Ethernet/IEEE 802.3

These two terms actually refer to different things:
Ethernet
is a communication technology and
IEEE802.3
is a variety of Ethernet. Ethernet, in the more specific sense, is a
carrier sense, multiple
access/collision detection (CSMA/CD)
local area network. An Ethernet network uses these
attributes—carrier sense, multiple access, and collision detection—to enhance communication. This
definitely does
not
mean that Ethernet is the only technology that uses these attributes. In today’s
technical jargon, however, the term
Ethernet
is getting closer to meaning
all
CSMA/CD technologies.
Both Ethernet and IEEE 802.3 are broadcast networks. All frames that cross a given segment
can be heard by all machines populating that segment. Because all machines on the segment have
equal access to the physical media, each station tries to wait for a quiet spot before it transmits its
data. If two machines talk at the same time, a collision occurs.
Ethernet services both the Physical and Data Link layers, whereas IEEE 802.3 is more concerned
with the Physical layer and how it talks to the Data Link layer. Several IEEE 802.3 protocols
exist; each one has a distinct name that describes how it is different from other IEEE 802.3
protocols. Table 36.3 summarizes these differences.
TABLE 3 6 . 3
IEEE 802.3 Characteristics
802.3
Values
Data Rate
(Mbps)
Signaling
Method
Maximum
Segment
Length (m) Media Topology
10Base5 10 Baseband 500 50 Ohm coax Bus
10Base2 10 Baseband 185 50 Ohm coax Bus
1Base5 1 Baseband 185 Unshielded twisted pair Star
10BaseT 10 Baseband 100 Unshielded twisted pair Star
100BaseT 100 Baseband 100 Unshielded twisted pair Star
10Broad36 10 Broadband 1800 75 Ohm coax Bus
1000BaseT 1000 Baseband 100 Unshielded twisted pair Star

Table 36.3 is an excerpt from Cisco documentation; for the full document,
please see
www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/
ethernet.htm
.
In Table 36.3, you will notice that the terms baseband and broadband are used to describe
the signaling type. In a baseband transmission, only a single frequency is used for sending data,
and therefore only a single signal can be sent over the same media. A broadband signal multiplexes
multiple signals of different frequencies together on the same physical media.
Though not specifically called out in the table, there are four different IP encapsulation types
supported by Cisco for Ethernet: ARPA, SNAP, Novell-Ether, and SAP. Of these, ARPA is the
default encapsulation type used.

Layer 2: Data Link Layer

Protocols and Applications
This section is dedicated to layer 2 protocols and applications. It is a very important section
because it provides specific information on how the layer 2 protocols work. What better
way to be able to troubleshoot a problem than by understanding the intricacies of the protocol
in question?

This section covers the following layer 2 protocols:

Ethernet/IEEE 802.3

PPP

SDLC

Frame Relay

ISDN

Connectionless Protocols

Now that connection-oriented protocols have been discussed, we’ll move on to connectionless
protocols.
Connectionless protocols
differ from connection-oriented protocols because they do
not provide for flow control.
Figure 36.7 shows you how connectionless protocols work. This figure looks somewhat
like Figure 36.3, except that there are no steps that involve a connection setup or
termination. It is also missing the flow control and error control information sent by the
receiving system.
Connectionless protocols do not send data relative to any other data units. The data
included in the PDU must contain enough information for the PDU to get to its destination
and for the receiving system to properly process it. Because there is no established connection,
flow and error control cannot be implemented. Without flow and error control,
the originating system has no way of knowing whether all of the transmitted data was
received by the destination system without errors. Table 36.2 shows examples of connectionless
protocols.

Error Control

Error control
is responsible for checking each transmission and verifying that all of the PDUs
are contiguous and not erroneous. If there are missing or damaged PDUs, the destination will
not send an ACK packet for the previous transmission. (Refer to Figure 36.6.)
Once all of the data is transferred without errors, the originating system sends a termination
request, which tells the destination system that no more data needs to be transmitted. The destination
system then responds with a termination acknowledgment.
As you can see, both systems do a lot of communicating, aside from the exchange of data.
From the connection request to the termination acknowledgment, every exchange is accompanied
with control information that keeps the data transfer reliable and error free. Table 36.1
gives examples of several connection-oriented protocols. 1102

Flow Control

Although flow control was briefly described earlier, this section contains more detail.
Flow control
is responsible for ensuring that the transmitting station does not send data faster than the
receiving station can process it. This is done by establishing a window size for the transmission.
Look at Figure 36.6 to see how windowing works. Notice that the originating system sends
out a specified number of PDUs. Once that number is reached, the originating system waits for
a response from the destination system. After the response is received, the system continues to
transmit data.