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Ethernet Essentials
Now that we understand the basic concepts of a LAN, let's explore Ethernet,
which is by far the most popular LAN standard.
Ethernet Origins
Robert Metcalfe and a number of his associates at the Xerox Palo Alto
Research Center (Xerox PARC) developed the LAN concept in the early 1970s. As
stated in the concept memorandum, it became clear to them that there was a
need to network terminals and minicomputers across the hall, just as it became
clear in the 1960s that there was a need to network terminals and mainframes
across the country.
That first LAN originally was known as the Altos Aloha Network, as it connected Altos
computers through a packet network based on the AlohaNet radio technology
developed some years before by the University of Hawaii. Metcalfe and his
associates later changed the name to Ethernet, taking the name from
luminiferous ether, which early scientists theorized was the conductive matter
that somehow magically supported the propagation of electromagnetic energy
through space.
Luminiferous ether doesn't exist, of course, but Ethernet was a clever name for a network
that, in its time, was truly incredible, if not magical. Xerox commercialized
the technology, renaming it The Xerox Wire.
Xerox, Digital Equipment Corporation and Intel later formed a joint venture that
standardized the technology at a raw data rate of 10 Mbps. In 1982, the IEEE
formalized the standard as 802.3, which is slightly different from true
Ethernet, even though we commonly refer to it as such. So, we'll use the term
Ethernet to refer to 802.3. It's easier to say and it's easier for me to type.
Ethernet Defined
Ethernet is a packet-based data network
designed to operate over relatively short distances, providing an aggregate
data rate 10 Mbps over a variety of physical media. Ethernet is a bus
technology involving a contentious access method that operates on a
decentralized basis. That's the short version, at least. Now let's deal with specifics.
Ethernet Evolution: Media Alternatives
Ethernet can, and does, run over just about any medium, including coaxial cable, twisted
pair, optical fiber and even radio.
Coaxial Cable:
The original Ethernet was commonly known as ThickNet, as it ran
over a thick coaxial cable. It also was known as 10Base5, which translates
into 10 Mbps, Baseband, with a maximum segment length of 500 meters.
10 Mbps is the raw aggregate data rate, which is shared among all devices connected to
the network. Baseband, as we discussed in the LAN Basics column, means that
one transmission at a time is supported. This original Ethernet could span a
total distance of up to 2,500 meters, and could comprise multiple physical
segments, each with a maximum length of 500 meters.
These maximum lengths are due to several factors. First, signal attenuation (i.e., loss of
signal strength) limits the length of a physical segment, even in the case of
a relatively low resistance coaxial cable.
The physical segments are interconnected via bridges, which, at least at the most basic
level function as digital signal repeaters, boosting signal strength and
filtering out any accumulated noise that might have distorted the signal. It
would be extremely unusual to find an old 10Base5 network still in place.
Second, the maximum length of 2,500 meters is due to issues of overall signal propagation
from end to end. Assuming that the Ethernet frame is of the maximum size of
1,518 octets, and that the transmit and receive devices are separated by the
full 2,500 meters, the transmit device will wait a predetermined length of
time before it sends the next frame of data.
That gives it enough time to receive a collision notification in the event that there was a
data collision somewhere along the way, and to retransmit the affected frame
before sending the next. We'll discuss collision notification in more detail later.
Variously dubbed ThinNet, Thin Ethernet and CheaperNet, the first media alternative was
10Base2, which translates into 10 Mbps, Baseband, with a maximum segment
length of 200 meters.
It uses a thinner and less expensive coaxial cable that offers more resistance to the
signal, which attenuates the signal to a greater extent, and which, therefore,
considerably reduces the maximum segment length. It would also be highly
unlikely to find an old 10Base2 network still in place.
Unshielded Twisted Pair (UTP): 10BaseT (T translates into Twisted pair) takes
advantage of the lower acquisition cost and much lower configuration (i.e.,
installation) cost of data grade UTP, as compared to coax.
