290 Networking Explained, Second Edition
LLC component. (Note: For completeness sake, the LLC sublayer also contains a source
service access point, SSAP, and a destination service access point, DSAP. These SAPs
provide the mechanism for source and destination nodes to communicate and are needed
for protocol-type identiﬁcation. See the Internet’s Request for Comment, RFC 1340, for
10. Are there any other differences?
Yes. Two other differences between V2.0 and 802.3 are topology and cable type. IEEE
802.3 supports both bus and star topologies, but Ethernet V2.0 supports only a bus topol-
ogy. Finally, 802.3 compliant networks can be either baseband or broadband, but in V2.0
only baseband Ethernet networks are supported.
11. Is IEEE 802.3 considered superior to Ethernet V2.0?
It depends on what you mean by superior. Rather than comment on whether one stan-
dard is better than another, we will say this: There is nothing intrinsically wrong with V2.0
other than it is a proprietary standard that does not comply with the prescribed ISO stan-
dard for Ethernet-like networks. Furthermore, although the two standards are similar—
802.3 was designed after V2.0—they are different enough to be considered incompatible.
This incompatibility resulted from political as well as technical issues and is best left to
historians and analysts.
12. So what’s the bottom line? Do I have to be concerned with V2.0?
The bottom line is that vendor support for IEEE 802.3 overshadows that for Ethernet
V2.0; any new “Ethernet” network you install probably will be based on the IEEE 802.3
protocol. Nevertheless, there are still many “old” Ethernets in existence and it is possible
that you might inherit one that incorporates both V2.0 and 802.3 compliant products.
13. How important is it to distinguish between V2.0 and 802.3 when discussing Ether-
In casual usage, IEEE 802.3 is commonly referred to as Ethernet. What you should
realize, though, is that technically it is not Ethernet—only V2.0 is considered Ethernet.
This is similar to the zero versus “oh” issue in mathematics when reading a number such
as 206. In casual, everyday usage we say “two-‘oh’-six.” Technically, though, the number
should be read as “two-zero-six” because zero is a number whereas “oh” is a letter. Never-
theless, in this book, to play it safe, we use the notation “Ethernet/802.3” when referring to
14. Let’s move on to the 802.3 protocol.
OK. At the physical layer (Chapters 4 and 6), the IEEE 802.3 standard addresses
issues such as cable type, cable length, and connector types. Ethernet/802.3’s physical
layer also encodes data prior to transmission using a technique called Manchester encod-
ing, whose purpose is to ensure that the end of a transmission (carrier-sense failure) is
properly detected. Manchester encoding differs from standard digital transmission in two
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