Gigabit Ethernet offers performance enhancement for existing
networks without having to change the cables, protocols and
applications already in use. But is ATM about to displace it?
The history
Ethernet was initially developed in the 1970s, and is now the most
widespread network technology in the world. As a result of the
standardisation of Gigabit Ethernet in June 1998, the scalability
of Ethernet was again significantly improved. With a bandwidth of
1000Mbit/s (1Gbit/s), Gigabit Ethernet is 100 times faster than the
original Ethernet. As Gigabit Ethernet uses the same packet format
and in half-duplex mode (shared media), also the CSMA/CD network
access method, the standard is compatible with Ethernet and Fast
Ethernet. From today's perspective, Gigabit Ethernet will be used
as the backbone technology in corporate networks. The technology is
further primarily suited to increasing the data transfer rate
between clients and server farms and to connecting Fast Ethernet
switches.One other area of application is linking workstations and
servers with very high bandwidth requirements, as in image editing
or CAD environments. It is to be assumed that Gigabit Ethernet will
be used above all in the more powerful full-duplex mode. Today,
Ethernet is synonymous with the IEEE 802.3 standard for a
1-persistent CSMA/CD LAN. The origin of the 802.3 standard can be
traced back to the ALOHA network at the University of Hawaii, the
forerunner of all shared-media networks. The original Ethernet
developed by Xerox was thus also based on the ALOHA system. This
first incarnation was a 2.94 Mbit/s CSMA/CD system which was used
to connect more than 100 workstations on a 1km-long cable. The
system was so successful that it was standardised by Xerox, DEC and
Intel in 1985 as IEEE 802.3 with a data rate of 10Mbit/s.In
Ethernet, network access by all stations is controlled by the
CSMA/CD method. When a station wishes to send data, it "listens" to
the line. If no other station is currently transmitting data, it
has the opportunity to transmit data itself. If two stations start
data transmission at the same time, this is recognised as a
collision. Both stations later try to repeat the process at
different times.Initially there were two types of coaxial cable
used, which were known as Thick Ethernet and Thin Ethernet. Later,
the main type of cable used was the unshielded twisted pair (UTP)
copper cable originating from the field of telecommunications. When
DEC, Intel and Xerox created what became known as the DIX Ethernet
standard in 1980, 10Mbit/s was an enormous bandwidth. As computer
technology progressed, however, the demand for more bandwidth in
the network grew constantly, leading to the Fast Ethernet standard
in the form of IEEE 802.3u in 1995. The standardisation of Fast
Ethernet was vigorously promoted by an economic consortium under
the name of the Fast Ethernet Alliance. As a result of the Fast
Ethernet standard, the conventional Ethernet achieved 10 times
greater bandwidth and other new features such as full-duplex
operation and auto-negotiation.Fast Ethernet was defined for
100Mbit/s and opened the way for the scalability of the original
Ethernet. Whereas traditional Ethernet networks operated in half
duplex mode, full-duplex Ethernet technology was added with the
introduction of Fast Ethernet. In full-duplex mode a station can
simultaneously transmit and receive data - something that was
previously impossible.In May 1996, shortly after the IEEE announced
the 802.3z Gigabit Ethernet standardisation project, 11 companies
founded the Gigabit Ethernet Alliance. At the last count more than
120 companies in the networking, computer and semiconductor
industries were members of this association.In July 1997, the IEEE
approved the 802.3z standard, currently the last in the series.
