CCN Activity Session for Section 5.1 – Wireless MAC
Today’s activities are on topics around the MAC layer in wireless networks.
Discussion
What does it mean for a wireless network to be operating in “Infrastructure mode”? If a network is not in infrastructure mode, what mode is it operating in? What are the key differences between the modes of operation?
In infrastructure mode of operation, each wireless host is connected to the larger network via a base station (access point). If not operating in infrastructure mode, a network operates in ad-hoc mode. In ad-hoc mode, wireless hosts have no infrastructure with which to connect. In the absence of such infrastructure, the hosts themselves must provide for services such as routing, address assignment, DNS-like name translation, and more.
Question
What are the differences between the following types of wireless channel impairments: path loss, multipath propagation, interference for other sources?
Path loss is due to the attenuation of the electromagnetic signal when it travels through matter. Multipath propagation results in blurring of the received signal at the receiver and occurs when portions of the electromagnetic wave reflect off objects and ground, taking paths of different lengths between a sender and receiver. Interference from other sources occurs when the other source is also transmitting in the same frequency range as the wireless network.
Discussion
What are the hidden and exposed terminal problems? Why are they important, and can they be solved?
Hidden terminal: Absence of carrier ≠ safe to transmit
• Node A transmits to node B
o Mountain blocks signal from C
o Likewise if C transmits to B
• Node A cannot sense node C
• Nodes A and C interfere at B
o Collision rate high, wastes bandwidth & energy
Exposed terminal: Presence of carrier ≠ unsafe to transmit
• Node W transmits to X
• Node Y wants to transmit to Z
o Senses transmission of W
o Defers transmission
• Potential for simultaneous transmission missed
o High idle rate, wastes bandwidth
Cannot be solved completely although 802.11 MAC helps with the issues.
Question
As a mobile node gets farther and farther away from a base station, why does the base station increase its transmission power, and decrease the transmission rate?
This is to compensate for the fall-off in SNR
Activity
Suppose an 802.11b station is configured to always reserve a channel with the RTS/CTS sequence. Suppose this station wants to transmit 1,000 bytes of data, and all other stations are idle at this time. Assume a transmission rate of 12Mbps. As a function of SIFS and DIFS, and ignoring propagation delays & bit errors, calculate the time required to transmit the frame and receive the acknowledgement. HINT: A conventional data frame is 34 bytes plus payload, a control data frame is 32 bytes.
A frame without data is 32 bytes long (256 bits). Assuming a transmission rate of 12 Mbps, the time to transmit a control frame (such as an RTS frame, a CTS frame, or an ACK frame) is (256 bits)/(12 Mbps) = 21.333• usec. The data frame is (1000 * 8) + (34 * 8) = 8272 bits (as we need to include the sequence control in this frame). The time required to transmit the data frame is (8272 bits)/(12 Mbps) = 689.333• usec.
The complete sequence therefore (as a function of SIFS and DIFS) is:
DIFS + RTS + SIFS + CTS + SIFS + FRAME + SIFS + ACK
= DIFS + 3SIFS + (3*21.333• + 689.333•) usec = DIFS + 3SIFS + 753.333• usec
Activity (you’ll need to look beyond the lecture notes!))
Describe the role of beacon frames in 802.11. What is the content of a beacon frame?
APs transmit beacon frames. An AP’s beacon frames will be transmitted over one of the 11 channels. The beacon frames permit nearby wireless stations to discover and identify the AP.
Further reading
CDMA is a commonly used channel partitioning protocol widely used in wireless LAN technologies. You can find out about CDMA by reading R. Pickholtz, D. Schilling, L. Milstein (1982) “Theory of Spectrum Spread Communication – A Tutorial” IEEE Transactions on Communications, Vol. 30, No. 5, pp. 855-884.