TCP/IP Link Layer (Data Link Plus Physical)
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The TCP/IP model’s original link layer defines the protocols and hardware required to
deliver data across some physical network. The term link refers to the physical connections,
or links, between two devices and the protocols used to control those links.
Just like every layer in any networking model, the TCP/IP link layer provides services to
the layer above it in the model. 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), with no mention of the underlying Ethernet. The Ethernet
network, which implements link-layer protocols, must then be used to deliver that packet
from host Larry over to router R1. Figure 1-11 shows four steps of what occurs at the link
layer to allow Larry to send the IP packet to 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 Larry Using Ethernet to Forward an IP Packet to Router R1
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, the link-layer processes on 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 link layer includes a large number of protocols and standards. For example, the link
layer includes all the variations of Ethernet protocols, along with several other LAN standards
that were more popular in decades past. The link layer includes wide-area network (WAN) standards for different physical media, which differ significantly compared to LAN
standards because of the longer distances involved in transmitting the data. This layer also
includes the popular WAN standards that add headers and trailers as shown generally in
Figure 1-11—protocols such as the Point-to-Point Protocol (PPP) and Frame Relay. Chapter
2, “Fundamentals of Ethernet LANs,” and Chapter 3, “Fundamentals of WANs,” further
develop these topics for LANs and WANs, respectively.
In short, the TCP/IP link layer includes two distinct functions: functions related to the
physical transmission of the data, plus the protocols and rules that control the use of the
physical media. The five-layer TCP/IP model simply splits out the link layer into two layers
(data link and physical) to match this logic.