The TCP/IP model’s data-link and physical layers define the protocols and hardware required to deliver data across some physical network. The two work together quite closely; in fact, some standards define both the data-link and physical layer functions. The physical layer defines the cabling and energy (for example, electrical signals) that flow over the cables. Some rules and conventions exist when sending data over the cable, however; those rules exist in the data-link layer of the TCP/IP model.
Focusing on the data-link layer for a moment, just like every layer in any networking model, the TCP/IP data-link layer provides services to the layer above it in the model (the network layer). When a host’s or router’s IP process chooses to send an IP packet to another router or host, that host or router then uses link-layer details to send that packet to the next host/router.
Because each layer provides a service to the layer above it, take a moment to think about the IP logic related to Figure 1-10. In that example, host Larry’s IP logic chooses to send the IP packet to a nearby router (R1). However, while Figure 1-10 shows a simple line between Larry and router R1, that drawing means that some Ethernet LAN sits between the two. Figure 1-11 shows four steps of what occurs at the link layer to allow Larry to send the IP packet to R1.
Figure 1-11 Larry Using Ethernet to Forward an IP Packet to Router R1
Note
Figure 1-11 depicts the Ethernet as a series of lines. Networking diagrams often use this convention when drawing Ethernet LANs, in cases where the actual LAN cabling and LAN devices are not important to some discussion, as is the case here. The LAN would have cables and devices, like LAN switches, which are not shown in this figure.
Figure 1-11 shows four steps. The first two occur on Larry, and the last two occur on Router R1, as follows:
Step 1. Larry encapsulates the IP packet between an Ethernet header and Ethernet trailer, creating an Ethernet frame.
Step 2. Larry physically transmits the bits of this Ethernet frame, using electricity flowing over the Ethernet cabling.
Step 3. Router R1 physically receives the electrical signal over a cable and re-creates the same bits by interpreting the meaning of the electrical signals.
Step 4. Router R1 de-encapsulates the IP packet from the Ethernet frame by removing and discarding the Ethernet header and trailer.
By the end of this process, Larry and R1 have worked together to deliver the packet from Larry to Router R1.
Note
Protocols define both headers and trailers for the same general reason, but headers exist at the beginning of the message and trailers exist at the end.
The data-link and physical layers include a large number of protocols and standards. For example, the link layer includes all the variations of Ethernet protocols and wireless LAN protocols discussed throughout this book.
In short, the TCP/IP physical and data-link layers include two distinct functions, respectively: functions related to the physical transmission of the data, plus the protocols and rules that control the use of the physical media.
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