Transcript Ethernet

Chapter 13
Wired LANs: Ethernet
13.1
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
LAN



13.2
LAN is a computer network that is
designed for a limited geographic area
such as a building or a campus.
It can be used as an isolated network to
connect computers in an organization for
the sole purpose of sharing resources.
LANs are also linked to a WAN or the
Internet.
LAN Technology


13.3
The LAN market has seen several
technologies such as Ethernet, Token
Ring, Token Bus, FDDI, and ATM LAN.
Some of these technologies survived for a
while, but Ethernet is by far the dominant
technology.
13-1 IEEE STANDARDS
In 1985, the Computer Society of the IEEE started a
project, called Project 802, to set standards to enable
intercommunication among equipment from a variety
of manufacturers. Project 802 is a way of specifying
functions of the physical layer and the data link layer
of major LAN protocols.
Topics discussed in this section:
Data Link Layer
Physical Layer
13.4
Relation of 802 Standard and
Traditional OSI model


13.5
The IEEE has subdivided the data-link
layer into two sub-layers: logical link
control (LLC) and Media Access Control
(MAC).
IEEE has also created several physical
layer standards for different LAN
protocols.
Figure 13.1 IEEE standard for LANs
13.6
Logical Link Layer





13.7
In IEEE Project 802, flow control, error control, and part
of the framing duties are collected into one sub-layer
called the logical link control.
Framing is handled in both the LLC sub-layer and the
MAC sub-layer.
The LLC provides one single data link control protocol for
all IEEE LANs.
MAC sub-layer provides different protocols for different
LANs.
A single LLC protocol can provide interconnectivity
between different LANs because it makes the MAC sublayer transparent.
Framing




13.8
LLC defines a protocol data unit (PDU)
that is somewhat similar to that of HDLC.
The header contains a control field like the
one in HDLC; this field is used for flow and
error control.
The two other header fields define the
upper-layer protocol at the source and
destination that uses LLC.
These fields are called the destination
service access point (DSAP) and the
source service access point (SSAP).
Figure 13.2 HDLC frame compared with LLC and MAC frames
13.9
Continued….


13.10
The other fields defined in a typical data
link control protocol such as HDLC are
moved to the MAC sub-layer.
In other words, a frame defined in HDLC
is divided into a PDU at the LLC sub-layer
and a frame at the MAC sub-layer.
Need for LLC


13.11
The purpose of the LLC is to provide flow
and error control for the upper-layer
protocols that actually demand these
services.
However, most upper-layer protocols such
as IP, do not use the services of LLC.
Media Access Control (MAC)


IEEE project 802 has created a sub-layer
called media access control that defines
access method (random access, controlled
access and channelization) for each LAN.
Example:


13.12
CSMA/CD for the Ethernet LANs
Token Passing for Token Ring and Token Bus
LANs.
Continued…..


13.13
A part of framing function is also handled
by the MAC layer.
In contrast to the LLC sub-layer, the MAC
sub-layer contains a number of distinct
modules; each defines the access method
and the framing format specific to the
corresponding LAN protocol.
Physical Layer



13.14
The physical layer is dependent on the
implementation and type of physical
media used.
IEEE defines detailed specifications for
each LAN implementation.
For example, although there is only one
MAC sub-layer for standard Ethernet,
there is different physical layer
specifications for each Ethernet
implementations (as we will see later).
13-2 STANDARD ETHERNET
The original Ethernet was created in 1976 at Xerox’s
Palo Alto Research Center (PARC). Since then, it has
gone through four generations. We briefly discuss the
Standard (or traditional) Ethernet in this section.
Topics discussed in this section:
MAC Sublayer
Physical Layer
13.15
Figure 13.3 Ethernet evolution through four generations
13.16
Standard Ethernet (MAC Sublayer)


13.17
In Standard Ethernet, the MAC sub-layer
governs the operation of the access
method.
It also frames data received from the
upper layer and passes them to the
Physical layer.
Frame Format



13.18
The Ethernet frame contains seven fields:
Preamble, SFD, DA, SA, length or type of
protocol data unit (PDU), upper-layer
data, and the CRC.
Ethernet does not provide any mechanism
for acknowledging received frames,
making it what is known as an unreliable
medium.
Acknowledgments must be implemented
at the higher layers.
Figure 13.4 802.3 MAC frame
13.19
Frame Length


