# Unit 3: error and access traps

Subject: Computer Networks

Exam: TEE

Reader: https://deep-learning-study-room.vercel.app/cn/#tee/revision/tee-revision/u3

Canonical: https://deep-learning-study-room.vercel.app/read/cn/tee/revision/tee-revision--u3/

TEE · Use after the full lessons and solved papers · current syllabus 702AI0C023

| Trap | Correction |
| --- | --- |
| Hamming distance calculated by decimal subtraction | XOR equal-length strings and count 1s |
| Pick largest pairwise distance | Find the minimum of every distinct pair |
| Detect and correct counts assumed identical | Detect dmin−1; correct floor((dmin−1)/2) |
| CRC generator has four bits, so append four zeros | Append degree r, one fewer than generator bit count |
| CRC receiver appends more zeros | Divide the received complete frame directly |
| Every lost ACK forces immediate retransmission | A later cumulative ACK may acknowledge the missing earlier ACK in GBN |
| SR sender skips a missing base after a later ACK | Window base cannot pass its earliest unacknowledged frame |
| GBN and SR ACK labels interpreted identically | State cumulative next-expected versus individual-frame convention |
| Use S as probability per attempted frame | S is successful frames per frame time; success probability is S/G |
| Carrier sense guarantees no collisions | Propagation delay creates an uncertainty interval |
| CSMA/CA called collision-free | Avoidance reduces risk; collisions remain possible |