A late collision is a network issue that occurs when a collision is detected after a significant portion of the data packet has already been transmitted.
Understanding Late Collisions
In networks like older Ethernet (using CSMA/CD - Carrier Sense Multiple Access with Collision Detection), stations listen to the network before transmitting. If two stations transmit simultaneously, their signals collide. Normally, sending stations are designed to detect these collisions quickly and stop transmitting to avoid wasting bandwidth and to allow retransmission later.
However, a late collision breaks this expected behavior. According to the provided reference, a late collision is:
"...a collision that is detected late in the transmission of the packet."
It specifically happens when:
"If a sending station does not detect the collision quickly (specifically, within a time period of 512 bit-times), it is a late collision."
This means the collision isn't noticed within the standard window for early detection, which is typically calculated based on the round-trip time across the network segment (often standardized to 512 bit-times for Ethernet).
Normal vs. Late Collisions
| Feature | Normal Collision | Late Collision |
|---|---|---|
| Detection Time | Detected early in packet transmission | Detected late in packet transmission |
| Detection Window | Within the first 512 bit-times | After the first 512 bit-times |
| Packet Status | Packet is aborted quickly | Significant portion of packet sent |
| Impact | Managed by CSMA/CD, packet re-sent | Often indicates an underlying issue |
Why Do Late Collisions Occur?
Unlike normal collisions which are an expected part of CSMA/CD operation under contention, late collisions usually point to a problem within the network segment. Common causes include:
- Network Segment Length: The network segment is too long, causing the collision signal to take too long to propagate back to the sending station within the 512 bit-time window. This violates the physical layer specifications.
- Faulty Network Interface Card (NIC): A defective NIC might not detect collisions properly or quickly enough.
- Repeater/Hub Issues: Problems with intermediate devices that affect signal timing or collision detection.
- Too Many Repeaters/Hubs: Exceeding the maximum allowed number of repeaters between two stations.
- Duplex Mismatch: A station configured for full-duplex communicating with a device (or port) configured for half-duplex. Full-duplex assumes no collisions will occur and disables collision detection, while half-duplex expects it.
Implications of Late Collisions
When a late collision happens, the sending station might not realize the packet was corrupted by the collision until it finishes transmitting or even later. This can lead to:
- Corrupted Data: The transmitted frame is damaged and unusable by the recipient.
- Lower Throughput: Bandwidth is wasted sending packets that will be discarded.
- Application Errors: Higher-layer protocols may time out or report errors due to missing or corrupted data.
- Troubleshooting Needed: Requires investigation into the network's physical layer or device configurations, as it's not a standard CSMA/CD event.
Troubleshooting Late Collisions
Identifying and resolving late collisions typically involves checking:
- Cable lengths and types against standards.
- The number of repeaters or hubs in the segment.
- The duplex settings on connected devices (NICs, switch/hub ports).
- NIC and driver health.
- Replacing potentially faulty cables or network devices.
Late collisions are a strong indicator that the fundamental rules of the network's physical layer (like Ethernet's 5-4-3 rule for repeaters or maximum segment length) are being violated or that equipment is malfunctioning.
