Day 29. Ethernet and Media Access Control

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 sublayer. 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.

Video 29.1—The Purpose of the LLC and MAC Data Link Sublayers

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.

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

1. A device with a frame to send listens until the Ethernet is not busy.

2. When the Ethernet is not busy, the sender(s) begin(s) sending the frame.

3. The sender(s) listen(s) to make sure that no collision occurred.

4. 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.

5. After the jamming is complete, each sender randomizes a timer and waits that long before trying to resend the collided frame.

6. 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.

• 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.

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.

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.

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).

Video 29.2—How CSMA/CD Work and How Switches Segment Collision Domains

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 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.

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.

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

Video 29.3—The Structure of a MAC Address and the Ethernet Frame Formats

Exercise: Order the Fields in a Frame

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.

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