Andrew D Boston.
DATA COM & WIRELESS NET (ACSG-525-01-2008F) , Fall 2008.
Sunday, December 07, 2008, Assignment #07 Chapter #07
Special Notes taken from SXU - Black Board:
Assignment # 7:
Answer all end of chapter exercises for Chapter 7 (pgs 264 thru 266). Due
on Thursday, December 4th.
6. 1 or 2.
15. Direct Sequence.
17. The PLCP frame preamble and header are always transmitted at 1 MPS. What are the
advantages and disadvantages of this?
The advantages are: This allows for a slower sending device to talk to a faster receiving device
because it is using the slowest speed possible for frames on this technology. The disadvantages are:
even if both networking devices are faster and can transmit at a faster rate, the two faster devices
must still fall back to the original 1 Mbps transmission rate, for the preamble and header networking,
go with a slower rate for connection. This helps specify that data can be transmitted at 1 or 2 Mbps as
well, the networking works.
18. Explain how a carrier sense multiple access with collision avoidance (CSMA/CA) is
different from carrier sense multiple access with collision detection (CSMA/CD)?
The Differences are:
Carrier Sense Multiple Access With Collision Detection (CSMA/CD): in computer networking, is a
network control protocol in which a carrier sensing scheme is used. A transmitting data station that
detects another signal while transmitting a frame, stops transmitting that frame, transmits a jam signal,
and then waits for a random time interval (known as "back off delay" and determined using the truncated
binary exponential back off algorithm) before trying to send that frame again.
CSMA/CA stands for: Carrier Sense Multiple Access With Collision Avoidance:
Major difference is:
Channel access method called carrier sense multiple access with collision avoidance
(CSMA/CA). CSMA/CA is based on a similar Ethernet technology.
In computer networking, CSMA/CA belongs to a class of protocols called multiple access methods.
CSMA/CA stands for: Carrier Sense Multiple Access With Collision Avoidance. In CSMA, a station
wishing to transmit has to first listen to the channel for a predetermined amount of time so as to check for
any activity on the channel. If the channel is sensed "idle" then the station is permitted to transmit. If the
channel is sensed as "busy" the station has to defer its transmission. This is the essence of both
CSMA/CA and CSMA/CD. In CSMA/CA (Local Talk), once the channel is clear, a station sends a signal
telling all other stations not to transmit, and then sends its packet. In Ethernet 802.3, the station continues
to wait for a time, and checks to see if the channel is still free. If it is free, the station transmits, and waits
for an acknowledgment signal that the packet was received.
19. Explain how packet acknowledgement works?
CSMA/CA also reduces collisions by using explicit packet acknowledgement (ACK). An
acknowledgement packet is sent by the receiving device, back in the sending devices to confirm that
the data packet arrived intact. If the ACK frame is not received by the sending device, either the
original data packet was not received or the ACK was not received intact. The sending devices
assumes that a problem has occurred and retransmits the data packet after waiting another random
amount of time. The explicit ACK mechanism handles interference and related problem, such as a
client device hearing the transmission from another client far away.
20. What RTS/CTS/ What are it advantages and how it does work?
What it is:
RTS/CTS (Request to Send / Clear to Send) is the optional mechanism used by the 802.11 wireless
networking protocol, the advantage is to reduce frame collisions introduced by the hidden terminal
problem. Originally the protocol fixed the exposed terminal problem as well, but modern RTS/CTS
includes ACKs and does not solve the exposed terminal problem.
Request to Send /Clear to Send (RTS/CTS) protocol also called virtual carrier sensing. The RTS/CTS
protocol I works by imposing additional overhead and is not used unless there is poor network
performance due to excessive collusion.
21. What is polling? How it it differ from contention?
Another type of channel access method is polling. With this method each computer is sequentially
polled, or asked if it wants to transmit, if the answer is yes, then it is given permission to transmit while all
other devices must wait. If the answer is no, then the next device in sequence is polled.
The interval between beacon between frames is divided into a contention-free period and a
contention period, during the contention free period the hybrid coordinator controls access to the
medium based upon the information received from the traffic.
22. How can 4-PPM transmit at twice the speed of 16-PPM
A modulation technique that translates two data bits into light impulses two data bits into 4 light
impulses. The other is a modulation technique that translates four data bits into light impulses.
4 PPM is two data bits, where as 16 PPM is four data bits, 16 PPM is twice the data bits of 4
PPM. That’s how 16 PPM is twice the transmittal of 4 PPM.