Evolving Internet: class #3

Course 4: Mobility

Slides

This course presents Internet mobility (IP level) and discusses the architectural interest of the HIP protocol. We also present the Mobile Ad-Hoc Networks and the related protocols. You can access the course slides here.

CDMA

Use orthogonal vectors to transmit at the same time without collisions: needs perfect synchronization. Each station has a code vector $V_i$, the size of the vector will depend on the amount of stations that share the channel. Each station has data to transmit $D_i$, the signal seen on the shared channel will be:

(1)

\begin{align} S = V_a \times D_a + V_b \times D_b \end{align}

Note that the size of the sent data is a matrix of size nm, where n and m are the sizes of the data and coding vectors.

802.11 frame address format

I think there was an error in the slide. From http://sss-mag.com/pdf/802_11tut.pdf I found this:

ToDS

This bit is set to 1 when the frame is addressed to the AP for forwarding it to the Distribution System (including the case where the destination station is in the same BSS, and the AP is to relay the frame). The Bit is set to 0 in all other frames.

FromDS

This bit is set to 1 when th eframe is comming from the Distribution System.

Address Fields

A frame may contain up to 4 Addresses depending on the ToDS and FromDS bits defined in the Control Field, as follows:

• Address-1 is always the Recipient Address (i.e. the station on the BSS who is the immediate recipient of the packet), if ToDS is set this is the Address of the AP, if ToDS is not set then this is the address of the end-station.
• Address-2 is always the Transmitter Address (i.e. the station who is physically transmitting the packet), if FromDS is set this is the address of the AP, if it is not set then it is the address of the Station.
• Address-3 is in most cases the remaining, missing address, on a frame with FromDS set to 1, then the Address-3 is the original Source Address, if the frame has the ToDS set then Address 3 is the destination Address.
• Address-4 is used on the special case where a Wireless Distribution System is used, and the frame is being transmitted from one Access Point to another, in this case both the ToDS and FromDS bits are set, so both the original Destination and the original Source Addresses are missing.

The following Table summarizes the usage of the different Addresses according to the ToDS and FromDS bits setting:

To DS	From DS	Address 1	Address 2	Address 3	Address 4
0	0	DA	SA	BSSID	N/A
0	1	DA	BSSID	SA	N/A
1	0	BSSID	SA	DA	N/A
1	1	RA	TA	DA	SA