Specifically, Category 3 (Cat 3) cable originally was used, as it can support a 10 Mbps
transmission over a distance up to 100 meters. More recently, Cat 5 has become
the medium of choice, since it performs much better and over longer distances.
(Cat 5 also will support 100 Mbps Ethernet and even Gigabit Ethernet, or GbE.)
Proper installation is critical, as UTP is highly susceptible to EMI (ElectroMagnetic
Interference), which can wreak havoc on a data transmission. 10BaseT, as you
may well know, has evolved into 100BaseT, which runs at an aggregate data rate of 100 Mbps.
10/100BaseT is everywhere. In any case, 10/100BaseT makes use of UTP to connect terminal
devices (e.g., workstations, servers and printers) to centralized hubs or switches.
Optical Fiber: Ethernet hubs, switches and routers oftentimes are interconnected
with optical fiber, especially at speeds of 100 Mbps or more and especially
over long distances greater than 100 meters.
Optical fiber costs a good deal more than UTP, but is considered to be well worth the extra
expense, at least in the backbone. Fiber-to-the-desktop is unusual at this
time, but undoubtedly will become commonplace in the future, as we seek to
satisfy our seemingly insatiable appetite for bandwidth.
Radio Frequency (RF): Wireless LANs generally are RF-based, as is the case with
the popular 802.11b and the more recent 802.11a standards. We'll discuss the
specifics of these standards in a future Web page.
Topology
Ethernet is a bus topology. In other words, it is designed around a single
electrical path that is shared by all connected devices. Transmission across
the bus is in both directions, and is digital in nature. Figure 1 illustrates
an original coax bus topology, which is both a physical bus and a logical bus.
Ethernet can also take the form of a physical star and a logical bus, as illustrated in Figure 2.
10/100BaseT, for example, has the physical appearance of a star, although it still operates as a
logical bus by virtue of the fact that a single shared electrical bus is collapsed
inside the 10/100BaseT hub.
Logically, the 10/100BaseT hub works just like a coax
segment, even though its physical appearance is that of a star.
Media Access Control (MAC)
Ethernet is based on a Media Access Control (MAC) scheme that is decentralized and
non-deterministic in nature.
Decentralized refers to the fact that there is no
central point of control. Rather, each Ethernet-attached device makes independent
decisions about when it is appropriate to access the network.
Non-deterministic refers to the fact that each device has absolutely no ability to
determine when, if ever, it will be able to accomplish access to the network.
Specifically, the MAC technique employed is that of
CSMA (
Carrier Sense Multiple
Access), of which there are two versions.
CSMA/CD (
Carrier Sense Multiple Access with Collision Detection) is the most
common implementation. Using this approach, each of the attached devices constantly senses
(i.e., monitors) the
carrier frequency (i.e., the frequency range that carries the data transmissions).
If the carrier frequency is clear, a device with data to transmit simply begins to transmit data a
frame at a time, and in both directions across the shared medium. The risk, of course, is that another
device located some distance away may access the network and begin to transmit about the same time.
In order to deal with this eventuality, each device
that detects a data collision transmits a collision notification on a sub-carrier
frequency (i.e., a frequency range lower than the carrier), which all devices also monitor.
If a transmitting device detects a collision
notification within a prescribed period of time, it simply backs off the network and
calculates a random number of milliseconds (i.e., thousandths of a second) before it
begins to sense the carrier again, access the network if clear, and retransmit the
affected frame.
A large geography (up to 2,500 meters) comprising a
number of segments (perhaps five or more) and supporting a large number of attached
devices (up to 1,024 per segment) and network-intensive users engaged in
bandwidth-intensive activities may incur a lot of collisions, backoffs and
retransmissions. That's why Ethernet advertises raw bandwidth of 10 Mbps, but may deliver
effective throughput of only 4 Mbps, for example.
