Tài liệu giảng dạy CCNA - module 01 chapter 03 - Data Link Layer Fundamentals

1
Module 01
InterNetwork Overview
Chapter 03
Data Link Layer Fundamentals: Ethernet LANs
2
Table of Content
1 Typical LAN Features for OSI Layer 1
2 Typical LAN Features for OSI Layer 2
3 Early Ethernet Standards
4 Recent Ethernet Standards
3
1. Typical LAN Features for OSI
Layer 1

The OSI physical layer, or Layer 1, defines the
details of how to move data from one device to
another.

The OSI physical layer defines the standards used
to send and receive bits across a physical
network.
4
2. Typical LAN Features for OSI
Layer 2
5
Data Link Function 1: Arbitration

The specifications for these data-link protocols
define how to arbitrate the use of the physical
medium to avoid collisions, or at least to recover

from the collisions when they occur.

Ethernet uses the carrier sense multiple access
with collision detection (CSMA/CD) algorithm for
arbitration.
6
CSMA/CD

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
begins sending the frame.

3. The sender listens to make sure that no
collision occurred.

4. Once the senders hear the collision, they
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.

6. When each timer expires, the process starts
over with Step 1.
7
Data Link Function 2: Addressing


Every computer has a unique way of identifying
itself : MAC address or physical address.

The physical address is located on the Network
Interface Card (NIC).

MAC addresses have no structure, and are
considered flat address spaces. MAC addresses
are sometimes referred to as burned-in
addresses (BIAs) because they are burned into
read-only memory (ROM) and are copied into
random-access memory (RAM) when the NIC
initializes.

0000.0c12.3456 or 00-00-0c-12-34-56.
8
MAC address format
The first six hexadecimal
digits, which are administered
by the IEEE, identify the
manufacturer or vendor.
The remaining six
hexadecimal digits comprise
the interface serial number.
9
Using MAC addresses
A DData A DData A DDataA DData
Source Address
Destination Address

1
0
Data Link Function 3: Error Detection

Most data-link protocols include a frame check
sequence (FCS) or cyclical redundancy check
(CRC) field in the data-link trailer. This field
contains a value that is the result of a
mathematical formula applied to the data in the
frame.

Error detection does not imply recovery. The FCS
allows the receiving device to notice that errors
occurred and then discard the data frame.
1
1
Data Link Function 4: Identifying the
Encapsulated Data
1
2
3. Early Ethernet Standards
1
3
10BASE2 and 10BASE5
Standard Topology Medium Maximum
cable
length
Transport
10BASE5 Bus Thick
coaxial

cable
500m Half-duplex
10BASE2 Bus Thin coaxial
cable
185m Half-duplex
1
4
A Repeater

Standards guarantee minimum bandwidth and
operability by specifying the maximum number of
stations per segment, maximum segment length,
maximum number of repeaters between stations,
etc.

Stations separated by repeaters are within the
same collision domain. Stations separated by
bridges or routers are in different collision
domains.
1
5
10BASE-T Ethernet

Topology: star (hub)

Medium: CAT3 UTP.

Maximum cable length: 100m.

Transport: half-duplex.

1
6
Ethernet 10BASE-T Cabling
Pin 1 Pin 1
Pin 2 Pin 2
Pin 3 Pin 3
Pin 4 Pin 4
Pin 5 Pin 5
Pin 6 Pin 6
Pin 7 Pin 7
Pin 8 Pin 8
Pin 1 Pin 3
Pin 2 Pin 6
Pin 3 Pin 1
Pin 4 Pin 4
Pin 5 Pin 5
Pin 6 Pin 2
Pin 7 Pin 7
Pin 8 Pin 8
Straight-through cable Crossover cable
1
7
10BASE-T Hubs

The hub repeats the
signal to all other
devices.
1
8
Reducing Collisions Through LAN

Switching

Switches do not create a single shared bus, like
a hub.

Switch treat each individual physical port as a
separate bus.

Switches use memory buffers to hold incoming
frames as well.

LAN switches with only one device cabled to
each port of the switch allow the use of full-
duplex operation.
1
9
4. Recent Ethernet Standards
2
0
Fast Ethernet

Two of the key features of Fast Ethernet,
as compared to 10-Mbps Ethernet, are
higher bandwidth and autonegotiation.

Medium: CAT5 UTP, fibre optic.

Transport: half or full-duplex.
2
1

Gigabit Ethernet

The most important similarity is that the
same Ethernet headers and trailers are
used, regardless of whether it’s 10 Mbps,
100 Mbps, or 1000 Mbps.

Gigabit Ethernet differs from the slower
Ethernet specifications in how it encodes
the signals onto the cable.
2
2
Summary
OSI Layer 2 Functions Ethernet Implementation
Arbitration CSMA/CD algorithm.
Addressing 6-bytes-long MAC address.
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