Tutorial 6 Flashcards
Suppose nodes A, B, and C each attach to the same broadcast LAN (through their adapters).
If A sends thousands of IP datagrams to B with each encapsulating frame addressed to the
MAC address of B, will C’s adapter process these frames? If so, will C’s adapter pass the IP
datagrams in these frames to the network layer C? How would your answers change if A sends
frames with the MAC broadcast address?
C’s adapter will process the frames, but the adapter will not pass the datagrams up the protocol stack.
If the LAN broadcast/ MAC address address is used, then Cs adapter will both process the frames and pass the datagrams up the protocol stack.
Why is an ARP (address resolution protocol) query sent within a broadcast frame? Why is an ARP response sent within a
frame with a specific destination MAC address??
-In order for a sender to send an IP datagram, the sender must know the IP address and the MAC address.
- To get the unknown MAC address, the sender will send an ARP query as a broadcast message across the LAN.
- Each node will receive the message and determine its a broadcast message
- Each Node/adapter will then send the message up to its own ARP module.
- Each ARP module will check to see if that IP address matches its own IP address.
- If it does, it will send the ARP response message back to the sender with the MAC address inside a frame of the packet
As a mobile node gets farther and farther away from a base station, what are two actions that
a base station could take to ensure that the loss probability of a transmitted frame does not
increase?
Two possible ways:
1.) increasing the transmission power
2.) reducing the transmission rate
Why are acknowledgments used in 802.11 (WLAN) but not in wired Ethernet?
- In a wireless LAN, when a station sends a frame, the frame may not reach the destination station for a variety of reasons. The main reason being high bit error rates of wireless channels. To deal with this, the 802.11 MAC protocol uses acknowledgements scheme.
- In wired ethernet, bit error rate is very negligible and transmission in wired ethernet is very reliable. Therefore, acknowledgements are not required in wired ethernet.
Suppose there are two ISPs providing WiFi access in a particular café, with each ISP operating
its own AP and having its own IP address block.
Further suppose that by accident, each ISP has configured its AP to operate over
channel 11. Will the 802.11 protocol completely break down in this situation? Discuss
what happens when two stations, each associated with a different ISP, attempt to
transmit at the same time?
No, the 802.11 protocol will not completely break down in this situation.
The MAC and SSID (service set identifier/name of wifi network) of every AP (access point) is different.
A wireless station arriving to the cafe will associate with one of the SSIDs (one of the APs). After association, theres a virtual link between the AP and the wireless station. Label the APs AP1 and AP2. Suppose the new station associates with AP1. When new station sends a frame, it will be addressed to AP1. Although AP2 will also receive the frame, it will not process it because the frame is not addressed to AP2. Thus, the two ISPs can work in parallel over the same channel. However, the two ISPs will be sharing the same wireless bandwidth. If wireless stations in different ISPs transmit at the same time, there will be a collision. For 802.11b, the maximum aggregate transmission rate for the two ISPs is 11 Mbps.
Suppose there are two ISPs providing WiFi access in a particular café, with each ISP operating
its own AP and having its own IP address block.
Now suppose that one AP operates over channel 1 and the other over channel 11. How
do your answers change?
Now if two wireless stations in different ISPs (and hence different channels) transmit at the same time, there will not be a collision. Thus the maximum aggregate transmission rate for the two ISPs is 22 Mbps for 802.11b.
