Ethernet and Wireless

By Raul Bernardino

Introduction:

Computer networks for the home to small business and from Small Business to Enterprises can be built by using either wired or wireless technology. The wired Ethernet is a physical and data link layer technology for local area networks (LANs). LAN supplies networking capability to a group of computers in close proximity to each other such as in an office building, a school, or a home. A LAN is useful for sharing resources like files, printers, games or other applications. A LAN in turn often connects to other LANs, and to the Internet or other WAN.

Ethernet was invented by engineer Metcalfe, R. It has been the traditional choice in homes, however nowadays Wi-Fi (Wireless Fidelity) technology allows to use at home as well. Wi-Fi is the industry name for wireless local area network (WLAN) communication technology related to the IEEE 802.11 family of wireless networking standards. Wi-Fi technology first became popular in 1999 with 802.11b standard.

Characteristic of IEEE 802.11 physical layer

Characteristics	802.11a	802.11b	802.11g
Frequency	5Ghz	2.4Ghz	2.4Ghz
Rate(Mbps)	6,9,12,18,24,36 ,48,54	1,2,5.5,11	1,2,5.5,6,9,11,12,18,24,36,48,5 4
Modulation	BPSK,QPSK,1 6QAM,64QA M (OFDM)	DBPSK,DQPS K,CCK (DSS,IR      and FH)	BPSK,DBPSK,QPSK,DQPSK, CCK 16QAM,64QAM(OFDM    and DSSS)
FEC Rate	1/2,2/3,3/4	NA	1/2,2/3,3/4
Basic Rate	6Mbps	1 or 2Mbps	1,2 or 6 Mbps

Today, Wi-Fi refers to any of the established standards: wireless technologies are gaining ground fast. Both wired and wireless can claim advantages over the other; both represent viable options for home and other LANs.

What are Wireless network elements?

1.     The end system or wireless host is a devices that running application such as laptop, desktop, PDA, and mobile phones. There are two types of the wireless host such as mobility or to fix. It shows in the below diagram: 

2.     The wireless link is a media where the communication link is established between wireless host and base station. The wireless link has two difference distinct characteristic which coverage area and link rate. The link rates are not permanent it is depending on distance, channel performance, and the numbers of the wireless users. It shows in the below diagram:

3.     The base station is the key element of the network infrastructure in the wireless communication system. The base station is coordination the transmission from and to wireless hosts to the destinations that base stations and wireless hosts are associated with. For instance the base station for cellular phones network is called Cell tower while the wireless network base station is called access point. As it shows in the below diagram:

The wireless hosts which are associated with the base stations are operating in the infrastructure mode because they (base stations) had all network services such as acting as routers and address assignment (IP address, NAT, Gateway, etc).

The ad hoc communication network is not operating in the infrastructure mode because there is no base station to connect. Every wireless host has to act and provide the services such as routing, address assignment, NAT, DNS, and etc as it shows in below diagram:

The difference between wireless hosts that operates in the infrastructure mode and in the ad hoc mode as follows:

1.     In the infrastructure mode it has base station (cell tower or access point) which give a services to the wireless hosts

2.     In the ad hoc mode there is no base station. Each hosts acting to provide services.

The diagram below shows the difference as well:

The wireless host types are Mobility or Fix or Both

It is not a necessary to have nodes which are connected wirelessly and got internet communications should be mobile. For instance, a wireless desktop that is host a data. It is fixed in one place and gets wireless link communication from the base stations or from the ad hoc bases. In other case, it is rooming from one access point to other access points. The mobility of the wireless hosts are communicating to the base stations which is starting from one the standpoint. Because, the access points or base stations have a limitation of the distance coverage area and media interference to the wireless nodes. The ad hoc standpoint as it shows in the below diagram:

·        The A nodes can hear or gets signals from B

·        The B nodes can hear or gets signals from C

·        The C nodes can’t hear or can’t get a signal from A

The diagram below shows the use of base stations to the stand point:

·        Frame broadcast from H1

·        Response from APs

·        Association request H1 to select AP

·        Association response from selected AP to H1

a.     How TCP bandwidth is working >> d, the client buffer as it shows in the below diagram:

The buffer has to be big enough for whole media streaming then x(t) is equal to d

b.     If the buffer is 1/3 then x (t) = 1/3d

c.      Therefore client buffer is 1/10 then x (t) = 1/10 d; and where b and c will be using diagrams below:

The diagram above is one of the streaming stored multimedia.

