Data Link Layer

1
Data Link Layer

• Wireless Networks
• Wi-Fi (Wireless LAN) Example Problems
• RTS/CTS

2
Example 1

• Consider an CSMA/CA 802.11 WLAN with two stations
  A and C and one Access Point (AP), B. At time,
  t0, station A has generated a frame of length 10
  slots and station C has generated a frame of
  length 5 slots both to be transmitted to B. What
  is the earliest time (T) that both the frames
  will be successfully received and acknowledged by
  B?
• Assumptions
• DIFS 2 slots
• SIFS 1 slot
• ACK frames 1 slot
• T ?

3
Example 2

• Consider an CSMA/CA 802.11 WLAN with two stations
  A and C and one Access Point (AP), B. At time,
  t0, station A has generated a frame of length 10
  slots and station C has generated a frame of
  length 5 slots both to be transmitted to B.
  Assume that after waiting for DIFS, both A and C
  generated a random backoff number equal to 1.
  What is the earliest time (T) that both the
  frames will be successfully received and
  acknowledged by B?
• Assumptions
• DIFS 2 slots
• SIFS 1 slot
• ACK frames 1 slot
• T ?

4
Example 3

• Consider an CSMA/CA 802.11 WLAN with two stations
  A and C and one Access Point (AP), B. At time,
  t0, station A has generated a frame of length 20
  slots and station C has generated a frame of
  length 10 slots both to be transmitted to B.
  Assume that after waiting for DIFS, both A and C
  generated a random backoff number equal to 1.
  What is the earliest time (T) that both the
  frames will be successfully received and
  acknowledged by B?
• Assumptions
• DIFS 2 slots
• SIFS 1 slot
• ACK frames 1 slot
• T ?

5
Avoiding collisions RTS/CTS

• idea allow sender to reserve channel rather
  than random access of data frames avoid
  collisions of long data frames
• sender first transmits small request-to-send
  (RTS) packets to BS using CSMA
• RTSs may still collide with each other (but
  theyre short)
• BS broadcasts clear-to-send CTS in response to
  RTS
• CTS heard by all nodes
• sender transmits data frame
• other stations defer transmissions

avoid data frame collisions completely using
small reservation packets!
6
Collision Avoidance RTS-CTS exchange
A
C
AP
defer
time
7
Example 4

• Consider an CSMA/CA 802.11 WLAN with two stations
  A and C and one Access Point (AP), B. At time,
  t0, station A has generated a frame of length 20
  slots and station C has generated a frame of
  length 10 slots both to be transmitted to B.
  Assume that after waiting for DIFS, both A and C
  generated a random backoff number equal to 1.
  What is the earliest time (T) that both the
  frames will be successfully received and
  acknowledged by B?
• Assumptions
• A and C are both RTS/CTS enabled
• RTS CTS 1 slot
• DIFS 2 slots
• SIFS 1 slot
• ACK frames 1 slot
• T ?

8
Wireless network characteristics

• Multiple wireless senders and receivers create
  additional problems (beyond multiple access)

• Hidden terminal problem
• B, A hear each other
• B, C hear each other
• A, C can not hear each other
• means A, C unaware of their interference at B

• Signal attenuation
• B, A hear each other
• B, C hear each other
• A, C can not hear each other interfering at B