Speed and Duplex

What speed and duplex mean

When you plug in a cable, the two ends of the link agree on two settings before any data is sent:

These are Layer 1 settings, handled by the network interface card (NIC) and the switch port. They decide how fast and how smoothly every frame travels over that one cable.

Bits and bytes: Mbps vs. MB/s

This is the most common source of confusion. Network speeds and file sizes are counted in different units:

One byte is 8 bits. So to turn a link speed into bytes per second, divide by 8:

• 100 Mbps ÷ 8 = 12.5 MB/s (about 11.8 MB/s of real data)
• 1 Gbps ÷ 8 = 125 MB/s (about 118 MB/s of real data)
• 10 Gbps ÷ 8 = 1250 MB/s (if your disks can keep up)

💡 Why is the real data rate a little lower? A full-size frame carries about 1460 bytes of TCP data, but each one also needs 78 bytes for the preamble, the Ethernet, IP and TCP headers, the FCS (frame check sequence) and the gap between frames. That is about 5% overhead.

Learn more: The Ethernet Frame

Common Ethernet speeds

The speed a link can reach depends on both ends and on the cable in between. A 1 Gbps PC on a 100 Mbps switch port runs at 100 Mbps. Each speed also needs the right category of cable.

Learn more: Copper Cabling · Ethernet Standards

Half duplex vs. full duplex

Half duplex works like a walkie-talkie: only one side can talk at a time. If both talk at once, the signals collide, and both sides must stop, wait a random time and try again. This is CSMA/CD (Carrier Sense Multiple Access with Collision Detection), the method used in the days of hubs and shared cables, where every shared segment was one collision domain.

Full duplex works like a phone call: both sides talk and listen at the same time. On 10 and 100 Mbps copper, each direction has its own wire pair. Gigabit copper sends in both directions on all four pairs at once and uses signal processing (echo cancellation) to separate them. Fibre uses one strand for each direction. There are no collisions, so CSMA/CD is switched off, and a 1 Gbps link can carry 1 Gbps in each direction.

Autonegotiation: how the two ends agree

Autonegotiation lets the two ends of a copper link tell each other what they support and pick the best match automatically. It is on by default on every modern NIC and switch port, and it is required for 1000BASE-T and faster copper standards.

1. The cable is plugged in, and each side sends bursts of pulses that list its abilities.
2. Each side compares its own list with the other side's.
3. Both sides use the same priority order and pick the highest speed both support, with full duplex if both support it.
4. The link comes up and the port light turns on. Frames can now be sent.

When only one end negotiates

If one end has autonegotiation turned off (speed and duplex hard-set by hand), it sends no list of abilities. The other end can still detect the speed from the type of signal it receives, a fallback called parallel detection. But it cannot detect the duplex, so the standard says it must assume half duplex at 10 or 100 Mbps. That rule is the root cause of most duplex mismatches.

Duplex mismatch: the classic problem

A duplex mismatch happens when one end of a link runs full duplex and the other runs half duplex. The link light is on and ping works, but the link is very slow under real load.

⚠️ The fix is almost never “set the other end by hand too”. The fix is to put both ends back on auto. If a device really cannot negotiate, hard-set both ends to the same speed and duplex.

A real-world example: the slow back-office PC

An office reports that one PC copies files from the server at 3 MB/s, while everyone else gets over 100 MB/s. You investigate and find:

Checking speed and duplex

Windows (PowerShell)

PS C:\> Get-NetAdapter | Format-Table Name, Status, LinkSpeed, FullDuplex
Name     Status       LinkSpeed FullDuplex
----     ------       --------- ----------
Ethernet Up           1 Gbps          True
Wi-Fi    Disconnected 0 bps          False

PS C:\> Get-NetAdapterAdvancedProperty -Name "Ethernet" -DisplayName "Speed & Duplex"
Name      DisplayName     DisplayValue     RegistryKeyword RegistryValue
----      -----------     ------------     --------------- -------------
Ethernet  Speed & Duplex  Auto Negotiation *SpeedDuplex    {0}

Linux (ethtool)

$ sudo ethtool enp3s0
Settings for enp3s0:
	Supported ports: [ TP ]
	Supported link modes:   10baseT/Half 10baseT/Full
	                        100baseT/Half 100baseT/Full
	                        1000baseT/Full
	Supports auto-negotiation: Yes
	Advertised link modes:  10baseT/Half 10baseT/Full
	                        100baseT/Half 100baseT/Full
	                        1000baseT/Full
	Advertised auto-negotiation: Yes
	Link partner advertised link modes:  10baseT/Half 10baseT/Full
	                                     100baseT/Half 100baseT/Full
	                                     1000baseT/Full
	Link partner advertised auto-negotiation: Yes
	Speed: 1000Mb/s
	Duplex: Full
	Auto-negotiation: on
	Port: Twisted Pair
	MDI-X: on (auto)
	Link detected: yes

$ ip -s link show enp3s0
2: enp3s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP mode DEFAULT group default qlen 1000
    link/ether 02:00:00:00:00:aa brd ff:ff:ff:ff:ff:ff
    RX:  bytes packets errors dropped  missed   mcast
    184467233  152311    417       0       0     412
    TX:  bytes packets errors dropped carrier collsns
     21983345   98012      0       0       0       0

macOS

$ ifconfig en0 | grep media
	media: autoselect (1000baseT <full-duplex>)

Seen on a switch

If you have access to the switch, its port list shows the same information from the other side. On a Cisco switch, an a- prefix means “autonegotiated”:

SW1#show interfaces status
Port      Name               Status       Vlan       Duplex  Speed Type
Gi1/0/4   Office-PC-12       connected    10         a-full a-1000 10/100/1000BaseTX
Gi1/0/5   Back-office        connected    10         a-half  a-100 10/100/1000BaseTX

Troubleshooting checklist

1. No link light at all? There is no signal, or the two ends cannot agree on a speed. Check the cable, the port, and that both ends support a common speed (a 10 Gbps-only port will not link with a 1 Gbps card).
2. Link at 100 Mbps on gigabit gear? Suspect the cable or a damaged pair first. Try a known-good patch lead.
3. Slow under load, but ping works? Check the duplex on both ends and look for late collisions, CRC errors and runts.
4. One end hard-set? Return both ends to auto.
5. Still slow? The bottleneck may not be the link at all. It could be Wi-Fi, a slow disk or the internet connection.

Learn more: Layer 1 and 2 Problems

Common mistakes

• Comparing Mbps with MB/s. Speeds are in bits and file sizes in bytes. A 100 Mbps internet plan that downloads at 11 MB/s is working perfectly.
• Hard-setting only one end. The other end, still on auto, falls back to half duplex, which causes an instant duplex mismatch.
• Blaming the network for a cable problem. A link stuck at 100 Mbps is usually a bad cable, not a slow switch.
• Thinking full duplex doubles your download speed. It lets you upload and download at full speed at the same time, but each direction is still limited to 1 Gbps on a gigabit link.
• Expecting Wi-Fi to deliver its link rate. A laptop showing a 1200 Mbps Wi-Fi rate usually moves far less real data, because the radio channel is shared and half duplex.

Check yourself

Where to go next

Next, learn why half duplex needed collision detection, and how to find physical faults step by step.

Learn more: Collision Domains · Layer 1 and 2 Problems