Typical LAN Features for OSI Layer

The OSI physical layer, or Layer 1, defines the details of how to move data from one device to another. In fact, many people think of OSI Layer 1 as "sending bits." Higher layers encapsulate the data and decide when and what to send. But eventually, the sender of the data needs to actually transmit the bits to another device. The OSI physical layer defines the standards used to send and receive bits across a physical network.

To keep some perspective on the end goal, consider the example of the web browser requesting a web page from the web server. Figure 3-1 reminds you of the point at which Bob has built the HTTP, TCP, IP, and Ethernet headers, and is ready to send the data to R2.

Figure 3-1 Data Link Frames Sent Using Physical Layer

HTTP GET

HTTP GET

HTTP GET

In the figure, Bob's Ethernet card uses the Ethernet physical layer specifications to transmit the bits shown in the Ethernet frame across the physical Ethernet. The OSI physical layer and its equivalent protocols in TCP/IP define all the details that allow the transmission of the bits from one device to the next. For instance, the physical layer defines the details of cabling— the maximum length allowed for each type of cable, the number of wires inside the cable, the shape of the connector on the end of the cable, and other details. Most cables include several conductors (wires) inside the cable; the endpoint of these wires, which end inside the connector, are called pins. So, the physical layer also must define the purpose of each pin, or wire. For instance, on a standard Category 5 (CAT5) unshielded twisted-pair (UTP) Ethernet cable, pins 1 and 2 are used for transmitting data by sending an electrical signal over the wires; pins 3 and 6 are used for receiving data. Figure 3-2 shows an example Ethernet cable, with a couple of different views of the RJ-45 connector.

Figure 3-2 CAT5 UTP Cable with RJ-45 Connector

The picture on the left side of the figure shows a Regulated Jack 45 (RJ-45) connector, which is a typical connector used with Ethernet cabling today. The right side shows the pins used on the cable when supporting some of the more popular Ethernet standards. One pair of wires is used for transmitting data, using pins 1 and 2, and another pair is used for receiving data, using pins 3 and 6. The Ethernet shown between Bob and R2 in Figure 3-1 could be built with cables, using RJ-45 connectors, along with hubs or switches. (Hubs and switches are defined later in this chapter.)

The cable shown in Figure 3-2 is called a straight-through cable. A straight-through cable connects pin 1 on one end of the cable with pin 1 on the other end, pin 2 on one end to pin 2 on the other, and so on. If you hold the cable so that you compare both connectors side by side, with the same orientation for each connector, you should see the same color wires for each pin with a straight-through cable.

One of the things that surprises people who have never thought about network cabling is the fact that many cables use two wires for transmitting data and that the wires are twisted around each other inside the cable. When two wires are twisted inside the cable, they are called a twisted pair (ingenious name, huh?). By twisting the wires, the electromagnetic interference caused by the electrical current is greatly reduced. So, most LAN cabling uses two twisted pairs—one pair for transmitting and one for receiving.

The OSI physical layer and its equivalent protocols in TCP/IP define all the details that allow the transmission of the bits from one device to the next. In later sections of this chapter, you will learn more about the specific physical layer standards for Ethernet. Table 3-2 summarizes the most typical details defined by physical layer protocols.

Table 3-2 Typical Physical Layer Functions

Function

Description

Cabling

Defines the number of wires and the type of shielding used (or not used).

Connectors

Defines the shape of the connectors and the number of pins.

Pins

Defines the purpose of the pins. For instance, one pin might be used to signal to the other device whether it is allowed to send.

Voltage and current

Defines the electrical characteristics of the endpoint devices that use a cable.

Encoding

Defines how a device signals a binary 0 or 1 onto the transmit pin(s). For instance, +5V might mean 1, and -5V might mean 0. (Many encoding schemes exist and are beyond the scope of CCNA.)

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