Ethernet efficiency; switched vs shared Ethernet

1. Minimum Ethernet frame size?
   Stuck? Peek at 2.4.1 Ethernet frame format, MAC address types, minimum and maximum frame sizes

   • Frame: preamble 7 + SFD 1 | dest 6 | src 6 | type/len 2 | data 46–1500 | CRC 4.
   • Frame 64–1518 bytes (preamble excluded); pad data to 46.
   • MTU = 1500 bytes.
   • First-byte LSB 1 → multicast; FF:…:FF → broadcast; else unicast.
   • 64 bytes = 512 bits = CSMA/CD slot time at 10 Mbps.

   Read lesson 2.4.1

2. A at range 2 and B at range 4: P(A wins)?
   Stuck? Peek at 2.3.4 Binary exponential backoff; probability of winning

   • After the n-th collision: K uniform in {0, …, 2^min(n,10) − 1}; wait K × slot; abort after 16.
   • Slot = 512 bit times (51.2 µs at 10 Mbps).
   • P(A wins) = #{K_A < K_B} / (a·b); both at range 2: win 1/4, tie 1/2.
   • Capture: A at range 2 vs B at range 4 → A wins 5/8.

   Read lesson 2.3.4

3. Send-and-wait utilisation in terms of a?
   Stuck? Peek at 1.3.3 RTT, bandwidth-delay product, utilisation and a = Tp/Tt

   • RTT = 2·T_p for tiny signals (add T_t / queuing if stated).
   • BDP = B × T_p bits in flight; B × RTT before any reply.
   • a = T_p / T_t.
   • Send-and-wait: U = T_t/(T_t + 2T_p) = 1/(1 + 2a); throughput = U·B.
   • To keep the link busy: about 1 + 2a units in flight.

   Read lesson 1.3.3

Classic Ethernet had every computer clamped onto one coaxial cable — one big classroom with CSMA/CD as the speaking rule. Today each computer has its own cable to a switch, and collisions have vanished. GATE asks both: the efficiency formula for the shared case, and how switches change collision and broadcast domains.

On a shared Ethernet, does efficiency get better or worse when the cable gets longer, with the same frame size and rate?

Worse. A longer cable means a larger T_p, so a = T_p/T_t grows: collisions take longer to notice and more time is lost in each contention period.

Efficiency of shared (CSMA/CD) Ethernet

Efficiency = fraction of time the channel carries successful frames when many stations are busy. The standard approximation:

As a → 0 (short cable or long frames), η → 1; as a grows, η falls.

Textbook variants

Shared vs switched Ethernet

• Shared (bus or hub): one medium, CSMA/CD needed, everyone in one collision domain, half-duplex.
• Switched: each host has a dedicated full-duplex link to a switch interface; the switch stores and forwards frames (Lesson 1.2.2) only toward the right interface. No collisions, so CSMA/CD is not needed, and each link gets its full rate.

Collision domains and broadcast domains

Putting it together

• Shared Ethernet efficiency falls with a: η ≈ 1/(1 + 5a).
• Switches give each interface its own collision-free, full-duplex link.
• Switches split collision domains; routers split broadcast domains.

• Switched full-duplex Ethernet has no collisions — CSMA/CD efficiency formulas do not apply to it.
• Switches do not split broadcast domains; routers do.
• A hub with many hosts is still one collision domain.

"Hubs share the floor, switches give everyone a private lane, routers build walls against shouting."

Phase 2 is complete: our frame can cross one link reliably and fairly. The packet inside still needs a route across many networks. Next (Phase 3): the network layer — what does the address that stays fixed from laptop to server look like?

1. NAT

   Shared Ethernet with a = 0.2. Efficiency using 1/(1 + 5a)?

   Write your answer before opening the key.

2. MCQ

   Which device splits broadcast domains?

   1. Hub
   2. Repeater
   3. Switch
   4. Router
3. NAT

   A switch has 8 interfaces, each connected to one host. How many collision domains?

   Write your answer before opening the key.

4. MSQ

   Which are true of switched full-duplex Ethernet?

   1. No collisions occur.
   2. CSMA/CD is not needed.
   3. All interfaces are one collision domain.
   4. Broadcast frames still reach every interface.
5. NAT

   Shared Ethernet: T_t = 51.2 µs, T_p = 5.12 µs. Efficiency with 1/(1 + 5a) (3 decimals)?

   Write your answer before opening the key.

• Shared Ethernet efficiency η = 1/(1 + 5a) (course default); variants 1/(1 + 5.44a), 1/(1 + 6.44a).
• Switched full-duplex: no collisions, no CSMA/CD, full rate per link.
• Collision domains: hub = 1; switch/router = 1 per interface.
• Broadcast domains: only routers split them.

• Ethernet efficiency 1/(1 + 5a) and variants
• shared vs switched Ethernet
• collision domain
• broadcast domain

1. Shared Ethernet efficiency (default)?
   Reveal answer

   1/(1 + 5a).
2. Which device splits broadcast domains?
   Reveal answer

   A router.
3. Collision domains on an 8-interface switch?
   Reveal answer

   8.

• η = 1/(1 + 5a) for shared Ethernet. — CSMA/CD with many active stations; course default unless the question states another constant.
• Counting domains: each switch/router interface = one collision domain (a hub and everything on it = one); each router interface = one broadcast domain. — hubs, switches and routers only (no VLANs).