31 Days Before Your CCENT Exam: Ethernet and Media Access Control

Date: Jan 6, 2014

To help you prepare for your CCENT exam. this chapter reviews Ethernet technologies and operation at both the data link and physical layers.

CCENT 100-101 ICND1 Exam Topics

Determine the technology and media access control method for Ethernet networks.

Key Topics

Ethernet has continued to evolve from the 10BASE2 flavor, capable of speeds up to 10Mbps, to the newest 10GigE (10 Gigabit Ethernet), capable of speeds up to 10Gbps. Since 1985, the IEEE has continued to upgrade the 802.3 standards to provide faster speeds without changing the underlying frame structure. This feature, among others, has made Ethernet the choice for LAN implementations worldwide. Today we review Ethernet technologies and operation at both the data link and physical layers.

Ethernet Overview

802.3 is the IEEE standard for Ethernet, and both terms are commonly used interchangeably. The terms Ethernet and 802.3 both refer to a family of standards that together define the physical and data link layers of the definitive LAN technology. Figure 29-1 shows a comparison of Ethernet standards to the OSI model.

Figure 29-1 Ethernet Standards and the OSI Model

Ethernet separates the functions of the data link layer into two distinct sublayers:

Logical Link Control (LLC) sublayer: Defined in the 802.2 standard
Media Access Control (MAC) sublayer: Defined in the 802.3 standard

The LLC sublayer handles communication between the network layer and the MAC sub-layer. In general, LLC provides a way to identify the protocol that is passed from the data link layer to the network layer. In this way, the fields of the MAC sublayer are not populated with protocol type information, as was the case in earlier Ethernet implementations.

The MAC sublayer has two primary responsibilities:

Data encapsulation: Includes frame assembly before transmission, frame parsing upon reception of a frame, data link layer MAC addressing, and error detection.
Media Access Control: Because Ethernet is a shared media and all devices can transmit at any time, media access is controlled by a method called Carrier Sense Multiple Access with Collision Detection (CSMA/CD) when operating in half-duplex mode.

At the physical layer, Ethernet specifies and implements encoding and decoding schemes that enable frame bits to be carried as signals across both unshielded twisted-pair (UTP) copper cables and optical fiber cables. In early implementations, Ethernet used coaxial cabling.

Legacy Ethernet Technologies

Ethernet is best understood by first considering the two early Ethernet specifications—10BASE5 and 10BASE2. With these two specifications, the network engineer installs a series of coaxial cables connecting each device on the Ethernet network, as shown in Figure 29-2.

Figure 29-2 Ethernet Physical and Logical Bus Topology

The series of cables creates an electrical circuit, called a bus, which is shared among all devices on the Ethernet. When a computer wants to send some bits to another computer on the bus, it sends an electrical signal and the electricity propagates to all devices on the Ethernet.

With the change of media to UTP and the introduction of the first hubs, Ethernet physical topologies migrated to a star, as shown in Figure 29-3.

Figure 29-3 Ethernet Physical Star and Logical Bus Topology

Regardless of the change in the physical topology from a bus to a star, hubs logically operate similarly to a traditional bus topology and require the use of CSMA/CD.

CSMA/CD

Because Ethernet is a shared media where every device has the right to send at any time, it also defines a specification for how to ensure that only one device sends traffic at a time. The CSMA/CD algorithm defines how the Ethernet logical bus is accessed.

CSMA/CD logic helps prevent collisions and also defines how to act when a collision does occur. The CSMA/CD algorithm works like this:

A device with a frame to send listens until the Ethernet is not busy.
When the Ethernet is not busy, the sender(s) begin(s) sending the frame.
The sender(s) listen(s) to make sure that no collision occurred.
If a collision occurs, the devices that had been sending a frame each send a jamming signal to ensure that all stations recognize the collision.
After the jamming is complete, each sender randomizes a timer and waits that long before trying to resend the collided frame.
When each random timer expires, the process starts again from the beginning.

When CSMA/CD is in effect, it also means that a device’s network interface card (NIC) is operating in half-duplex mode—either sending or receiving frames. CSMA/CD is disabled when a NIC autodetects that it can operate in—or is manually configured to operate in—full-duplex mode. In full-duplex mode, a NIC can send and receive simultaneously.

