Foundation Summary

Study Material For Cisco Ccna Ccnp And Ccie Students

Study Material For Cisco Students

Get Instant Access

The "Foundation Summary" section of each chapter lists the most important facts from the chapter. Although this section does not list every fact from the chapter that will be on your CCNA exam, a well-prepared CCNA candidate should know, at a minimum, all the details in each "Foundation Summary" section before going to take the exam.

Table 3-8 lists the various protocol specifications for the original three IEEE LAN standards. Table 3-8 MAC and LLC Details for Three Types of LANs

Name

MAC Sublayer Spec

LLC Sublayer Spec

Ethernet Version 2 (DIX Ethernet)

Ethernet

IEEE Ethernet

IEEE 802.3

IEEE 802.2

IEEE Token Ring

IEEE 802.5

IEEE 802.2

ANSI FDDI

ANSI X3T9.5

IEEE 802.2

Figure 3-14 depicts the cabling and basic operation of an Ethernet network built with 10BASE-T cabling and an Ethernet hub.

Figure 3-14 Small Ethernet 10BASE-T Network

Larry

10BASE-T, Using Shared Hub - Acts like Single Bus

Hub 1

Solid Lines Represent Twisted Pair Cabling

Archie

Figures 3-15 and 3-16 show Ethernet straight-through and crossover cabling.

Figure 3-15 Straight-Through Ethernet Cable

RJ-45

RJ-45

RJ-45

Figure 3-16 Crossover Ethernet Cable

RJ-45

Larry ^ ul Bob

^12 Crossover Cable 1 2

RJ-45

RJ-45

RJ-45

Larry ^ ul Bob

^12 Crossover Cable 1 2

Cross-over Cable Conceptual View

Cross-over Cable Conceptual View

Full-duplex Ethernet cards can send and receive concurrently. Figure 3-17 shows the full-duplex circuitry used with a single PC cabled to a LAN switch.

Figure 3-17 10BASE-T Full-Duplex Operation

Receive

>ocooo«oooooooooooc>o<

Transmit

Full-Duplex NIC

oocjoooooooooooooooo;

Transmit

Receive

Switch NIC

Table 3-9 summarizes some of the key points as they relate to what is covered in this initial section of the chapter.

Table 3-9 Summary of Some Basic Ethernet Features

Ethernet Environment

Description

10BASE2, 10BASE5

Single bus cabled serially between devices using coaxial cable

10BASE-T with a Hub

One electrical bus shared among all devices creating a single collision domain, cabled in a star topology using twisted-pair cabling.

10BASE-T with a Switch

One electrical bus per switch port creating multiple collision domains, cabled in a star topology using twisted-pair cabling.

Half Duplex

Logic that requires a card to only send or receive at a single point in time. Used to avoid collisions.

Full Duplex

Logic that enables concurrent sending and receiving, allowed device is attached to a switch port, ensuring that no collision

when one can occur.

Figure 3-18 shows the details of several types of LAN frames.

Figure 3-18 LAN Header Formats

Ethernet (DIX)

8 6 6 2 Variable 4

Ethernet (DIX)

8 6 6 2 Variable 4

T

Dest.

Source

Preamble

Data

FCS

Address

Address

p

e

IEEE Ethernet (802.3)

7

1

6

6

2

1

1

1-2 Variable

4

Preamble

SD

Dest. address

Source address

Length

D S A P

S S A P

Control

Data

FCS

802.3

802.2

802.3

IEEE 802.3 with SNAP Header

7

1

6

6

2

1

1

1-2

5 Variable 4

Preamble

SD

Dest. address

Source address

Length

D S A P

S S A P

Control

SNAP

Data

FCS

802.3

802.2

802.3

Table 3-10 summarizes the fields that are used for identifying the types of data contained in a frame.

Table 3-10 Protocol Type Fields in LAN Headers

Field Name

Length

LAN Type

Ethernet Type

2 bytes

Ethernet

802.2 DSAP and SSAP

1 byte each

IEEE Ethernet, IEEE Token Ring, ANSI FDDI

SNAP Protocol

2 bytes

IEEE Ethernet, IEEE Token Ring, ANSI FDDI

Ethernet also defines many important OSI Layer 2 functions. In this chapter, four of these features were emphasized, as shown in Table 3-11.

Table 3-11 OSI Layer 2 Feature Summary: Ethernet

OSI Layer 2 Function

Ethernet Implementation

Arbitration

CSMA/CD algorithm

Addressing

6-byte-long MAC addresses

Error detection

FCS in Ethernet trailer

Identifying the type of packet inside the frame

Protocol Type (2 bytes)—DIX Ethernet DSAP (1 byte)—IEEE 802.2

SNAP Protocol Type (2 bytes)—IEEE 802.2 with SNAP header

Was this article helpful?

0 0

Post a comment