Final standardisation was achieved in June 1998. The proposal for
the 802.3z Gigabit Ethernet specification was submitted to the IEEE
802.3 committee in March 1996 for examination. This was followed in
May 1996 by the founding of the Gigabit Ethernet Alliance by 11
companies: 3Com, Bay Networks, Cisco, Compaq, Granite, Intel, LSI
Logic, Packet Engines, Sun, UB Networks and VLSI Technology.The
objective of the alliance was to create an open standard and
interoperable products. This meant:1) Developing extensions for
Ethernet and Fast Ethernet with a view to greater bandwidth.2)
Drawing up technical proposals which were suitable for
standardisation.3) Setting up processes and procedures for
interoperability tests.Today, more than 120 companies are united in
the Gigabit Ethernet Alliance. The physical basis of Gigabit
Ethernet is provided by well-proven technologies from the original
Ethernet and the ANSI X3T11 Fibre Channel specifications. The
physical layer of the Gigabit Ethernet standard 802.3z was adopted
from Fibre Channel, for example, and supports transmission via
multimode and single- node optical fibers as well as twinaxial
cables. The standard for twisted pair cabling is expected to be
finalised in late 1999; an independent project was set up for this
within IEEE 802.3ab. Gigabit Ethernet will therefore be based on
four physical media types, which are defined as 802.3z (1000Base-X)
and 802.3ab (1000Base-T).
Transmitting dataThe 1000Base-X
standard is based on the physical layer of Fibre Channel, a
technology for connecting workstations, supercomputers, storage
systems and peripheral devices. The architecture of Fibre Channel
consists of four layers; the two lower layers, FC-0 (interface and
medium) and FC-1 (encode/decode), are used in Gigabit
Ethernet.Ethernet uses a minimum packet size of 64 bytes. The
reason for introducing a minimum packet size was that a station has
to be capable of detecting a collision at the remote end of the
cable. The minimum length of time required to detect this collision
is called the slot time. Looked at the other way, the data that can
be transmitted within the slot time amounts to the slot size.The
maximum cable length with Ethernet is 2.5km with a maximum number
of four repeaters in the path. If the bit rate is increased, the
sending station transmits the packets faster. Given the same packet
size and cable length, this would mean that the packets would be
transmitted faster than a detected collision could be reported.In
order to obtain the same cable lengths, the slot time, and hence
the minimum packet size, would have to be increased. Alternatively,
to be able to use the same packet size, the maximum cable length
would have to be reduced. In Fast Ethernet, the maximum cable
length was reduced to approximately 205m, while the packet sizes
and the slot time were left unchanged.However, as Gigabit Ethernet
is 10 times faster than Fast Ethernet, the maximum cable length
would have to be reduced to less than 20m. Instead of that, the
slot size was increased to 512 bytes. To maintain the required
compatibility with the minimum packet size despite this increase,
it was decided to introduce what is known as the carrier extension.
This entails padding packets smaller than 512 bytes with symbolic
values. These symbols must not appear in the actual data area, and
are referred to as the carrier extension.
1) Carrier
extensionThe carrier extension was introduced to guarantee the
interoperability of Gigabit Ethernet with existing 802.3 Ethernet
networks. If packets are smaller than 512 bytes, extension symbols
are added to them until the size of 512 bytes is reached. In this
way the transmitter is in a position to detect any collisions that
occur with that packet. The extension symbols are removed from the
packet at the receiver before the packet checksum (Frame Check
Sequence - FCS) is calculated. The logical level (LLC - Logical
Link Control) is unaffected by the carrier extension.The use of a
carrier extension meant that a means was found to remain compatible
with the existing Ethernet packet sizes while retaining an
acceptable cable length. However, this approach is inefficient and
wastes bandwidth. A second technique, known as packet bursting,
aims to counteract this disadvantage.