Legacy Ethernet Summary

Today, you might occasionally use LAN hubs, but you will more likely use switches instead of hubs. However, keep in mind the following key points about the history of Ethernet:

The original Ethernet LANs created an electrical bus to which all devices connected.
10BASE2 and 10BASE5 repeaters extended the length of LANs by cleaning up the electrical signal and repeating it—a Layer 1 function—but without interpreting the meaning of the electrical signal.
Hubs are repeaters that provide a centralized connection point for UTP cabling—but they still create a single electrical bus, shared by the various devices, just like 10BASE5 and 10BASE2.
Because collisions could occur in any of these cases, Ethernet defines the CSMA/CD algorithm, which tells devices how to both avoid collisions and take action when collisions do occur.

Current Ethernet Technologies

Refer to Figure 29-1 and notice the different 802.3 standards. Each new physical layer standard from the IEEE requires many differences at the physical layer. However, each of these physical layer standards uses the same 802.3 header, and each uses the upper LLC sublayer as well. Table 29-1 lists today’s most commonly used IEEE Ethernet physical layer standards.

Table 29-1 Today’s Most Common Types of Ethernet

Common Name	Speed	Alternative Name	Name of IEEE Standard	Cable Type,Maximum Length
Ethernet	10Mbps	10BASE-T	802.3	Copper, 100 m
Fast Ethernet	100Mbps	100BASE-TX	802.3u	Copper, 100 m
Gigabit Ethernet	1000Mbps	1000BASE-LX	802.3z	802.3z
Gigabit Ethernet	1000Mbps	1000BASE-T	802.3ab	Copper, 100 m
10GigE (Gigabit Ethernet)	10Gbps	10GBASE-T	802.3an	Copper, 100 m

UTP Cabling

The three most common Ethernet standards used today—10BASE-T (Ethernet), 100BASE-TX (Fast Ethernet, or FE), and 1000BASE-T (Gigabit Ethernet, or GE)—use UTP cabling. Some key differences exist, particularly with the number of wire pairs needed in each case and in the type (category) of cabling.

The UTP cabling used by popular Ethernet standards includes either two or four pairs of wires. The cable ends typically use an RJ-45 connector. The RJ-45 connector has eight specific physical locations into which the eight wires in the cable can be inserted, called pin positions or, simply, pins.

The Telecommunications Industry Association (TIA) and the Electronics Industry Alliance (EIA) define standards for UTP cabling, color-coding for wires, and standard pinouts on the cables. Figure 29-4 shows two TIA/EIA pinout standards, with the color-coding and pair numbers listed.

Figure 29-4 TIA/EIA Standard Ethernet Cabling Pinouts

For the exam, you should be well prepared to choose which type of cable (straight-through or crossover) is needed in each part of the network. In short, devices on opposite ends of a cable that use the same pair of pins to transmit need a crossover cable. Devices that use an opposite pair of pins to transmit need a straight-through cable. Table 29-2 lists typical devices and the pin pairs they use, assuming that they use 10BASE-T and 100BASE-TX.

Table 29-2 10BASE-T and 100BASE-TX Pin Pairs Used

Devices That Transmit on 1,2 and Receive on 3,6	Devices That Transmit on 3,6 and Receive on 1,2
PC NICs	Hubs
Routers	Switches
Wireless access points (Ethernet interfaces)	—
Networked printers (printers that connect directly to the LAN)	—

1000BASE-T requires four wire pairs because Gigabit Ethernet transmits and receives on each of the four wire pairs simultaneously.

However, Gigabit Ethernet does have a concept of straight-through and crossover cables, with a minor difference in the crossover cables. The pinouts for a straight-through cable are the same—pin 1 to pin 1, pin 2 to pin 2, and so on. The crossover cable crosses the same two-wire pair as the crossover cable for the other types of Ethernet—the pair at pins 1,2 and 3,6—as well as crossing the two other pairs (the pair at pins 4,5 with the pair at pins 7,8).

Benefits of Using Switches

A collision domain is a set of devices whose frames could collide. All devices on a 10BASE2, 10BASE5, or any network using a hub risk collisions between the frames that they send, so all devices on one of these types of Ethernet networks are in the same collision domain and use CSMA/CD to detect and resolve collisions.

LAN switches significantly reduce, or even eliminate, the number of collisions on a LAN. Unlike hubs, switches do not create a single shared bus. Instead, switches do the following:

They interpret the bits in the received frame so that they can typically send the frame out the one required port, rather than all other ports.
If a switch needs to forward multiple frames out the same port, the switch buffers the frames in memory, sending one at a time, thereby avoiding collisions.

