Übersicht Ethernet Typen Seite 2
FAQ Ethernet Seite 4
Multicast und Broadcast Adressen Seite 20
Herstellerkennungen Seite 23
Standard Ethernet Koaxialkabel, „Thick Cable“, Bus-Topologie
Ein Segment darf maximal 500m lang sein. Pro Segment können über MAU‘s (Medium Access Unit) 100 Stationen angeschlossen werden, deren Abstand am Kabel mindestens 2,5m betragen muß. Es sind zusätzlich maximal vier Repeater möglich, womit die Gesamtlänge eines 10base5-Netzes höchstens 2500m beträgt.
Flexibles Ethernet Koaxialkabel, „Thin Cable“, Bus-Topologie
Auch als Cheapernet bezeichnetes Ethernet mit dünnem Koaxialkabel. Ein Segment darf maximal 185m lang sein. Pro Segment dürfen 30 Stationen angeschlossen werden. Die MAU’s befinden sich normalerweise auf der Netzwerkkarte, so daß diese dann über T-Stücke angeschlossen werden. Die Leitungsenden müssen mit einem 50 Widerstand abgeschlossen werden, um Reflexionen zu verhindern. Es sind wiederum vier Repeater zulässig, somit ist eine maximale Länge von 925m möglich.
10baseT – UTP
Paarweise verdrillte 4-Draht-Leitung, nicht geschirmt ,UTP (unshilded twisted pair), Stern-Topologie
Hier ist der Einsatz eines Hub (Sternverteiler) notwendig. Jede Station wird über ein eigenes Kabel, welches max. 100m lang sein darf, an den Hub angeschlossen.
10baseT – STP
Paarweise verdrillte 4-Draht-Leitung, abgeschirmt, STP (shilded twisted pair), Stern-Topologie
Wie 10baseT – UTP, aber das Kabel ist abgeschirmt.
Lichtwellenleiter (LWL), Stern-Topologie
Die maximale Länge eines Segments beträgt ca. 2000m . Es können sowohl die Multimode- oder die Monomode Faser verwendet werden. Beim Einsatz von Monomode Faser kann die Segmentlänge bis zu 25km betragen.
modulierte Breitbandübertragung, Koaxialkabel, Bus-Topologie
Der Abstand zwischen zwei Stationen darf max. 3600m betragen.
„StarLan“ 1MByte Punkt-zu-Punkt-Verbindung, Stern-Topologie
Die Kabellänge zwischen zwei Stationen darf 250m nicht überschreiten.
100MBit/s Ethernet über vier verdrillte Paare (UTP), Stern-Topologie
Die Kabellänge zwischen Hub und Station beträgt maximal 100m. Die Datenübertragung erfolgt auf vier Leitungspaaren mind. der Kabelkategorie 3 (d.h. für Datenübertragungen bis 10MBit/s). Diese Kabel sind günstiger als die für 100baseTX notwendigen, dafür sind aber Hubs und Adapter schwerer zu bekommen.
100baseTX – UTP
100MBit/s Ethernet über zwei verdrillte Paare, UTP, Stern-Topologie
Die Kabellänge zwischen Hub und Station beträgt maximal 100m. Die Datenübertragung erfolgt auf zwei Leitungspaaren der Kabelkategorie 5 (d.h. Übertragungsraten bis 100MBit/s). 100baseTX ist der weitverbreitetste Fast-Ethernet-Typ.
100baseTX – STP
100MBit/s Ethernet über zwei verdrillte Paare, abgeschirmt (STP), Stern-Topologie
Wie 100baseTX – UTP, nur abgeschirmt.
100MBit/s über Glasfaserverkabelung, Stern-Topologie
Die maximale Länge zwischen Hub und Station beträgt 400m. Beim Einsatz von Monomode Faser kann die Segmentlänge bis zu 25km betragen.
1000MBit/s Ethernet über UTP, Stern-Topologie
Die Kabellänge zwischen Hub und Station beträgt maximal 100m. Es müssen Kabel der Kategorie 5 eingesetzt werden.
