1. 13.1
Wired LANs:
Ethernet
We learned that a local area network (LAN) is a computer
network that is designed for a limited geographic area such
as a building or a campus.
Although a LAN can be used as an isolated network to connect
computers in an organization for the sole purpose of sharing
resources, most LANs today are also linked to a wide area
network (WAN) or the Internet.
The LAN market has 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.
2. What is Ethernet?
Ethernet is a type of communication protocol that is created at Xerox PARC in 1973 by
Robert Metcalfe and others, which connects computers on a network over a wired
connection. It is a widely used LAN protocol, which is also known as Alto Aloha Network.
It connects computers within the local area network and wide area network. Numerous
devices like printers and laptops can be connected by LAN and WAN within buildings,
homes, and even small neighborhoods.
3. 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
13.3
4. 13.4
The relationship of the 802 Standard to the traditional
OSI (open source interconnection )model is shown in
Figure 13.1. The IEEE has subdivided the data link
layer into two sublayers: logical link control (LLC)
and media access control (MAC). IEEE has also
created several physicallayer standards for different
LAN protocols.
6. 13.6
Data Link Layer:
As we mentioned before, the data link layer in the IEEE
standard is divided into two sublayers: LLC and MAC.
Logical Link Control (LLC) In Chapter 11, we discussed data
link control. We said that data link control handles framing,
flow control, and error control. In IEEE Project 802, flow
control, error control, and part of the framing duties are
collected into one sublayer called the logical link control.
Framing is handled in both the LLC sublayer and the MAC
sublayer.
The LLC provides one single data link control protocol for
all IEEE LANs. In this way, the LLC is different from the
media access control sublayer MAC, which provides
different protocols for different LANs
7. 13.7
The purpose of the LLC is to provide flow and error control for
the upper-layer protocols that actually demand these
services. Media Access Control (MAC) In Chapter 12, we
discussed multiple access methods including random access,
controlled
access, and channelization. IEEE Project 802 has created a
sublayer called media access control that defines the specific
access method for each LAN.
For example, it defines CSMA/CD as the media access method
for Ethernet LANs and the token passing method for Token
Ring and Token Bus LANs. As we discussed in the previous
section, part of the framing function is also handled by the
MAC layer.
In contrast to the LLC sublayer, the MAC sublayer contains a
number of distinct modules; each defines the access method
and the framing format specific to the corresponding LAN
protocol.
8. 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
13.8
10. 13.10
MAC Sublayer
In Standard Ethernet, the MAC sublayer governs the operation
of the access method. It also frames data received from the
upper layer and passes them to the physical layer.
12. 13.12
Frame
Format
Preamble. The first field of the 802.3 frame contains 7 bytes
(56 bits) of alternating Os and 1s that alerts the receiving
system to the coming frame and enables it to synchronize its
input timing. The pattern provides only an alert and a timing
pulse. The 56-bit pattern allows the stations to miss some
bits at the beginning of the frame. The preamble is actually
added at the physical layer and is not (formally) part of the
frame.
Start frame delimiter (SFD). The second field (l byte:
10101011) signals the beginning of the frame. The SFD warns
the station or stations that this is the last chance for
synchronization. The last 2 bits is 11 and alerts the receiver
that the next field is the destination address.
13. 13.13
Destination address (DA). The DA field is 6 bytes and contains the
physical address of the destination station or stations to receive the
packet. We will discuss addressing shortly.
Source address (SA). The SA field is also 6 bytes and contains the
physical address of the sender of the packet. We will discuss
addressing shortly.
Length or type. This field is defined as a type field or length
field. The original Ethernet used this field as the type field to define
the upper-layer protocol using the MAC frame. The IEEE standard
used it as the length field to define the number of bytes in the data
field. Both uses are common today.
Data. This field carries data encapsulated from the upper-layer
protocols. It is a minimum of 46 and a maximum of 1500 bytes, as
we will see later.
CRC. The last field contains error detection information, in this
case a CRC-32
16. 13.16
Addressing
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. As shown in Figure 13.6, the
Ethernet address is 6 bytes (48 bits), nonnally written in
hexadecimal notation, with a colon between the bytes.
Unicast, Multicast, and Broadcast Addresses A source
address is always a unicast address-the frame comes from
only one station. The destination address, however, can be
unicast, multicast, or broadcast. Figure 13.7 shows how to
distinguish a unicast address from a multicast address. If the
least significant bit of the first byte in a destination address is
0, the address is unicast; otherwise, it is multicast.
19. 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.
Note
13.19
20. The broadcast destination address is a
special case of the multicast address in
which all bits are 1s.
Note
13.20
21. Define the type of the following destination
addresses: b.
47:20:1B:2E:08:EE
a. 4A:30:10:21:10:1A
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.
Example
13.1
22. 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:
Example
13.2
13.22
23. 13.23
Physical
Layer
The Standard Ethernet defines several
physical layer implementations; four of
the most common, are shown in Figure
13.8.
