Information (data) is broken into packets prior to being transported to their destination(s).
The OSI model outlines the various layers that data must traverse on it's journey from its source to its destination(s).
Each layer has it's own set of rules (protocols) for handling data packets and adds or removes it's own header (and sometimes a footer) before passing data packets to the next layer. It treats each packet, with or without formatting from other layers, as a simple payload, ignoring it's contents. This process is referred to as encapsulation.
The software and hardware associated with each individual layer is not concerned with, and in fact, not even aware of what happens to the packets in the other layers. It's only function is to format the packets it receives from the layer above or below in a predictable fashion. In this way a very complex process is broken down into a series of simple steps which allows all kinds of different technologies to simply and reliably interact with one another.
The physical layer always concerns itself with the medium the signal traverses through (or over) while the link layer could be split into two sub-layers: Media Access Control and Logical Link Control. The Media Independent Interface is again something different, this is used to connect the distinct switch-chip to the SoC.
How big a packet is, and how it's header looks like depends on the protocol or standard it abides by. Read about this: , IP packet, for an overview. For real precise technical data, read the corresponding RFCs.
These protocols, in turn,
|Protocol||Header Size in Bytes||Total Size in Bytes|
|Ethernet frame||38–42||Size: 84–1542Bytes, with Jumbo frame up to 9042|
|Cisco Picture WLAN mac mode PPDU Frame|
|IPv4 Packet Structure||20–60||Size: 20– (20-byte header + 0 bytes data) 65.535|
|IPv6 Packet Structure||40 fixed, Optional Extension Header possible||up to 65.535|
|TCP Segment Structure||20–60||up to 65.535|
|UDP Packet Structure||4–8 with IPv4 and 6–8 with IPv6||up to 65.535|
Please see MTU
|Ethernet||Preamble||Start of frame delimiter||MAC dest||MAC source||802.1Q tag (opt.)||Ethertype or length||Payload||CRC||Interframe gap|
|IPv4||Version||Header Length||Differentiated Services Code Point||Explicit Congestion Notification||Total Length||Identification||Flags||Fragment Offset||Time to Live||Protocol||Header Checksum||Source IP Address||Destination IP Address||Options ( if Header Length > 5 )||Payload|
|TCP||Source port||Destination port||Sequence number||Acknowledgment number||Data offset||Reserved||Flag||Window Size||Checksum||Urgent pointer||Options (if Data Offset > 5)||padding||Payload|
|Bits||16||16||32||32||4||4||8||16||16||16||Options (if Data Offset > 5)||8||Payload|
Please note, that the Ethernet protocol family comprises standards of Layer 2 and also Layer 1. The latter are e.g. 100Base-TX or 1000Base-T. PPPoE is yet another Layer 2 communication protocol! And DSL is another Layer 1 communication protocol. So the Modem on the customers side communicates over some DSL-protocol with the DSLAM on the ISP side and the router communicates with the DSL-AC over PPPoE.
Being Layer 1 protocols, 1000Base-T or DSL don't care about packets or whatever. Their logic is only concerned with transmitting the data over the specified medium.