal of Network and Computer Applications.

Glenda, K. (2009). AARNet -20 Years of the Internet in Australia.

Wu, H., & Pan, Y. (2010). Medium access control in wireless networks.