1000MBit/s Ethernet über STP, Stern-Topologie
Die Kabellänge zwischen Hub und Station beträgt maximal 25m.
1000MBit/s Ethernet über Multimode Glasfaser, Sterntopologie
Die Kabellänge zwischen Hub und Station beträgt maximal 550m.
1000MBit/s Ethernet über Monomode Glasfaser, Sterntopologie
Die Kabellänge zwischen Hub und Station beträgt maximal 5km.
02: General information about Ethernet and standards.
03: Ethernet Cabling Information.
04: Ethernet Devices and Components.
05: Errors and Related Terms.
06: Testing and Troubleshooting.
07: Additional Information.
02.07Q: What is the difference between an Ethernet frame and a IEEE802.3
frame? Why are there two types? Why is there a difference?
A: Ethernet was invented at Xerox Palo Alto Research Center and later
became an international standard. IEEE handled making it a
standard; and their specifications are slightly different from the
original Xerox ones. Hence, two different types. 802.3 uses the
802.2 LLC to distinguish among multiple clients, and has a "LENGTH"
field where Ethernet has a 2-byte "TYPE" field to distinguish among
multiple client protocols.
TCP/IP and DECnet (and others) use Ethernet_II framing, which is
that which Xerox/PARC originated.
02.08Q: What is SNAP
A: Sub-Network Access Protocol, an extention to the original 802.2
data link level format. (SNAP is described in IEEE 802-1990) The
802.2 data link format replaced the Ethernet Protocol Type concept
with two 8 bit fields; Source SAP, and Destination SAP.
Unfortunately that causes problems with migration of protocols, and
the lack of SAP space that is available. So one SAP as allocated
for this scheme which greatly expands the available protocol space.
When using the SNAP SAP the first 5 bytes of data are used as a
protocol ID. The first 3 bytes should be a value allocated to you
as a vendor id, the same as you get for Source address values. The
is called the OUI (Organizationally Unique ID) The second 2 bytes
is a protocol type.
Note that this is 802.2 and applies across all 802 LAN media types.
For translation bridging, there is a convention, if you set the OUI
to zero, you are representing a mapped Ethernet frame. So that a
bridge will translate such a frame back into Ethernet format, and
not into an 802.3 frame format.
802.2 SNAP frame:
| MAC | DSAP | SSAP | UI | OUI | Type | data |
| Header| 0xAA | 0xAA | 0x03 | 3bytes|2bytes| |
This will appear the same on all 802 compliant LAN media. On
802.3, there will be a Length field between the SA and the DSAP but
not on 802.5 or FDDI.
02.09Q: Where can I find out which Protocols use which Ethernet type
A: Look at IETF RFC-1700 - Assigned Numbers RFC.
02.10Q: What is a MAC address?
A: It is the unique hexadecimal serial number assigned to each Ether-
net network device to identify it on the network. With Ethernet
devices (as with most other network types), this address is
permanently set at the time of manufacturer, though it can usually
be changed through software (though this is generally a Very Bad
Thing to do).
02.11Q: Why must the MAC address to be unique?
A: Each card has a unique MAC address, so that it will be able to
exclusively grab packets off the wire meant for it. If MAC
addresses are not unique, there is no way to distinguish between
two stations. Devices on the network watch network traffic and
look for their own MAC address in each packet to determine whether
they should decode it or not. Special circumstances exist for
broadcasting to every device.
02.12Q: Is there a special numbering scheme for MAC addresses?
A: The MAC addresses are exactly 6 bytes in length, and are usually
written in hexadecimal as 12:34:56:78:90:AB (the colons may be
omitted, but generally make the address more readable). Each
manufacturer of Ethernet devices applies for a certain range of MAC
addresses they can use. The first three bytes of the address
determine the manufacturer. RFC-1700 (available via FTP) lists
some of the manufacturer-assigned MAC addresses. A more up-to-date
listing of vendor MAC address assignments is available on
ftp.lcs.mit.edu in pub/map/Ethernet-codes.
02.13Q: What is a preamble ?
A: A seven octet field of alternating one and zero binary bits sent
prior to each frame to allow the PLS circuitry to reach its steady
state synchronization with received frame timing. (802.3 standard,
02.14Q: What is a Start Frame Delimiter (SFD)?