2) Packet burstingWhen
using a carrier extension with small data packets, it is possible
that up to 448 additional bytes will be sent. The result would be a
data throughput only insignificantly larger than with Fast
Ethernet. Packet bursting was introduced to correct this
disadvantage, at least in part. If a station wishes to send several
packets, the first packet is filled according to the carrier
extension method. The subsequent packets can then be added with the
shortest possible spacing, the inter-packet gap (IPG). Packet
bursting is limited to 8Kb of data.Carrier extension and packet
bursting are only of relevance to half-duplex operation. Neither of
these methods are necessary for full-duplex operation, to which
considerably greater importance is attached.Full-duplex mode,
standardised in IEEE 802.3x, is an important element in the
deployment of Ethernet at gigabit speeds. It allows Ethernet
network access and the extension of the network without limitation
by the CSMA/CD procedure, which is subject to collisions. The
channel capacity can be fully exploited and total throughput is
increased.Full-duplex modeTo be able to utilise full-duplex mode, a
dedicated link is necessary between two nodes. Separate send and
return circuits are implemented for this, as well as two MAC
controllers, one at each end. The carrier sense and collision
detect functions are deactivated, and the loop-back function is
suppressed, which in half-duplex mode sends the received data back
out onto the transmit path. In this way, a station is able to
receive and transmit data simultaneously.The use of full-duplex
mode makes sense for switch-to-switch links, server and router
links, and links over long distances. Nowadays in the Ethernet and
Fast Ethernet field, even full-duplex links between workstations
are no longer a rarity.A full-duplex port provides buffer storage
on the input and output path in case the transmitting stations send
more data than the receiving stations can process at any one time.
To prevent all subsequently arriving packets from being discarded
in the event of the buffer store of a full-duplex port overflowing,
which would result in a considerable overhead as a consequence of
repeated sending of the lost packets, flow control for Ethernet was
introduced. The effect of packet loss is covered by the protocols
at the higher levels. There are two bodies involved in determining
the management capabilities of Ethernet: the IEEE, which defines
the hardware-dependent specifications in the Ethernet standard, and
the IETF (Internet Engineering Task Force), an interest group which
concerns itself with problems relating to TCP/IP and the Internet.
The specifications of the IEEE are usually found implemented in the
hardware as counters or timers. In contrast, the IETF occupies
itself with the structure of the management information, as found
in MIBs (Management Information Bases) for example. The outcome of
this work is seen in such important standards as SNMP (Simple
Network Management Protocol) and RMON (Remote Monitoring).As was
the case in the transition from Ethernet to Fast Ethernet, the
management objects are also the same in Gigabit Ethernet. SNMP, for
example, defines a standardised method of collecting and presenting
Ethernet information at the device level. In relation to this, SNMP
uses the corresponding MIBs to gather important statistics such as
collision counters, the number of packets received or sent, error
rates etc. Further information can be collected by RMON agents and
displayed in network management systems.As Gigabit Ethernet also
uses the familiar Ethernet packets, the same MIBs and RMON agents
as in Ethernet can be used to offer management capabilities at
gigabit speeds.
Applications of Gigabit Ethernet( Upgrading
an Ethernet / Fast Ethernet networkThe simplest way of connecting
networks running Ethernet and Fast Ethernet via Gigabit Ethernet
backbones is by using Fast Ethernet switches. These make both
10Mbit/s and 100Mbit/s available at each port, and Gigabit Ethernet
switches, which allow the selection of 100 or 1000 Mbit/s. In this
way it is also very easy to integrate existing hubs and routers.(
Upgrading an FDDI backbone or Token Ring networkFor connecting FDDI
(Fiber Distributed Data Interface) networks or installations using
Token Ring it is advisable to use modular switches which allow
switching between different technologies. In this way, the existing
backbone structure can be segmented or parts of the backbone can be
incorporated step by step into the Gigabit Ethernet backbone.When
FDDI components are transferred into Gigabit Ethernet, it must be
ensured that existing redundant links are replaced with suitable
Gigabit Ethernet links. If routers are used, it is very often
possible to deploy VLAN-capable switches that exploit both the
functionality of the routers and the performance advantages of the
