Transceivers and Ethernet Standards
Unit 3: Media, Connectors, and Ethernet. Lesson 9 of 49 in NetPlus Academy.
Switches and routers use pluggable transceivers so one port can be copper or fiber, short or long reach, depending on the module you insert.
Form factors
- SFP: small form-factor pluggable, 1 Gbps.
- SFP+: same size, 10 Gbps.
- QSFP and QSFP+: quad, four lanes, 40 Gbps.
- QSFP28: 100 Gbps. QSFP-DD and OSFP push to 400 Gbps.
Both ends of a link need matching wavelength, fiber type, and speed. A single-mode module on multimode fiber, or an SR module at one end and an LR module at the other, will not link.
Naming the standards
Ethernet names read speed, signaling, medium: 1000BASE-T is 1 Gbps over twisted pair. Common ones:
- 1000BASE-T: 1 Gbps, Cat 5e or better, 100 m.
- 10GBASE-T: 10 Gbps, Cat 6A, 100 m.
- 1000BASE-SX: 1 Gbps, multimode, up to about 550 m.
- 1000BASE-LX: 1 Gbps, single-mode, up to 10 km.
- 10GBASE-SR: short reach, multimode, about 300 m (400 m on OM4).
- 10GBASE-LR: long reach, single-mode, 10 km.
- 10GBASE-ER: extended reach, single-mode, 40 km.
Pattern: S means short and multimode, L means long and single-mode, T means twisted pair.
How Ethernet shares the wire
Classic Ethernet used CSMA/CD: listen before sending, and back off if a collision is detected. Full-duplex switched links have no collisions, so CSMA/CD is effectively retired. A duplex mismatch (one side full, one side half) causes late collisions and CRC errors. Auto-negotiation should agree on speed and duplex; hard-coding one side and leaving the other on auto is a classic cause of mismatch.
MAC addresses
48 bits written as 12 hex digits, such as 3C:52:82:1A:9F:07. The first 24 bits are the OUI, the manufacturer code. The address is burned into the network card and is used only on the local segment.
Transceivers and Ethernet Standards, explained slowly
The plug that decides what the port is
Modern switches have empty cages instead of fixed ports. You slide in a transceiver that decides whether the port is copper or fiber, short reach or long reach. Swap the module, change the port.
Form factor Speed Lanes SFP 1 Gbps 1 SFP+ 10 Gbps 1 (same size as SFP) QSFP / QSFP+ 40 Gbps 4 (Q = quad) QSFP28 100 Gbps 4 x 25 QSFP-DD, OSFP 400 Gbps
Both ends of a link must match: same speed, same wavelength, same fiber type. A short-reach module on one end and a long-reach module on the other will not link. Neither will a single-mode module plugged into multimode fiber.
Decoding an Ethernet standard name
The names look cryptic until you know the pattern: speed, BASE, medium code.
10GBASE-SR
| | |
10G | S = short reach, multimode
BASE = baseband (always)The letter after the dash tells you the medium:
- T: twisted pair copper.
- S: short reach over multimode fiber.
- L: long reach over single-mode fiber.
- E: extended reach over single-mode.
Standard Speed Medium Reach 1000BASE-T 1 Gbps Cat 5e+ 100 m 10GBASE-T 10 Gbps Cat 6A 100 m (Cat 6: 55 m) 1000BASE-SX 1 Gbps multimode about 550 m 1000BASE-LX 1 Gbps single-mode 10 km 10GBASE-SR 10 Gbps multimode about 300 m (400 m on OM4) 10GBASE-LR 10 Gbps single-mode 10 km 10GBASE-ER 10 Gbps single-mode 40 km
How Ethernet shares a wire: CSMA/CD
On old shared Ethernet, everyone was on one cable, so two devices could talk at once and garble each other: a collision. The rule was CSMA/CD: listen before sending (carrier sense), anyone may send (multiple access), and if you detect a collision, stop, wait a random moment, and try again (collision detection). Switched full-duplex links give every device its own private lane, so collisions cannot happen and CSMA/CD is effectively retired.
Duplex, and the mismatch that will not go away
Think of a walkie-talkie versus a phone. Half duplex is the walkie-talkie: one side talks at a time. Full duplex is the phone: both talk at once.
Both ends of a link should agree, and normally auto-negotiation sorts it out. The classic mistake: one side is hard-coded to full duplex and the other is left on auto. Auto-negotiation needs both sides to participate; when one side is silent, the auto side falls back to half duplex. Now one end thinks it is a phone call and the other thinks it is a walkie-talkie. The result is late collisions and CRC errors, and a link that works but crawls. Fix: configure both ends the same way.
MAC addresses
A MAC address is 48 bits, written as 12 hex digits such as 3C:52:82:1A:9F:07. The first 24 bits (first six hex digits) are the OUI, which identifies the manufacturer. The rest is unique to the card. It is burned in at the factory and only matters on the local segment.
How the exam asks it
- "10 Gbps to a server 250 m away over existing multimode": 10GBASE-SR.
- "Link is up but shows late collisions and CRC errors": duplex mismatch.
- "Which module for 100 Gbps": QSFP28.
- "SR module, single-mode fiber, no link": mismatch.
- "First six hex digits of a MAC": the manufacturer's OUI.
What to memorize
- SFP 1G, SFP+ 10G, QSFP 40G, QSFP28 100G.
- S short and multimode, L long and single-mode, T twisted pair.
- Duplex mismatch = late collisions and CRC errors.