BCC-4TDC Assistant Programmer Written Exam: 2019 (BCC)প্রশ্ন ও সমাধান

Piggybacking: in two-way (full-duplex) communication, when a data frame is received, the receiver delays sending back the acknowledgement (ACK) and instead attaches it to its own next outgoing data frame to the sender. This technique of "hooking" the ACK onto an outgoing data frame — instead of sending a separate ACK frame — is called piggybacking; it reduces the number of frames on the network and improves bandwidth utilisation.

sequenceDiagram
  participant A as Host A
  participant B as Host B
  A->>B: Data frame 1
  Note over B: delays ACK, waits for own data to send
  B->>A: Data frame 2 + ACK for frame 1 (piggybacked)

Difference among Hub, Switch and Router:

Device	OSI Layer	Function	Collision Domain
Hub	Physical (1)	Broadcasts every incoming signal to all ports (no intelligence)	One shared domain for all ports
Switch	Data Link (2)	Learns MAC addresses, forwards frames only to the intended port	Separate domain per port
Router	Network (3)	Routes packets between different networks using IP addresses	Separates broadcast domains too

Exam takeaway: piggybacking saves bandwidth by combining an ACK with outgoing data; among the three devices, intelligence and addressing scope increase from Hub (dumb, physical) to Switch (MAC-aware) to Router (IP-aware, inter-network).