13.20
Ethernet has imposed restrictions on both
the minimum and maximum lengths of a
frame.
The minimum length restriction is required
for the correct operation of CSMA/CD.
Figure 13.5 Minimum and maximum lengths
13.21
Note
Frame length:
Minimum: 64 bytes (512 bits)
Maximum: 1518 bytes (12,144 bits)
13.22
Addressing



13.23
Each station on an Ethernet network(such
as a PC, workstation, or printer) has its
own network interface card (NIC).
The NIC fits inside the station and
provides the station with a 6-byte physical
address.
Ethernet address is 6 bytes (48 bits),
normally written in hexadecimal notation,
with a colon between the bytes.
Figure 13.6 Example of an Ethernet address in hexadecimal notation
13.24
Unicast, Multicast, and
Broadcast Addresses


13.25
A source address is always a unicast
address, the frame comes from only one
station.
The destination can be unicast , multicast
or broadcast.
Figure 13.7 Unicast and multicast addresses
13.26
Note
The least significant bit of the first byte
defines the type of address.
If the bit is 0, the address is unicast;
otherwise, it is multicast.
13.27
Note
The broadcast destination address is a
special case of the multicast address in
which all bits are 1s.
13.28
Example 13.1
Define the type of the following destination addresses:
a. 4A:30:10:21:10:1A
b. 47:20:1B:2E:08:EE
c. FF:FF:FF:FF:FF:FF
Solution
To find the type of the address, we need to look at the
second hexadecimal digit from the left. If it is even, the
address is unicast. If it is odd, the address is multicast. If
all digits are F’s, the address is broadcast. Therefore, we
have the following:
a. This is a unicast address because A in binary is 1010.
b. This is a multicast address because 7 in binary is 0111.
c. This is a broadcast address because all digits are F’s.
13.29
Example 13.2
Show how the address 47:20:1B:2E:08:EE is sent out on
line.
Solution
The address is sent left-to-right, byte by byte; for each
byte, it is sent right-to-left, bit by bit, as shown below:
13.30
Figure 13.8 Categories of Standard Ethernet
13.31
10Base 5 : Thick Ethernet



13.32
First implementation.
Nick name derives from the size of the
cable, which is roughly the size of a
garden hose and too stiff to bend with
your hands.
10 base 5 was the first Ethernet
specification to use a bus topology with an
external transceiver (transmitter/receiver)
connected via a tap to a thick coaxial
cable.
Figure 13.10 10Base5 implementation
13.33
10 base 2: Thin Ethernet






13.34
Second Implementation
Cheaper-net
Uses bus topology
Cable is much thinner and more flexible.
The cable can be bent to pass very close
to the stations.
In this case, transceiver is normally part of
the network interface card (NIC), installed
inside the station.
Figure 13.11 10Base2 implementation
13.35
Continued…..




13.36
There is collision in thin coaxial.
Implementation is inexpensive, thin coax
is inexpensive than thick.
Tee connections are much cheaper than
taps.
Length of each segment must do not
exceed 185m due to high level of
attenuation in thin coax cable.
10 Base T: (Twisted Pair
Ethernet)






13.37
3rd implementation
Uses a physical star topology.
The stations are connected to a hub via
two pairs of twisted cable. (one for
sending and one for receiving)
Any collision here happens in the hub.
Compared to 10 base 5 or 10 base 2, the
hub actually replaces the coaxial cable.
The maximum length of the twisted cable
here is 100m.
Figure 13.12 10Base-T implementation
13.38
10 Base F: Fiber Ethernet