25. 13.25
lOBase5: Thick Ethernet
The first implementation is called 10Base5, thick
Ethernet, or Thicknet. The nickname derives from
the size of the cable. lOBase5 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 shows a
schematic diagram of a 10Base5 implementation.
27. Baseband Transmission is a signaling technology that sends digital signals over
a single frequency as discrete electrical pulses. ... The baseband signal is
bidirectional so that a baseband system can both transmit and receive signals
simultaneously.
Broadband system use modulation techniques to reduce the effect of noise in the
environment. Broadband transmission employs multiple channel unidirectional
transmission using combination of phase and amplitude modulation.
Baseband is a digital signal is transmitted on the medium using one of the signal codes
like NRZ, RZ Manchester biphase-M code etc. is called baseband transmission.
28. Baseband transmission –
Digital signalling.
Frequency division multiplexing is not possible.
Baseband is bi-directional transmission.
Short distance signal travelling.
Entire bandwidth is for single signal transmission.
Example: Ethernet is using Basebands for LAN.
Broadband transmission –
Analog signalling.
Transmission of data is unidirectional.
Signal travelling distance is long.
Frequency division multiplexing possible.
Simultaneous transmission of multiple signals over different
frequencies.
Example : Used to transmit cable TV to premises.
29. 13.29
10Base2: Thin Ethernet
The second implementation is called lOBase2, thin Ethernet,
or Cheapernet. 1OBase2 also uses a bus topology, but the
cable is much thinner and more flexible. The cable can be bent
to pass very close to the stations. In this case, the transceiver
is normally part of the network interface card (NIC), which is
installed inside the station. Figure 13.11 shows the schematic
diagram of a IOBase2 implementation.
Note This implementation is more cost effective than
10Base5 because thin coaxial cable is less expensive than
thick coaxial and the tee connections are much cheaper than
taps. Installation is simpler because the thin coaxial cable is
very flexible. However, the length of each segment cannot
exceed 185 m (close to 200 m) due to the high level of
thin coaxial cable.
31. 13.31
lOBase-T: Twisted-Pair Ethernet
The third implementation is called lOBase-T or twisted-pair
Ethernet. 1OBase-T uses a physical star topology. The stations
are connected to a hub via two pairs of twisted cable, as
shown in Figure 13.12.
Note that two pairs of twisted cable create two paths (one
for sending and one for receiving) between the station and
the hub. Any collision here happens in the hub.
The maximum length of the twisted cable here is defined
as 100 m, to minimize the effect of attenuation in the
twisted cable.
33. 13.33
lOBase-F: Fiber Ethernet
Although there are several types of optical fiber lO
Mbps Ethernet, the most common is called 10Base-F.
lOBase-F uses a star topology to connect stations to
a hub. The stations are connected to the hub using
two fiber-optic cables, as shown in Figure 13.13.
36. 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
13.36
37. 13.37
Bridged Ethernet
The first step in the Ethernet evolution was the division of
a LAN by bridges.
Bridges have two effects on an Ethernet LAN: They raise
the bandwidth and they separate collision domains. In an
unbridged Ethernet network, the total capacity (10 Mbps) is
shared among all stations with a frame to send; the stations
share the bandwidth of the network.
If only one station has frames to send, it benefits from the
total capacity (10 Mbps). But if more than one station needs
to use the network, the capacity is shared. We can say that, in
this case, each station on average, sends at a rate of 5 Mbps.
38. 13.38
A bridge divides the network into two or more networks.
Bandwidth-wise, each network is independent. For
example, in Figure 13.15, a network with 12 stations is
divided into two networks, each with 6 stations.
Now each network has a capacity of 10 Mbps. The lO-
Mbps capacity in each segment is now shared between 6
stations (actually 7 because the bridge acts as a station in
each segment), not 12 stations.
40. 13.40
Another advantage of a bridge is the
separation of the collision domain. Figure
13.16 shows the collision domains for an
unbridged and a bridged network. You can see
that the collision domain becomes much
smaller and the probability of collision is
reduced .
42. 13.42
Switched Ethernet
The idea of a bridged LAN can be extended to a
switched LAN. Instead of having two to four
networks, why not have N networks, where N is the
number of stations on the LAN? In other words, if
we can have a multiple-port bridge, why not have
an N- port switch? In this way, the bandwidth is
shared only between the station and the switch (5
Mbps each). In addition, the collision domain is
divided into N domains.
44. 13.44
Full-Duplex Ethernet
One of the limitations of 10Base5 and lOBase2 is that
communication is half-duplex (lOBase-T is always full-
duplex); a station can either send or receive, but may not
do both at the same time.
The next step in the evolution was to move from switched
Ethernet to full-duplex switched Ethernet. The full-duplex
mode increases the capacity of each domain from 10 to 20
Mbps. Figure 13.18 shows a switched Ethernet in full-duplex
mode.
Note that instead of using one link between the station and
the switch, the configuration uses two links: one to transmit
and one to receive.
46. 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
13.46
51. 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
13.51
52. In the full-duplex mode of Gigabit
Ethernet, there is no collision;
the maximum length of the cable is
determined by the signal
attenuation in the cable.
Note
13.52
