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.
5th 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)
