Circuit switching: setup, reserved resources, teardown

1. PDU names from application down to data link?
   Stuck? Peek at 1.1.3 Encapsulation, headers, PDU names, addresses and devices per layer

   • Encapsulation: payload + header (+ trailer at data link); decapsulation in reverse.
   • PDUs: message (app) → segment (transport) → packet/datagram (network) → frame (data link) → bits.
   • Addresses: link (per hop, changes) · network (end to end, fixed) · which-program id · names.
   • Devices: hub/repeater 1 · bridge/switch 2 · router 3 · gateway/host up to 7.
   • Efficiency = data / (data + all headers + trailer).

   Read lesson 1.1.3

2. What does a layer's service describe?
   Stuck? Peek at 1.1.1 Why layer: protocol, service, interface, peers

   • Layer: one job in the stack; serves the layer above, uses the layer below.
   • Protocol: rules between peers (same layer, different machines): format, meaning, behaviour.
   • Service: what a layer does for the layer above. Interface: how the layer above asks for it.
   • Peer talk is logical; the real path is down → link → up.
   • Layering = modularity + reuse − some overhead.

   Read lesson 1.1.1

3. 1 KB at 1 kbps (default units): time?
   Stuck? Peek at 0.2.1 Bits, bytes and prefixes: powers of 10 for rates, powers of 2 for sizes

   • 1 B = 8 b. Lower-case b = bit, upper-case B = byte.
   • Rates: k = 10³, M = 10⁶, G = 10⁹ (bps).
   • Sizes: K = 2¹⁰, M = 2²⁰, G = 2³⁰ (bytes) — unless the question says 1000.
   • Time to push out = (size in bits) ÷ (rate in bit/s).
   • ms = 10⁻³ s, µs = 10⁻⁶ s, ns = 10⁻⁹ s.

   Read lesson 0.2.1

A classic phone call: before anyone says "hello", the telephone network reserves a path through every exchange between you and your friend. That path is yours until you hang up — even during the silences. This is circuit switching, the first answer to "how does data cross many links?".

During a 10-minute phone call you are silent half of the time. What happens to the reserved capacity during your silences?

It is wasted: nobody else can use it, because the circuit is reserved for you until teardown. That waste is the main weakness of circuit switching for bursty computer traffic.

Three phases

1. Setup: a signalling message travels hop by hop; each switch reserves capacity on its outgoing link for this call.
2. Data transfer: bits flow along the fixed path at a guaranteed rate, in order, with no waiting in queues.
3. Teardown: the resources are released.

Reserved resources

A link is shared among circuits by giving each one either a repeating time slot or its own band of frequencies. Either way the share is fixed: a link of rate R split into n circuits gives each circuit R/n.

Properties

• Guaranteed rate and constant delay once set up — good for voice.
• Data arrives in order (single path).
• Setup delay before the first bit.
• Idle capacity is wasted; few users can be admitted.
• If any switch on the path fails, the call drops.

Putting it together

• Circuit switching reserves a dedicated share of every link for the whole conversation.
• Setup first, then guaranteed in-order transfer, then teardown.
• Great for steady voice, wasteful for bursty computer traffic.

• The circuit's capacity is reserved even when idle — circuit switching wastes capacity on bursty traffic.
• Do not forget the setup time; it is paid before the first data bit.
• Each circuit gets R/n, not R: use the circuit's share as the sending rate.

"Book the whole road before you drive" — circuit switching is a reserved lane from start to finish.

Circuit switching suits steady voice. Computer traffic is bursty: long silences, sudden bursts. Next (1.2.2): what if we reserve nothing, chop the message into packets, and let each packet queue for links only when it actually has something to send?

1. NAT

   A 2.048 Mbps link is split into 32 equal circuits. Rate of one circuit in kbps?

   Write your answer before opening the key.

2. NAT

   A 1,280,000-bit file over a 64 kbps circuit with 1 s setup. Total time in seconds?

   Write your answer before opening the key.

3. MSQ

   Which are properties of circuit switching?

   1. Resources are reserved for the whole call.
   2. Bits wait in queues at every switch.
   3. Data arrives in order.
   4. Idle reserved capacity can be used by others.
4. MCQ

   In which phase of circuit switching are resources reserved?

   1. Setup
   2. Data transfer
   3. Teardown
   4. None — they are never reserved

• 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.

• circuit switching
• setup / data transfer / teardown
• reserved resources (time slot or frequency band)
• dedicated path
• circuit rate = R/n

1. Three phases of circuit switching?
   Reveal answer

   Setup, data transfer, teardown.
2. Rate of one of n circuits on a link of rate R?
   Reveal answer

   R/n.
3. Main weakness of circuit switching for computers?
   Reveal answer

   Reserved capacity is wasted during idle periods.

• Circuit rate = link rate ÷ number of circuits sharing it. — equal time-slot or frequency shares; the circuit's rate is fixed for the whole call.
• Circuit total time = setup + file bits ÷ circuit rate (+ signal travel time when given). — once set up, bits stream without waiting at switches.