Lecture 2 Flashcards
What is a Broadcast Network?
A single transmission medium shared by many users.
Also referred to as multiple access networks.
What is the purpose of Medium Access Control (MAC)?
To coordinate the access to the channel.
What are the two schemes for sharing a transmission medium?
- Channelization scheme (static)
- MAC scheme (dynamic)
What is the difference between channelization scheme and MAC scheme?
- Channelization scheme partitions the medium into separate channels (static)
- MAC scheme minimizes or eliminates collisions (dynamic)
What factors influence MAC performance?
- Propagation delay
- Transmission bit rate
- Frame length
- Throughput
How is propagation delay calculated?
tprop = d / v, where d is distance in meters and v is the speed of light (3x10^8 m/s).
What is the definition of Throughput?
The actual rate at which information is sent over the shared channel.
What is the relationship between Reff and R?
Reff < R due to coordination information and collisions.
What does normalized maximum throughput or efficiency represent?
ρmax = Reff / R, where Reff < 1.
What is the normalized delay-bandwidth product (α)?
α = tprop / X, where X is the time required to transmit a frame.
What happens to ρmax as α increases?
ρmax decreases.
What is the ALOHA protocol?
A protocol where messages are transmitted as soon as they are available, with collisions treated as transmission errors.
What are the steps of the ALOHA protocol?
- Transmit messages as they become available
- Handle collisions with retransmission
- Use back off algorithm to schedule retransmissions
What is the vulnerable period in ALOHA?
The time period during which a collision can occur, from t0-X to t0+X.
What is the maximum throughput for ALOHA when G=0.5?
Smax ≈ 18.4%.
What is Slotted ALOHA?
A variation of ALOHA where frames occupy one time slot and transmissions can only start at the beginning of a time slot.
What is the maximum throughput for Slotted ALOHA when G=1?
Smax = 36.8%.
What is Carrier Sense Multiple Access (CSMA)?
A protocol where a station senses the medium for carrier signals before transmitting.
What are the three types of CSMA?
- 1-persistent CSMA
- Non-persistent CSMA
- P-persistent CSMA
What is the main advantage of CSMA/CD?
It reduces wasted bandwidth by aborting transmission when a collision is detected.
What is the time required to resolve contention in CSMA/CD?
On average, 1/Pmax = e = 2.718 time slots.
How is throughput calculated for CSMA/CD?
ρmax = 1 / (1 + 6.44a), where a = tprop / X.
Which protocol has better throughput for small a?
CSMA-CD.
Which protocols perform better for larger a?
ALOHA and Slotted ALOHA.
