7 · Layer 1 optics & physical infrastructure¶

At hyperscale network sites (Google, AWS, Meta), over 40% of intermittent link flaps, silent packet drops, and degraded latency stem directly from physical layer issues. This module covers optical transceivers, fiber physics, Digital Optical Monitoring (DOM), Forward Error Correction (FEC), and physical link diagnostics.

Form Factors & Transceiver Optics¶

Modern data centers use standardized pluggable optical transceivers.

Form Factor	Max Speed	Typical Use Case	Connector Type
SFP+	10 Gbps	Legacy top-of-rack host links	LC Duplex
SFP28	25 Gbps	Server NIC to Leaf switch links	LC Duplex
QSFP28	100 Gbps	Leaf-to-Spine links, 100G host connections	LC / MPO-12
QSFP-DD	400 Gbps	High-density Spine & Fabric interconnects	MPO-16 / CS / SN
OSFP	400G / 800G	Next-gen hyperscale & AI/ML cluster fabrics	MPO-16 / Dual LC

Transceiver Optical Specifications & Designations¶

Selecting the proper transceiver requires matching optical reach, fiber type, modulation, connector type, and multiplexing standard.

Designation	Reach	Wavelength ( $\lambda$ )	Fiber Type	Connector	Optical Mux / Modulation	Typical Application
SR / SR4 / SR8	Short Reach (70m–100m)	850 nm	MMF (OM3/OM4)	MPO-12 / MPO-16 / LC	Parallel VCSEL lasers	Host-to-Leaf, intra-rack patching
DR / DR4	Data Center Reach (500m)	1310 nm	SMF (OS2)	MPO-12 / SN / CS	Parallel PAM4 (1 lane/fiber pair)	Leaf-to-Spine intra-datacenter
FR / FR4	Fiber Reach (2 km)	1271–1331 nm	SMF (OS2)	LC Duplex	CWDM4 (4 wavelengths on 1 pair)	Campus & inter-building fabric
LR / LR4 / LR8	Long Reach (10 km)	1295–1309 nm	SMF (OS2)	LC Duplex	LAN-WDM (Tight 800GHz grid)	Metro & inter-facility link
ER / ER4	Extended Reach (40 km)	1550 nm / LAN-WDM	SMF (OS2)	LC Duplex	EML Laser + APD Receiver	Regional DCI backbone links
ZR / ZR4	Zephyr Reach (80 km)	1550 nm	SMF (OS2)	LC Duplex	Amplified SMF / High Sensitivity	Long-haul regional transit
400G ZR / OpenZR+	Coherent (80km–120km+)	C-Band (1550 nm)	SMF (OS2)	LC Duplex	Coherent DSP / QPSK & 16-QAM	IP-over-DWDM direct router-to-router DCI
BiDi (Bidirectional)	Single-Strand (100m–10km)	1270nm / 1330nm	MMF / SMF	Simplex LC	WDM Tx/Rx split on single strand	Doubling fiber density on legacy cable runs
CWDM / DWDM	Multi-Channel (10km–80km+)	1270–1610nm (CWDM) 1528–1565nm (DWDM)	SMF (OS2)	LC Duplex	20nm spacing (CWDM) / 50-100GHz grid (DWDM)	Passive optical multiplexing

Optical Technology & Wavelength Multiplexing Mechanics¶

Understanding how light is multiplexed and modulated dictates cost and optic compatibility:

Multi-Mode VCSEL (SR / SR4):
Uses vertical-cavity surface-emitting lasers operating at 850nm.
Requires wider 50µm core Multi-Mode Fiber (OM3/OM4).
High speed (100G/400G) uses parallel ribbon fibers via MPO connectors (e.g., 4 Tx + 4 Rx fibers for 100GBASE-SR4).
Uncooled CWDM4 (FR4 / CWDM):
Coarse Wavelength Division Multiplexing uses 4 distinct wavelengths spaced 20nm apart (1271nm, 1291nm, 1311nm, 1331nm).
Combines 4 optical channels inside the transceiver onto a single duplex LC Single-Mode Fiber pair, saving structured cabling.
Uncooled lasers reduce power consumption and module cost.
Precision LAN-WDM (LR4 / ER4):
Uses tighter 800GHz (~4.5nm) channel spacing around the zero-dispersion 1310nm window (1295.56nm, 1300.05nm, 1304.58nm, 1309.14nm).
Requires TEC (Thermoelectric Cooler) inside the optic to maintain exact laser temperature and prevent channel overlap across 10km+ distances.
Coherent IP-over-DWDM (400G ZR / OpenZR+):
Replaces intensity modulation with Coherent Phase Modulation (QPSK or 16-QAM) driven by an onboard Digital Signal Processor (DSP).
Allows 400Gbps interfaces to plug directly into switch/router QSFP-DD ports and bridge 120km+ over DWDM line systems without external transponder boxes.

