Domain 1 of 5 · Chapter 6 of 9

Transmission Media

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Included in this chapter:

  • Choosing media: distance, bandwidth, EMI, and cost
  • Copper: twisted-pair categories, coax, and DAC
  • Fiber: single-mode vs multimode and the OM grades
  • Wireless media and plenum-rated cabling

Transmission media by reach, speed, EMI immunity, and typical use

MediumTypical max distanceTypical speedEMI immunityCommon use
Cat 5e UTP100 m1 GbpsLowLegacy/standard desktop Ethernet
Cat 6 UTP100 m (10G only to 55 m)1 Gbps (10G short)LowHorizontal runs to endpoints
Cat 6a UTP100 m10 GbpsLow (better with STP)10G to the desk / access layer
Cat 8 (twinax-class)~30 m25/40 GbpsLowTop-of-rack data-center links
DAC (direct attach copper)~1-7 m10-100 GbpsLowSwitch-to-server in a rack
Coaxial (RG-6)~100s of mBroadband (CATV/cable)MediumCable-internet / CATV drop
Multimode fiber (OM3/4/5)~300-550 m10-100 GbpsFull (immune)Data-center / in-building backbone
Single-mode fiberKilometers to tens of km10-100+ GbpsFull (immune)WAN / campus / carrier backbone
Wireless (802.11 / cellular / satellite)Meters to globalVaries, sharedNone (RF interference)Mobility, remote, WAN failover

Decision tree

Can a cable be run?cable feasible vs mobilityNoWirelessWi-Fi (LAN) / cellular (WAN) / satelliteYesOver 100 m, or needs EMI immunity?distance or noisy environmentYesKilometers / WAN reach?long haul vs short reachYesSingle-mode fiber (SMF)~9 um core, laser, km to tens of kmNoMultimode fiberOM3/OM4/OM5, short reachNoShort in-rack link (a few m)?copper within 100 mYesCat 8 or DAC cable25/40 Gbps, ~30 m / DAC ~few mNoNeed 10 Gbps to the desk?speed over the 100 m runYesCat 6a10 Gbps to 100 mNoCat 5e or Cat 61 Gbps to 100 m
Cheat sheet

Cheat sheet

  • Pick transmission media by distance, bandwidth, EMI immunity, and cost
  • Every twisted-pair category stops at a 100 m channel
  • Cat 6 does 10 Gbps only to 55 m; Cat 6a does 10 Gbps to 100 m
  • Cat 8 is the fastest twisted-pair but only for ~30 m in a rack
  • Shielded twisted-pair (STP) fights EMI that UTP cannot
  • Single-mode fiber is the long-haul choice; multimode is short reach
  • Higher OM grade buys more multimode reach and bandwidth
  • Fiber light sources: laser for single-mode, LED/VCSEL for multimode
  • Use a DAC cable for cheap short links inside a rack
  • Coax survives mainly as the cable-internet drop
  • Wireless splits into Wi-Fi for LAN, cellular for WAN, satellite for remote
  • Plenum-rated cable is required in air-handling spaces
  • Fiber is immune to EMI and resists tapping
  • Match the constraint in the stem to the medium
  • Lower wireless frequencies travel farther and penetrate walls; higher frequencies carry more data but stop short
  • Wi-Fi 6 (802.11ax) adds OFDMA for efficiency in dense client environments, and 6E opens the 6 GHz band

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References

  1. Category 6 cable Community
  2. Coaxial cable Community
  3. Single-mode optical fiber Community
  4. Multi-mode optical fiber Community
  5. Plenum cable Community