A: A binary sequence of '10101011' immediately following the preamble
and indicating the beginning of a frame. (802.3 standard, page
02.15Q: What does CRC mean?
A: Cyclical Redundancy Check - A method of detecting errors in a
message by performing a mathematical calculation on the bits in the
message and then sending the results of the calculation along with
the message. The receiving work-station performs the same
calculation on the message data as it receives it and then checks
the results against those transmitted at the end of the message.
If the results don't match, the receiving end asks the sending end
to send again.
02.13Q: What is a broadcast address?
A: The unique address that identifies a packet as appropriate to all
receiveing stations. In 802.3 any address in which the second byte
is an odd number. (1,3,...F).
02.14Q: What exactly do 10Base5, 10BaseT, 10Base2, 10Broad36, etc mean?
A: These are the IEEE names for the different physical types of
Ethernet. The "10" stands for signalling speed: 10MHz. "Base"
means Baseband, "broad" means broadband. Initially, the last
section as intended to indicate the maximum length of an unrepeated
cable segment in hundreds of meters. This convention was modified
with the introduction of 10BaseT, where the T means twisted pair,
and 10BaseF where the F means fiber (see the following Q&A for
specifics). This actually comes from the IEEE committee number for
In actual practice:
10Base2 Is 10MHz Ethernet running over thin, 50 Ohm baseband
coaxial cable. 10Base2 is also commonly referred to
as thin-Ethernet or Cheapernet.
10Base5 Is 10MHz Ethernet running over standard (thick) 50
Ohm baseband coaxial cabling.
10BaseF Is 10MHz Ethernet running over fiber-optic cabling.
10BaseT Is 10MHz Ethernet running over unshielded, twisted-
10Broad36 Is 10MHz Ethernet running through a broadband cable.
02.15Q: What does FOIRL mean?
A: Fiber Optic Inter Repeater Link. A "IEEE 802 standard" worked out
between many vendors some time ago for carrying Ethernet signals
across long distances via fiber optic cable. It has since been
adapted to other applications besides connecting segments via
repeaters (you can get FOIRL cards for PCs). It has been
superseded by the larger 10BaseF standard.
02.16Q: What is LattisNet?
A: LattisNet is a pre-10BaseT quasi-standard for running Ethernet over
twisted-pair cabling. It was developed by Synoptics, and several
other vendors made compatible equipment for a while. LattisNet is
not compatible with 10BaseT, but you can have LattisNet hubs and
10BaseT hubs in the same hub chassis or connected to the same
network backbone. The primary difference is that 10BaseT synchron-
izes the signals at the sending end, while LattisNet syncrhonizes
at the receiving end.
02.17Q: What is StarLAN-10?
A: StarLAN-10 is AT&T's variety of Ethernet over twisted-pair cabling.
Older StarLAN-10 is not 100% 10BaseT compliant, as it does not
provide link integrity to the AUI. However, many 10BaseT
interfaces can be configured to work with StarLAN-10 hubs,
alongside StarLAN-10 NICs. Beware, though, that the original
StarLAN-10 is NOT in any way compatible with 10BaseT, and worse,
there seems to be no way to tell other than trying it to see what
The current StarLAN products supported by AT&T/NCR are fully 802.3
compliant. This includes the SmartHUB model E, SmartHUB model B,
SmartHUB XE, and the other fiber and wire SmartHUB models.
03.01Q: What is coax?
A: Coaxial cable (coax) is a metallic electrical cable used for RF
(radio frequency) and certain data communications transmission.
The cable is constructed with a single solid or stranded center
conductor that is surrounded by the dielectric layer, an insulating
material of constant thickness and high resistance. A conducting
layer of aluminum foil, metallic braid or a combination of the two
encompass the dielectric and act as both a shield against
interference (to or from the center conductor) and as the return
ground for the cable. Finally, an overall insulating layer forms
the outer jacket of the cable. Coaxial cable is generally
superior in high-frequency applications such as networking.