13.39
Uses a star topology to connect stations to
a hub.
The stations are connected to the hub
using two fiber optic cables.
Figure 13.13 10Base-F implementation
13.40
Table 13.1 Summary of Standard Ethernet implementations
13.41
13-3 CHANGES IN THE STANDARD
The 10-Mbps Standard Ethernet has gone through
several changes before moving to the higher data
rates. These changes actually opened the road to the
evolution of the Ethernet to become compatible with
other high-data-rate LANs.
Topics discussed in this section:
Bridged Ethernet
Switched Ethernet
Full-Duplex Ethernet
13.42
Figure 13.14 Sharing bandwidth
13.43
Figure 13.15 A network with and without a bridge
13.44
Figure 13.16 Collision domains in an unbridged network and a bridged network
13.45
Figure 13.17 Switched Ethernet
13.46
Figure 13.18 Full-duplex switched Ethernet
13.47
13-4 FAST ETHERNET
Fast Ethernet was designed to compete with LAN
protocols such as FDDI or Fiber Channel. IEEE
created Fast Ethernet under the name 802.3u. Fast
Ethernet is backward-compatible with Standard
Ethernet, but it can transmit data 10 times faster at a
rate of 100 Mbps.
Topics discussed in this section:
MAC Sublayer
Physical Layer
13.48
Continued…..

The goal of Fast Ethernet can be
summarized as follows:





13.49
Upgrade the data rate to 100 Mbps
Make it compatible with standard Ethernet
Keep the same 48-bit address
Keep the same frame format
Keep the same minimum and maximum frame
lengths
MAC Sublayer





13.50
A main consideration in the evolution of
Ethernet from 10 to 100 Mbps was to
keep the MAC sublayer untouched.
The bus topology was replaced with star
topology.
For the star topology there are two
topologies: half duplex and full duplex
In half duplex the stations are connected
via a HUB.
In full duplex the connection is made via a
switch with buffers at each port.
Continued…..



13.51
Access method for CSMA/CD is half duplex
For full duplex there is no need of
CSMA/CD.
However, the implementation keep
CSMA/CD for backward compatibility with
Standard Ethernet.
New Feature (autonegotiation)


Allows a station or a hub a range of
capabilities.
(devices can negotiate with one
another)Negotiation of mode and data
rate of operation.


13.52
To allow incompatible devices to connect one
another. e.g, a device with a maximum capacity of
10 Mbps can communicate with a device of
100Mbps capacity.
To allow a station to check a hub’s capabilities.
Physical Layer


In Fast Ethernet the physical layer is more
complicated than the one in standard
Ethernet.
Topology:


13.53
If there are two stations they can be
connected point to point.
Three or more stations need to be connected
in a star topology with a HUB or a SWITCH.
Figure 13.19 Fast Ethernet topology
13.54
Figure 13.20 Fast Ethernet implementations
13.55
Table 13.2 Summary of Fast Ethernet implementations
13.56
13-5 GIGABIT ETHERNET
The need for an even higher data rate resulted in the
design of the Gigabit Ethernet protocol (1000 Mbps).
The IEEE committee calls the standard 802.3z.
Topics discussed in this section:
MAC Sublayer
Physical Layer
Ten-Gigabit Ethernet
13.57
Continued….

The goal of the gigabit ethernet design is:






13.58
Upgrade the data rate to 1 Gbps
Make it compatible with Standard or Fast
Ethernet.
Use the same 48-bit address.
Use the same frame format
Keep the same minimum and maximum frame
lengths
To support autonegotiation as defined in Fast
Ethernet.
MAC Sublayer




13.59
Gigabit Ethernet has two kinds of
implementations:
Half duplex
Full duplex
But now almost all implementations of
gigabit ethernet follow the full duplex
approach. The half duplex is there to be
compatible with the previous generations.
Figure 13.22 Topologies of Gigabit Ethernet
13.60
Figure 13.23 Gigabit Ethernet implementations
13.61
Table 13.3 Summary of Gigabit Ethernet implementations
13.62
TEN-Gigabit Ethernet


The IEEE committee created ten-gigabit
ethernet and called it standard 802.3ae.
The goals of the ten-gigabit ethernet
design are:






13.63
Upgrade the data rate to 10 Gbps
Make it compatible with standard, fast, and gigabit
ethernet.
Use the same 48-bit address.
Use the same frame format.
Keep the same minimum and maximum frame
lengths.
Works with only fiber optics.
MAC Sublayer




13.64
Allow the interconnection of existing LANs into
MAN or WAN.
Make ethernet compatible with technologies such
as frame relay and ATM.
Ten-gigabit ethernet operates only in full
duplex mode which means there is no
need for contention.
CSMA/CD is not used in Ten-Gigabit
Ethernet.
Table 13.4 Summary of Ten-Gigabit Ethernet implementations
13.65