Data Link Overhead
Another contributing factor to bandwidth is the Layer 2 protocol used to transport VoIP.
VoIP alone carries a 40 byte IP/UDP/RTP header, assuming uncompressed RTP.
Depending on the Layer 2 protocol used, the overhead could grow substantially. More
bandwidth is required to transport VoIP frames with larger Layer 2 overhead. The following
illustrates the Layer 2 overhead for various protocols:
■ Ethernet II: Carries 18 bytes of overhead—6 bytes for source MAC, 6 bytes for
destination MAC, 2 bytes for type, and 4 bytes for cyclic redundancy check (CRC)
■ MLP: Carries 6 bytes of overhead—1 byte for flag, 1 byte for address, 2 bytes for
control (or type), and 2 bytes for CRC
■ Frame Relay Forum Standard 12 (FRF.12): Carries 6 bytes of overhead—2 bytes
for data-link connection identifier (DLCI) header, 2 bytes for FRF.12 header, and 2
bytes for CRC