Q1(b) · ALOHA and vulnerable periods

Answer

Definition. Pure ALOHA is a random-access protocol in which a station transmits whenever a frame is ready and retransmits after a random delay if the transmission is unsuccessful.

Simple meaning. Frame ready → bhej do. Collision hua → random wait karke retry.

Swipe across the diagram to read its labels ↔

Diagram: Pure ALOHA: arbitrary start times can overlap. Labels: Pure ALOHA: arbitrary start times can overlap | Pure ALOHA: arbitrary start times can overlap | Station A | Station B | Frame A | Frame B | Overlap: both fail in the basic collision model | A retry begins after a random waiting interval.

Understand it step by step

1. Several stations share the same channel. Pure ALOHA does not first check whether the channel is idle.
2. A station transmits immediately. In the basic collision-channel model, any overlapping frame makes the involved frames unsuccessful.
3. An acknowledgement indicates successful reception. If it is not received before timeout, the station treats the attempt as unsuccessful. An absent ACK alone cannot distinguish a collision from another loss.
4. The station waits a random backoff time and retries. If every station used the same fixed delay, they could collide again together.

Yaad rakho: Pure = kisi bhi time start; retry ka wait random.

Definition. The vulnerable period is the interval of start times of other frames that would cause a collision with a chosen frame.

Simple meaning. Apne frame se ek frame-time pehle start hua frame bhi takra sakta hai, aur baad mein bhi.

Swipe across the diagram to read its labels ↔

Diagram: Pure ALOHA: interfering starts from −T to +T. Labels: Pure ALOHA: interfering starts from −T to +T | Pure ALOHA: interfering starts from −T to +T | −T | 0 | +T | Our frame: 0 to T | Vulnerable start-time interval length = 2T

Understand it step by step

1. Let your frame start at time 0 and finish at T. All frames have the same transmission duration T.
2. Another frame starting between −T and 0 is still being transmitted when your frame begins.
3. Another frame starting between 0 and T begins before your frame ends.
4. Therefore interfering start times occupy an interval of length 2T. Exact boundary contact is treated as non-overlap in this idealized timing model.

Yaad rakho: Pure: before T + after T = 2T.

Definition. Slotted ALOHA is a random-access protocol that divides time into slots of one frame duration and permits transmissions only at the beginning of a slot.

Simple meaning. Frame ready hai tab bhi next slot ki start-line ka wait karo.

Swipe across the diagram to read its labels ↔

Diagram: Slotted ALOHA: one frame duration per slot. Labels: Slotted ALOHA: one frame duration per slot | Slotted ALOHA: one frame duration per slot | 0T | 1T | 2T | 3T | 4T | 5T | Station A | Station B | A1 | A2 | B1 | B2 | A2 and B1 choose the same slot: collision. Starts stay aligned.

Understand it step by step

1. All stations agree on slot boundaries; time synchronization is required.
2. A station with a frame waits for a slot boundary before starting. A slot can be idle, successful or a collision.
3. Exactly one station transmitting in a slot produces success in the ideal model. Two or more simultaneous attempts collide.
4. Frames cannot partially overlap adjacent slots under the assumptions. The vulnerable interval is T instead of 2T. Randomized retries are still needed.

Yaad rakho: Slotted = aligned starts; collision risk half-window, not zero.