In addition, switches with only one device cabled to each port of the switch allow the use of full-duplex operation. Full-duplex means that the NIC can send and receive concurrently, effectively doubling the bandwidth of a 100Mbps link to 200Mbps—100Mbps for sending and 100Mbps for receiving.

These seemingly simple switch features provide significant performance improvements as compared with using hubs. In particular:

If only one device is cabled to each port of a switch, no collisions can occur.
Devices connected to one switch port do not share their bandwidth with devices connected to another switch port. Each has its own separate bandwidth, meaning that a switch with 100Mbps ports has 100Mbps of bandwidth per port.

Ethernet Addressing

The IEEE defines the format and assignment of LAN addresses. To ensure a unique MAC address, the first half of the address identifies the manufacturer of the card. This code is called the organizationally unique identifier (OUI). Each manufacturer assigns a MAC address with its own OUI as the first half of the address. The second half of the address is assigned by the manufacturer and is never used on another card or network interface with the same OUI. Figure 29-5 shows the structure of a unicast Ethernet address.

Figure 29-5 Structure of a Unicast Ethernet Address

Ethernet also has group addresses, which identify more than one NIC or network interface. The IEEE defines two general categories of group addresses for Ethernet:

Broadcast addresses: The broadcast address implies that all devices on the LAN should process the frame and has a value of FFFF.FFFF.FFFF.
Multicast addresses: Multicast addresses are used to allow a subset of devices on a LAN to communicate. When IP multicasts over an Ethernet, the multicast MAC addresses used by IP follow this format: 0100.5exx.xxxx. The xx.xxxx portion is divided between IPv4 multicast (00:0000–7F.FFFF) and MPLS multicast (80:0000–8F:FFFF). Multiprotocol Label Switching (MPLS) is a CCNP topic.

Ethernet Framing

The physical layer helps you get a string of bits from one device to another. The framing of the bits allows the receiving device to interpret the bits. The term framing refers to the definition of the fields assumed to be in the data that is received. Framing defines the meaning of the bits transmitted and received over a network.

The framing used for Ethernet has changed a couple of times over the years. Each iteration of Ethernet is shown in Figure 29-6, with the current version shown at the bottom.

Figure 29-6 Ethernet Frame Formats

The fields in the last version shown in Figure 29-6 are explained further in Table 29-3.

Table 29-3 IEEE 802.3 Ethernet Field Descriptions

Field	Field Length in Bytes	Description
Preamble	7	Synchronization
Start Frame Delimiter (SFD)	1	Signifies that the next byte begins the Destination MAC field
Destination MAC Address	6	Identifies the intended recipient of this frame
Source MAC Address	6	Identifies the sender of this frame
Length	2	Defines the length of the data field of the frame (either length or type is present, but not both)
Type	2	Defines the type of protocol listed inside the frame (either length or type is present, but not both)
Data and Pad	46–1500	Holds data from a higher layer, typically a Layer 3 PDU (generic), and often an IP packet
Frame Check Sequence (FCS)	4	Provides a method for the receiving NIC to determine whether the frame experienced transmission errors

The Role of the Physical Layer

We have already discussed the most popular cabling used in LANs—UTP. But to fully understand the operation of the network, you should know some additional basic concepts of the physical layer.

The OSI physical layer accepts a complete frame from the data link layer and encodes it as a series of signals that are transmitted onto the local media.

The delivery of frames across the local media requires the following physical layer elements:

The physical media and associated connectors
A representation of bits on the media
Encoding of data and control information
Transmitter and receiver circuitry on the network devices

There are three basic forms of network media on which data is represented:

Copper cable
Fiber
Wireless (IEEE 802.11)

Bits are represented on the medium by changing one or more of the following characteristics of a signal:

Amplitude
Frequency
Phase

The nature of the actual signals representing the bits on the media will depend on the signaling method in use. Some methods might use one attribute of a signal to represent a single 0 and use another attribute of a signal to represent a single 1. The actual signaling method and its detailed operation are not important to your CCNA exam preparation.

Study Resources

For today’s exam topics, refer to the following resources for more study.

Resource	Location	Topic
Primary Resources
Network Basics	10	All
Introduction to Networks	5	All
ICND1 Official Cert Guide	2	Building Physical Ethernet Networks with UTP Sending Data in Ethernet Networks
ICND1 Foundation Learning Guide	3	All
Supplemental Resources
CCENT Practice and Study Guide	5	All
Flash Cards	3	Relevant Questions
CCNA R&S Portable Command Guide	4	All