However, for shorter distances (up to 100 meters), UTP or STP cable
is generally just as reliable when using differential modulation
techniques (such as with 10BaseT).
There are three types of RG-58 cable, as far as I can tell. There
are probably other subtle differences, but for 10BASE2, impedance
and velocity of propagation are the important ones. The table
Cable Impedance Velocity
---------- ---------- --------------
RG-58A/U 50 ohms .66 or .78
RG-58C/U 50 ohms .66
RG-58/U 53.5 ohms .66 or .695
03.02Q: What is UTP, STP?
A: Twisted pair cables. UTP is for UNshielded, twisted pair, while
STP is for SHIELDED, twisted pair. UTP is what's typically
installed by phone companies (though this is often not of high
enough quality for high- speed network use) and is what 10BaseT
Ethernet runs over. UTP is graded according to its data carrying
ability (e.g., Level 3, Level 4, Level 5). 10BaseT Ethernet
requires at least Level 3 cable. Many sites now install only
Level-5 UTP, even though level 4 is more than sufficient for
10BaseT, because of the greater likelihood that emerging high-speed
standards will require cable with better bandwidth capabilities.
STP is typically used for Token-Ring networks, where it is commonly
referred to IBM Type 1 (or 2, 3, 6, 8, etc); however there are
several manufacturers of Ethernet equipment and interfaces that
support Ethernet over STP. Nevertheless, Ethernet over STP is not
officially defined in any standards. While there is a good level
of interoperability with Ethernet over STP, (Lattisnet, developed
by Synoptics, is the recognized de facto standard in this area),
one should consider the long-term availability and cost of this
non-standard scheme before planning new networks around it.
03.03Q: Are there any restrictions on how Ethernet is cabled?
A: Yes, there are many, and they vary according to the media used.
First of all, there are distance limitations:
10Base2 limited to 185 meters (607 ft) per unrepeated cable
10Base5 limited to 500 meters (1,640 ft) per unrepeated cable
10BaseF depends on the signaling technology and medium used
but can go up to 2KM.
10BaseT generally accepted to have a maximum run of 100-150M,
but is really based on signal loss in Db's (11.5db
maximum loss source to destination).
10Broad36 limited to 3,600 meters (almost 2.25 miles).
Then there are limitations on the number of repeaters and cable
segments allowed between any two stations on the network. There
are two different ways of looking at the same rules:
1. The Ethernet way:
A remote repeater pair (with an intermediate point-to-point
link) is counted as a single repeater (IEEE calls it two
repeaters). You cannot put any stations on the point to point
link (by definition!), and there can be two repeaters in the
path between any pair of stations. This seems simpler to me
than the IEEE terminology, and is equivalent.
2. The IEEE way:
There may be no more than five (5) repeated segments, nor more
than four (4) repeaters between any two Ethernet stations; and
of the five cable segments, only three (3) may be populated.
This is referred to as the "5-4-3" rule (5 segments, 4
repeaters, 3 populated segments).
It can really get messy when you start cascading through 10BaseT
hubs, which are repeaters unto themselves. Just try to remember,
that any possible path between two network devices on an
unbridged/unrouted network cannot pass through more than 4
repeaters or hubs, nor more than 3 populated cable segments.
Finally, 10Base2 is limited to a maximum of 30 network devices per
unrepeated network segment with a minimum distance of 0.5m (1.5ft)
between T-connectors. 10Base5 is limited to a maximum of 100
network devices per unrepeated segment, with a minimum distance of
2.5m (8.2ft) between taps/T's (usually indicated by a marker
stamped on the cable itself every 2.5m). 10BaseT and 10BaseF are
star-wired, so there is no minimum distance requirement between
devices, since devices cannot be connected serially. You can
install up to the Ethernet maximum of 1024 stations per network
with both 10BaseT and 10BaseF.
03.04Q: Can I mix 10Base2 and 10Base5 cabling on a single segment?
A: It is not "legal", but the network police will not read you your
rights and drag you away. Ideally, you should use a repeater (or
bridge, router, etc...) between the different cabling types.