switches.( Upgrading a server-switch linkIn most networks, servers
are set up centrally in arrangements known as server farms. Because
individual servers usually serve a large number of clients,
bandwidth requirements tend to add up at this point. On the network
side Gigabit Ethernet is able to meet these requirements, although
certain additional important points need to be taken into account
in the selection of the network - server link. If switching to
Gigabit Ethernet for performance reasons, one should pay attention
not only to network performance but also to the performance of the
server system as a whole when selecting the server card. Here it is
above all important to implement the interface between the network
card and the motherboard - generally the PCI interface - with very
high-performance ASICs. This is where the bus usage and CPU load
resulting from the network adapter will determine the overall
performance of the server system.( Upgrading high-performance
workstationsThe integration of high-performance workstations into a
Gigabit Ethernet network should be very easy if attention is paid
to the points described above relating to overall performance. For
cost reasons it is also possible to use full-duplex repeaters for
this purpose.( ATM vs. Gigabit EthernetWhen ATM (Asynchronous
Transfer Mode) was introduced, its speed of 155Mbit/s made it 1.5
times faster than Fast Ethernet. ATM was therefore ideally suited
to new applications with large bandwidths, such as multimedia
applications. The consequence was that demand for ATM grew in both
the LAN and WAN market.On the one hand, the ATM vendors attempt to
emulate Ethernet network by means of a LAN emulation (LANE) and IP
via ATM (IPOA) ( while on the other hand, the advocates of Ethernet
also make ATM functionality such as RSVP (Resource Reservation
Protocol) and RTSP (Real Time Streaming Transport Protocol)
available for Ethernet. There is no question that both technologies
have their desirable features and strengths, but it seems that the
two widely differing technologies are constantly converging in
terms of individual characteristics.Whereas ATM originally came on
the scene as a technology that could be deployed seamlessly from
the LAN through the backbone and into the WAN, the reality today is
rather different. The scalability achieved by Fast Ethernet and
Gigabit Ethernet has once again made established Ethernet
technology attractive, particularly in the field of the LAN and
backbone.As yet, however, most installed PCs and workstations are
not capable of making use of the large bandwidths that both
technologies provide. The scene of the contest has therefore
shifted to the switches and server links in the backbone field,
where Gigabit Ethernet appears to be well equipped. The swift
approval of the standard and the speed with which interoperable and
standard-compliant products have become available have helped
Gigabit Ethernet to reach a good starting position.ATM has the
advantage of being around longer. Although it is true that
installed products do not yet offer gigabit speeds, faster versions
are already in the pipeline. ATM is better suited to applications
such as the real-time transmission of video signals, through the
implementation of QoS (Quality of Service) e.g: with CBR (Constant
Bit Rate). Despite the efforts of the IETF (Internet Engineering
Task Force), the association working to standardise Internet
protocols, it will be difficult to catch up on the ATM lead. The
development of RSVP, which is intended to implement a kind of QoS
on Ethernet, likewise exhibits limitations. It will remain a
"best-effort" protocol, which, although it can detect and confirm a
QoS inquiry, cannot guarantee provision. With ATM, however, there
is the possibility of making data available with a defined delay
time.The greatest advantage of Gigabit Ethernet is its origin in
the proven technology of Ethernet. Users can therefore work on the
basis that migration from Ethernet to Gigabit Ethernet will be very
easy and transparent. Applications that operate via Ethernet will
also work via Gigabit Ethernet. If a user wants to run today's
applications using ATM, on the other hand, there is often a
significant overhead involved in integration into the ATM layers.
The fastest ATM products at the moment operate at 622Mbit/s. At
1000Mbit/s, Gigabit Ethernet is almost twice as fast.At present, it
is impossible to foresee whether one of the two technologies will
win alone. Probably both technologies will complement each other
and continue to exist side by side.So, to conclude, Gigabit
Ethernet, the third generation Ethernet technology with a speed of
1000Mbit/s, is fully compatible with existing Ethernet technologies
and promises a seamless transition to higher speeds. This means a
performance enhancement for existing networks without having to
change the cables, protocols and applications already in
use.
Ajith Ram