Formula sheet
No cheat sheet allowed in the exam, so these have to be memorized.
| What | Formula |
|---|---|
| Transmission delay | \(d_{trans} = \dfrac{L}{R}\) |
| Propagation delay | \(d_{prop} = \dfrac{d}{s}\), with \(s \approx 2\text{–}3 \times 10^8\) m/s |
| Nodal delay | \(d_{nodal} = d_{proc} + d_{queue} + d_{trans} + d_{prop}\) |
| Store-and-forward, 1 packet, N links | \(N \cdot \dfrac{L}{R}\) |
| P packets over N links (pipelined) | \((N + P - 1) \cdot \dfrac{L}{R}\) |
| Traffic intensity | \(\dfrac{La}{R}\) (→ 1 means huge delay, > 1 means infinite) |
| Avg queuing delay, N pkts arrive at once | \(\dfrac{(N-1)\,L}{2R}\) |
| Throughput | \(\min(R_s,\ R_c,\ R/n)\) (the bottleneck) |
| Bandwidth-delay product | \(R \cdot d_{prop}\) = max bits in the link |
| Width of a bit | \(\dfrac{s}{R}\) |
| Circuit-switching users | \(\dfrac{C}{r}\) = link rate / per-user rate |
| Packet switching, P(k active) | \(\dbinom{N}{k}\, p^k (1-p)^{N-k}\) |
| Non-persistent HTTP | 2 RTT per object (+ DNS RTTs) |
| Persistent HTTP (pipelined) | 2 RTT for base + 1 RTT for all objects |
| Access delay (caching) | \(\dfrac{\Delta}{1 - \Delta\beta}\), with \(\Delta = L / R_{access}\) |
| Client-server distribution | \(D_{cs} = \max\left\{ \dfrac{NF}{u_s},\ \dfrac{F}{d_{min}} \right\}\) |
| P2P distribution | \(D_{p2p} = \max\left\{ \dfrac{F}{u_s},\ \dfrac{F}{d_{min}},\ \dfrac{NF}{u_s + \sum u_i} \right\}\) |
| Sender utilization | \(U = \dfrac{N \cdot L/R}{RTT + L/R}\) (N = 1 for stop-and-wait) |
| GBN max window | \(2^k - 1\) |
| SR max window | \(2^{k-1}\) |