Data Link Overhead
Transmitting an IP packet over a link requires encapsulation of the IP packet in a frame that is appropriate for the data link layer protocol provisioned on that link. For instance, if the data link layer protocol used on a link is PPP, the interface connected to that link must be configured for PPP encapsulation. In other words, any packet to be transmitted out of that interface must be encapsulated in a PPP frame. When a router routes a packet, the packet can enter the router via an interface with a certain encapsulation type such as Ethernet, and it can leave the router through another interface with a different encapsulation such as PPP. After the Ethernet frame enters the router via the ingress interface, the IP packet is de-encapsulated. Next, the routing decision directs the packet to the egress interface. The packet has to be encapsulated in the frame proper for the egress interface data link protocol before it is transmitted.
Different data link layer protocols have a different number of bytes on the frame header; for VoIP purposes, these are referred to as data link overhead bytes. Data link overhead bytes for Ethernet, Frame Relay, Multilink PPP (MLP), and Dot1Q (802.1Q) are 18, 6, 6, and 22 bytes in that order, to name a few. During calculation of the total bandwidth required for a VoIP call, for each link type (data link layer protocol or encapsulation), you must consider the appropriate data link layer overhead.
Continue reading here: Security and Tunneling Overhead
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