Datagram Structures

Please read Datagram and OSI model.

OSI Layer 2 datagrams are called frames.

• IEEE 802.3
• IEEE 802.1q: VLANs

• Wireless LAN.

Layer 2 datagrams of the ATM protocol are called cells.

• ATM cell structure

The PPP (Point-to-point protocol) is a somewhat of a partial Layer 2 protocol. It is always used in conjunction with a second Layer 2 protocol, working as a carrier.

Such PPP frames are encapsulated into the carrier frame: common are Ethernet and ATM. So we talk about

• PPPoE Frame: TODO (or simply combine the already given information)
• PPPoA Cell: TODO (or simply combine the already given information)

But since ATM protocol is ever present, you only seldom have pure PPPoE over the whole distance from your Modem to the DSLAM. You have ATM as well as Ethernet. And this gives you two possibilities to combine the two protocols. You can either encapsulate or bridge:

• Encapsulating: PPPoEoA Frame: Point-to-Point_Protocol_over_Ethernet#How_PPPoE_fits_in_the_DSL_Internet_access_architecture
• Briding: PPPoE-2-PPPoA this means that the Modem, strips the PPPoE header of every frame and replaces it with a PPPoA header.

• Pure PPPoE: it is being claimed that many VDSL2 connections relinquish the ATM protocols, and use pure PPPoE.

Datagrams at OSI Layer 3 are called packets.

• IPv4 Packet
• IPv6 Packet

• ICMPv4 Packet
• ICMPv6 Packet

Some OSI Layer 4 datagrams are referred to as packets. TCP datagrams are called segments. By definition UDP datagrams remain datagrams because they are stateless but are often referred to as packets.

• TCP

• UDP

TCP segment in IPv4 packet in Ethernet frame