However, in reality, it will work fine, as long as none of the
other network parameters (lengths, numbers of stations, repeaters,
etc) are near the limit of the specification.
03.05Q: What about wireless Ethernets? Are there any?
A: Yes, and no. Many vendors offer equipment for Ethernet across a
variety of unbounded, or wireless, connections using lasers,
microwaves, and spread-spectrum radio transmissions. However, none
of these methods are organized by any standards body, so it is
unlikely to find equipment from any two different manufacturers
that work together.
03.06Q: When should I choose 10BaseT, when 10Base2 (or others)?
A: The specific environment and application must be considered when
selecting your media type. However, there are some general
rules-of-thumb that you can consider:
Avoid using copper between buildings. The electrical disturbances
caused by lightning, as well as naturally occurring differences in
ground potential over distance, can very quickly and easily cause
considerable damage to equipment and people. The use of
fiber-optic cabling between buildings eliminates network cabling as
a safety risk. There are also various wireless media available for
inter-building links, such as laser, spread-spectrum RF and
microwave. However, wireless media is much more expensive and less
reliable than fiber-optic, and should only be considered when it is
impossible to get right-of-way for fiber-optic cable.
10Base2 (thin Ethernet or Cheapernet) is the least expensive way to
cable an Ethernet network. However, the price difference between
10Base2 and 10BaseT (Ethernet over UTP) is rapidly diminishing.
Still, for small, budget-conscious installations, 10Base2 is the
most economical topology. The disadvantages of 10Base2 is that any
break in the cable or poor connection will bring the entire network
down, and you need repeaters if you have more than 30 devices
connected to the network or the cable length exceeds 185 meters
10Base5 is generally used as a low-cost alternative to fiber-optic
media for use as a backbone segment within a single building. It's
extended length (500m or 1640ft), higher attached device count
(100) and better noise resistance make 10Base5 well suited for use
as a network trunk for one or more floors in a building. However,
the high cost of connecting each device (in addition to the
interface, you also need an external transceiver, or MAU, and an
AUI cable) makes 10Base5 too expensive for most LAN installations,
and like 10Base2, a single break or bad connection in the cable can
bring the entire network down.
10BaseT is the most flexible topology for LANs, and is generally
the best choice for most network installations. 10BaseT hubs, or
multi-hub concentrators, are typically installed in a central
location to the user community, and inexpensive UTP cabling is run
to each network device (which may be 100m, or 330ft, from the hub).
The signalling technology is very reliable, even in somewhat noisy
environments, and 10BaseT hubs will usually detect many network
error conditions and automatically shut-down the offending port(s)
without affecting the rest of the network (unless, of course, the
offending port was your server, shared printer, or router to the
rest of the world). While the hardware is more expensive than
10Base2, the cabling is cheaper and requires less skill to install,
making 10BaseT installation costs only slightly higher than
10Base2. The flexibility and reliability more than offset the
marginally higher price.
10BaseF, and its predecessor, FOIRL, are the only recommended
topologies for inter-building links. However, they need not be
limited to this role. 10BaseF can also be run to the desktop,
though the cost is prohibitively high in all but the most
0 specialized environments (generally, extremely noisy manufacturing
facilities, or very security-conscious installations). More
commonly, FOIRL (and now, 10BaseF) is used inside buildings to form
backbone networks and to connect wiring closets together.
03.07Q: What are the advantages/disadvantages of a star like cabling?
A: Old style Ethernet bus wiring (ie, taking the cable from one
machine to the next, and then to the next, etc) is prone to cable
failure and quickly consumes allowed distances due to aesthetic
wiring needs. If the wiring connection is broken at any point, the
entire network (segment) fails - and the much greater number of
connections increases the probability of a failure or break. On the
other hand, it's pretty easy to do for a layman and may involve
less actual wiring for small segments.
Star wiring eliminates the single point of failure of a common
wire. A central hub has many connections that radiate out to
hosts, if one of these hosts connections fails it usually doesn't
affect the others. Obviously, however, the hub becomes a central
point of failure itself, but studies show a quality hub is less
likely to fail before a heavily used strand of coax.
There are a bunch of other reasons hubs are desirable, but this is