Bandwidth, data rate, throughput and transmission delay

1. Does a virtual circuit reserve link capacity?
   Stuck? Peek at 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.

   Read lesson 1.2.3

2. Rate of one of 24 circuits on a 1.536 Mbps link?
   Stuck? Peek at 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.

   Read lesson 1.2.1

3. How many bits are in 1500 bytes?
   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 toll booth lets cars through at one car every 12 seconds. A convoy of 10 cars needs 2 minutes just to get through the booth, no matter how fast the road beyond is. Pushing bits onto a link is the toll booth of networking: it takes time proportional to how many bits you have and inversely proportional to the link's speed.

A 1000-byte packet is put onto a 1 Mbps link. How long until its last bit has left the sender: 1 ms, 8 ms, or 1 s?

8 ms. 1000 bytes = 8000 bits; at 10⁶ bits per second that takes 8000 / 10⁶ = 0.008 s. This is the transmission delay.

Bandwidth and data rate

(Engineers also use "bandwidth" for a range of frequencies in Hz; in GATE networking numericals it always means the bit rate.)

Transmission delay

It depends only on the size and the link rate — not on distance.

Throughput

End to end it can never exceed the slowest link on the path, the bottleneck: throughput ≤ min(B₁, B₂, …).

Putting it together

• Bandwidth is capacity; throughput is what is actually achieved.
• Transmission delay L/B is the time to push all bits onto the link.
• The slowest link caps end-to-end throughput.

• Transmission delay does not depend on distance — a 1000 km link and a 1 m link of the same rate have the same T_t.
• Use bits on top: forgetting × 8 makes T_t eight times too small.
• Throughput is measured, bandwidth is capacity; a 100 Mbps link can show 40 Mbps throughput.

"Transmission = toll booth": how long the whole convoy takes to pass the booth, however long the road.

We can now time the first part of our message's trip: getting onto the wire. Next (1.3.2): once the last bit is on the wire it still has to travel, and at routers it may wait — how do all the delays add up across several hops?

1. NAT

   Transmission delay (ms) of a 2000-byte packet on a 4 Mbps link?

   Write your answer before opening the key.

2. NAT

   A 5 MB file (MB = 10⁶ bytes as stated) is received in 8 s. Throughput in Mbps?

   Write your answer before opening the key.

3. MCQ

   Doubling the length of a link (same rate) changes the transmission delay by a factor of:

   1. 2
   2. 1/2
   3. 1 (no change)
   4. 4
4. MSQ

   A path has links of 100, 20 and 50 Mbps. Which are true?

   1. End-to-end throughput cannot exceed 20 Mbps.
   2. End-to-end throughput can reach 100 Mbps.
   3. The 20 Mbps link is the bottleneck.
   4. A 1250-byte packet has T_t = 0.5 ms on the 20 Mbps link.
5. NAT

   Transmission delay (µs) of a 64-byte unit at 10 Mbps?

   Write your answer before opening the key.

• Bandwidth / data rate B = link capacity in bps (decimal prefixes).
• T_t = L / B (L in bits) — independent of distance.
• Throughput = bits delivered / time ≤ min link rate on the path (bottleneck).

• bandwidth (bit rate of a link)
• data rate
• throughput
• bottleneck link
• transmission delay T_t = L/B

1. Transmission delay formula?
   Reveal answer

   T_t = L / B (bits ÷ bits per second).
2. Does T_t depend on distance?
   Reveal answer

   No.
3. End-to-end throughput bound?
   Reveal answer

   The minimum link rate on the path (bottleneck).

• T_t in ms = (bytes × 8) ÷ (rate in kbps). — quick mental maths: 1000 B at 1000 kbps = 8000 / 1000 = 8 ms.
• End-to-end throughput ≤ min link rate. — single path, steady transfer; ignores waiting and losses (which only lower it further).