CSMA/CA (
Carrier Sense Multiple Access with Collision Avoidance) is an
improvement over CSMA/CD, but it's more overhead-intensive and expensive. CSMA/CA requires
that each attached device broadcast a
Request To Send (
RTS) frame before
transmitting. If the RTS frame gets through, the destination device responds with a
Clear To Send (
CTS) frame.
All other devices on the network honor this
reservation, and the transmission ensues. CSMA/CA is much more reliable, but the RTS/CTS
frames consume some bandwidth, so this approach is somewhat overhead-intensive.
Also, the additional programmed logic makes CSMA/CA
somewhat more expensive. CSMA/CA is used in wireless Ethernet, since the allotted RF
spectrum is so limited between the shared access point and the client workstations that
collisions just aren't acceptable.
Segmentation
Up to this point, we have considered Ethernet to comprise a single
collision domain,
which was part of the original specification. Well, that no longer has to be the case.
Filtering bridges and hubs have the ability to read
the destination addresses on incoming frames, and to forward them only in the event that
the address is determined to be on another physical segment. Switches and routers
inherently have this ability.
Therefore, contemporary Ethernets are segmented into
multiple collision domains, which greatly reduces collisions and greatly improves
throughput.
TCP/IP
Despite these various mechanisms and such, Ethernet still gets congested, collisions
occur and throughput is affected. CSMA/CD works, but it's not very sophisticated. CSMA/CA
works, but it's bandwidth-intensive and expensive, and it doesn't scale well.
Segmentation helps a lot, but it's not a perfect
solution and it can get complicated in a large enterprise. So, we almost invariably run
TCP/IP inside of LAN frames.
If you'll refer to TCP/IP Essentials, you'll note
that TCP (Transmission Control Protocol) provides great reliability of datastream
transport. As TCP requires IP (Internet Protocol) as the underlying protocol, the use of
TCP/IP means 40octets of overhead, which eats up some bandwidth, but it's considered a
small price to pay for the increased reliability.
Note: If we're supporting realtime, compressed voice or video over the Ethernet, we
substitute UDP (User Datagram Protocol) for TCP, thereby reducing the overhead by 12
octets although sacrificing some reliability in the process.
Frame Structure
While I earlier described Ethernet as a packet-based network, it actually forwards
data in a frame format.
Frame is the term we use at the local link level, and LANs
run at Layer 1 (Physical Layer) and Layer 2 (Data Link Layer) of the OSI Reference Model.
Packet is the term we use at Layer 3 to describe a data unit in an internetworking
context. So, LANs, Frame Relay, T-carrier, HDLC, SLIP and PPP all forward frames of data.
IP forwards packets of data.
Ethernet frames are a minimum of 64 octets and a
maximum of 1518 octets, in total. Of that total, 18 octets are consumed by overhead in the
form of source and destination addresses (6 octets each), data type (2 octets) and error
control (4 octets).
That leaves between 46 and 1,500 octets of payload,
or actual user data. The smallest frames are used for applications like realtime voice and
video, since they can be formed quickly and transmitted frequently. The larger frames are
used for various data applications, as they are less overhead intensive and, therefore,
more efficient.
Network Interface Cards (NICs)
The interface between the attached device and the network is accomplished through the
NIC (
Network Interface Card), which is specifically tuned to the physical
medium involved (e.g., UTP, optical fiber or RF).
The NIC is also responsible for forming the frames
before they are placed on the network, and for taking frames addressed to it off of the
network. Each NIC, at least theoretically, has a unique MAC address.
Conclusion
Ethernet is not elegant or pretty. As a matter of fact, it's relatively simple and
pretty ugly. But it works. Its relative simplicity makes it easily understandable. And
with the help of filtering, segmentation, TCP/IP, switching and a few other tricks, it
works great, even in the most demanding environments. It's also relatively
inexpensive. Put it all together, and Ethernet is downright beautiful.
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