The multimedia streaming for client buffering, it shows in the diagram above

The UDP as follows:

·        The servers send appropriate rate to the client and it is depending on the network congestion. Usually send rate = encoding rate=constant rate; then fill rate x(t) = constant rate – packet loss

·        Short play out delay is between 2 to 5 seconds to remove network jitter

·        Error recover: time permitting

The TCP as follows:

·        Sends maximum possible rates

·        The fill rate fluctuate due to the congestion control

·        Lager play out delay : smooth TCP delivery rate

·        Http/TCP passes more easy through firewall

Preemptive queuing:

The preemptive queuing services are always severing the high priorities first then low priorities. For instance, customer A arrives first with the low priority. The server is immediately serving customer A. The server while is serving customer A, the customer B arrives with high priority; the server then stop the service to customer A and star serving customer B. After server is serving B finished then server again customer A. As it shows in the below diagram:

In that diagram above packet 1, 3, and 4 are have high priorities while packet 2 and 5 have low priorities. How the packets have been served. The Packet 1 arrive in the server immediately because it is high priority. When packet 2 and 3 arrived, the packet 3 will server first after packet 1 finished because it is high priority too. After packet 3 served, the server start to serve packet 2 while serving packet 2 the packet 4 arrived, the server stop serving packet 2 and start serving packet 4 as a high priority. When packet 4 served then moves to packet 2. After packet 2 finish then start serving packet 5. 

The preemptive queuing sequences packet departures are packet 1, packet 3, packet 4, packet 2, and packet 5. As it shows in the below diagram:

While use non-preemptive queuing the sequences of the packet departure are packet 1, packet 3, packet 2, packet 4, and packet 5 as it show in below diagram:

While, it is processing packet 2 which is low priority is not disturbed or not dropped when packet 4 the high priority arrived.

The loss packet

 

The sender generates packets in every 20 msec during talk spurt. The first packet received at time r, and the first play-out schedule: begins at p in which at missing packets, the second play-out schedule: begins at p’.

The objective is minimizing the play-out delay and keeping late loss rate low

Other approach is adapting play-out delay adjustment such as:

·        Estimating network delay

·        Silent periods of compressing and elongating

·        Chunking play-out every 20 msec

The dynamic average of delay receiver:

In which u is constant e.g. 0.01

Reference List:

1.     Kurose, J.F. & Ross, K.W. (2010) Computer Networking: A Top-Down Approach. 5^th ed. Boston: Addison Wesley

2.     University of Liverpool/Laureate Online Education (2011) Lecture notes from Computer Networking Module Seminar 6 [Online]. Available from: University of Liverpool/Laureate Online Education VLE (Accessed: 13 September 2011).

3.     Chang, W. (8 August 1964), IBM: Preemptive Priority Queues, [Online]. Available from: http://www.jstor.org/pss/167731  (Accessed: 14 September 2011)

4.     Mitchell, B. LAN - Local Area Network, http://compnetworking.about.com/cs/lanvlanwan/g/bldef_lan.htm (Accessed: 14 September 2011)

5.     Mitchell, B.  Wi-Fi - Wireless Fidelity, [Online]. Available from: http://compnetworking.about.com/cs/wireless80211/g/bldef_wifi.htm (Accessed: 14 September 2011)

6.     Mitchell, B.  Ethernet, [Online]. Available from: http://compnetworking.about.com/cs/ethernet1/g/bldef_ethernet.htm (Accessed: 14 September 2011)

7.     Mitchell, B.  Wired vs Wireless Networking, [Online]. Available from: http://compnetworking.about.com/cs/ethernet1/g/bldef_ethernet.htm (Accessed: 14 September 2011)