Never mix MMF and SMF

Multi-mode fiber uses a wider 50 µm core for LED/VCSEL transmitters; Single-mode uses a narrow 9 µm core for precision lasers. Connecting an MMF patch cable to an SMF optic causes severe insertion loss, optical reflection, and complete link failure.

Digital Optical Monitoring (DOM / DDM)¶

DOM allows switches and hosts to read internal optical metrics in real time via I2C bus.

Key DOM metrics: 1. Tx Power (Transmitter): Optical power emitted by laser (in dBm or µW). 2. Rx Power (Receiver): Optical power received at local optic (in dBm or µW). 3. Laser Bias Current: Electrical current driving the laser diode (mA). High bias signals laser degradation. 4. Temperature: Operating temperature of optical module (°C).

Optical Power Math: mW to dBm¶

Optical power is measured logarithmically in dBm (decibel-milliwatts):

$P_{\text{dBm}} = 10 \cdot \log_{10}\left(\frac{P_{\text{mW}}}{1\text{ mW}}\right)$

$0\text{ dBm}$ = $1.0\text{ mW}$
$-3\text{ dBm}$ = $0.5\text{ mW}$ (3 dB loss = 50% power drop)
$-10\text{ dBm}$ = $0.1\text{ mW}$

Reading DOM in EOS and Linux¶

# Arista cEOS CLI: Inspect optical DOM metrics and alarm thresholds
show interfaces transceiver
show interfaces Ethernet1/1 transceiver detail

Ethernet1/1:
  Parameter                 Value        High Alarm   High Warn    Low Warn     Low Alarm
  ------------------------- ------------ ------------ ------------ ------------ ------------
  Temperature               34.21 C      75.00 C      70.00 C      -5.00 C      -10.00 C
  Tx Power                  -1.20 dBm    3.00 dBm     2.00 dBm     -6.00 dBm    -7.00 dBm
  Rx Power                  -3.85 dBm    3.00 dBm     2.00 dBm     -10.00 dBm   -11.00 dBm
  Tx Bias Current           6.45 mA      15.00 mA     12.00 mA     2.00 mA      1.00 mA

# Linux host CLI: Query EEPROM and DOM from host NIC
ethtool -m eth0

Forward Error Correction (FEC) & Bit Error Rate (BER)¶

At speeds of 100G and 400G, high frequency electrical signals experience inter-symbol interference. FEC adds parity bits to correct random bit errors in real time.

Correctable vs Uncorrectable FEC Errors¶

Correctable FEC Errors: Bit flips detected and successfully repaired by the receiver hardware. Low rates are normal.
Uncorrectable FEC Errors: Packet corruption too severe for FEC algorithms to repair. Result: Frame dropped at PHY layer.

# Inspecting FEC statistics on Arista EOS
show interfaces Ethernet1/1 phy detail

Ethernet1/1:
  FEC Mode: RS-FEC (Reed-Solomon)
  Correctable Codewords:   142850
  Uncorrectable Codewords: 0

Uncorrectable FEC = Physical Degradation

If Uncorrectable Codewords is incrementing, the link is dropping packets before IP routing or TCP can see them. Check for dirty optical connectors, bent fiber patch cables, or degrading transceivers.

Channelization & Port Breakouts¶

High-density switch ports can be broken out into multiple lower-speed interfaces using MPO breakout cables.

                 ┌─── Ethernet1/1/1 (25G)
                 ├─── Ethernet1/1/2 (25G)
QSFP28 100G Port ┼─── Ethernet1/1/3 (25G)
                 └─── Ethernet1/1/4 (25G)

# Configuring 100G port breakout into 4x 25G ports on EOS
configure
interface Ethernet1/1
  speed forced 4x25gfull

Next: Interview Questions → — test your overall knowledge on Linux, Shell, SSH, Kernel Networking, and Layer 1 Physical Optics.