Ethernet link speeds and end-to-end capacity

Ethernet link speed describes negotiated link-layer operation, while frame overhead, duplex and slower intermediate links determine its relationship to useful end-to-end media delivery.

A “gigabit” adapter can connect through a path containing a slower port or negotiated link. The label on one component is only one part of the route.

Use an original path inventory:

Link Declared negotiated rate
Server to switch 1000 Mbit/s
Switch uplink 100 Mbit/s
Viewing device link 1000 Mbit/s

The one-hundred-Mbit/s link is the lowest declared rate. Faster end links do not raise its continuous service capacity.

A cable, port, transceiver or negotiation result can account for a rate differing from a component’s maximum advertised capability.

Useful payload has an on-wire budget

Use an original full-size untagged frame exercise:

{
  "ipPacketBytes": 1500,
  "ipv4HeaderBytes": 20,
  "tcpHeaderBytes": 20,
  "ethernetFrameBytes": 1518,
  "preambleAndStartBytes": 8,
  "interFrameGapBytes": 12,
  "linkMbitPerSecond": 1000
}

The IP packet contains 1460 bytes of TCP payload under the minimum-header assumptions. The frame and stated timing allowance occupy 1538 byte-times.

The conditional payload rate is 1000 × 1460 / 1538, approximately 949.285 Mbit/s.

This omits tags, options, encryption/application framing, reverse traffic, loss and other work. It explains the accounting difference rather than predicting a real Emby transfer at that rate.

Full duplex describes two directions

A one-Gbit/s full-duplex link supports its declared operation in each direction. It is not a two-Gbit/s one-direction payload path.

Requests and acknowledgments use a return direction with its own work. A switch’s aggregate fabric capacity also differs from one port’s rate.

A link can negotiate and remain up while errors, drops or another path bottleneck reduce useful progress.

The client’s decode and server’s preparation/storage can limit playback further. Throughput follows the delivered bytes after those constraints.

See Also

References