Why do the header and trailer fields differ from one data link protocol to another?

Because the amount of control information a frame carries depends on the access control method and the logical topology, so different protocols need different fields.

The Frame structure is not fixed across all protocols:

• The fields of the header and trailer vary according to the data link layer protocol in use (e.g., Ethernet vs PPP vs HDLC).
• The amount of control information carried within a frame varies according to the access control requirements and the logical topology.

Why it matters: A simple point-to-point link (only two nodes) needs little addressing or media-access control, so its frames can be lean. A shared-access topology where many nodes contend for the medium needs richer control fields. Each protocol is shaped to its topology, which is why Ethernet, PPP, and HDLC frames look different.

Go deeper:

• High-Level Data Link Control (Wikipedia) — HDLC's Flag/Address/Control/Info/FCS layout, a direct contrast to Ethernet's fields illustrating how control info depends on the protocol.
• Data link layer (Wikipedia) — explains why LAN protocols (Ethernet, 802.11) carry MAC+LLC fields while point-to-point WAN protocols (PPP, HDLC) frame differently.