Chapter recap
Circuit switching: setup → transfer → teardown; each link's share (R/n) is reserved; in order, no queues, idle capacity wasted. Packet switching: packets with headers, no reservation, store-and-forward at every router, queues and possible loss; datagrams are routed independently and may reorder; statistical multiplexing lets many bursty users share. Virtual circuits: setup chooses a path, packets carry short VC labels, no link capacity reserved, in order.
Merged rule card
From 1.2.1 Circuit switching: setup, reserved resources, teardown
- Circuit switching: setup → data transfer → teardown.
- Dedicated path; each link's share (time slot or frequency band) is reserved: circuit rate = R/n.
- Total time = setup + bits/circuit rate (+ travel time if given).
- + guaranteed rate, in order, no queues; − setup delay, idle capacity wasted, fragile.
From 1.2.2 Packet switching: store-and-forward, datagrams, statistical multiplexing
- Packet switching: packets with headers; no reservation; full link rate while sending.
- Store-and-forward: receive whole packet → buffer/queue → forward; full buffer → loss.
- Datagram: each packet routed independently; may reorder; robust to failures.
- Statistical multiplexing: share capacity on demand; P(exactly k of n) = C(n,k)pᵏ(1−p)ⁿ⁻ᵏ.
From 1.2.3 Virtual circuits and the full comparison
- Virtual circuit: setup chooses a path, switches store (in-link, in-VCI → out-link, out-VCI); packets carry short VCIs; no link capacity reserved.
- Connection-oriented: circuit, VC. Connectionless: datagram.
- Setup? reserved? per-packet routing? → circuit Y/Y/N · VC Y/N/N · datagram N/N/Y.
- Order kept by circuit and VC; datagrams may reorder.
Practice questions
- NAT · 2 marks
A 3 Mbps link is time-slot shared by 30 circuits. A 600,000-bit file is sent after a 0.25 s setup. Total time in seconds?
Write your answer before opening the key.
- MCQ · 1 mark
Which method has no setup phase?
- Circuit
- Virtual circuit
- Datagram
- All have one
- MSQ · 1 mark
Which can lose packets when buffers overflow?
- Circuit switching
- Virtual-circuit switching
- Datagram switching
- None of them
- NAT · 1 mark
One packet needs 5 ms to be sent on each link; it crosses 4 links store-and-forward (nothing else counted). Arrival time in ms?
Write your answer before opening the key.
- NAT · 1 mark
20 users each active with probability 0.1. P(no user active)? (4 decimals)
Write your answer before opening the key.
- MCQ · 1 mark
Two packets of the same connection follow different paths. The network must be using:
- circuit switching
- virtual circuits
- datagrams
- any of the three
- MCQ · 2 marks
The VC identifier in a packet usually:
- is the same on every link
- changes at each hop according to switch tables
- is the full destination address
- is chosen randomly per packet
- MSQ · 1 mark
Advantages of packet switching over circuit switching for bursty data:
- Better use of idle capacity
- No setup delay (datagrams)
- Guaranteed constant delay
- More users per link
Answer key
Try the questions first. Open when you are ready to check.
- 6.25 — Circuit rate 100 kbps; 6 s + 0.25 s = 6.25 s.
- C — Datagrams are sent immediately.
- A: Circuits need setup.
- B: VCs need setup.
- D: Datagrams do not.
- BC — Both packet-based methods queue at switches.
- A: No queues on a reserved circuit.
- D: VC and datagram can drop.
- 20 — 4 × 5 = 20 ms.
- 0.1215..0.1216 — 0.9²⁰ ≈ 0.1216.
- C — Only datagrams route each packet independently.
- A: One fixed path.
- B: One path after setup.
- D: Only datagrams allow it.
- B — VCIs are local to links; switches swap labels.
- A: Labels are link-local.
- C: The full address is only in setup.
- D: All packets of a VC use the table entry.
- ABD — Sharing on demand, immediate sending, more users.
- C: Queueing makes delay variable.