HP 180 Degree Turn User manual

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2005/03/11(C) Herbert Haas
The Ethernet Evolution
The 180 Degree Turn

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“Use common sense in routing cable.
Avoid wrapping coax around sources
of strong electric or magnetic fields.
Do not wrap the cable around
flourescent light ballasts or
cyclotrons, for example.”
Ethernet Headstart Product, Information and Installation Guide,
Bell Technologies, pg. 11

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History: Initial Idea
Shared media CSMA/CD as access algorithm
COAX Cables
Half duplex communication
Low latency No networking nodes
(except repeaters)
One collision domain and also one broadcast domain
10 Mbit/s shared
by 5 hosts 2
Mbit/s each !!!
The initial idea of Ethernet was completely different than what is used today
under the term "Ethernet". The original new concept of Ethernet was the use of
ashared media and an Aloha based access algorithm, called Carrier Sense
Multiple Access with Collision Detection (CSMA/CD). Coaxial cables were
used as shared medium, allowing a simple coupling of station to bus-like
topology.
Coax-cables were used in baseband mode, thus allowing only unicast
transmissions. Therefore, CSMA/CD was used to let Ethernet operate under the
events of frequent collisions.
Another important point: No intermediate network devices should be used in
order to keep latency as small as possible. Soon repeaters were invented to be the
only exception for a while.
An Ethernet segment is a coax cable, probably extended by repeaters. The
segment constitutes one collision domain (only one station may send at the same
time) and one broadcast domain (any station receives the current frame sent).
Therefore, the total bandwidth is shared by the number of devices attached to the
segment. For example 10 devices attached means that each device can send 1
Mbit/s of data on average.
Ethernet technologies at that time (1975-80s): 10Base2 and 10Base5

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History: Multiport Repeaters
Demand for structured cabling (voice-grade
twisted-pair)
10BaseT (Cat3, Cat4, ...)
Multiport repeater ("Hub") created
Still one collision domain
("CSMA/CD in a box")
Later, Ethernet devices supporting structured cabling were created in order to
reuse the voice-grade twisted-pair cables already installed in buildings. 10BaseT
had been specified to support Cat3 cables (voice grade) or better, for example
Cat4 (and today Cat5, Cat6, and Cat7).
Hub devices were necessary to interconnect several stations. These hub devices
were basically multi-port repeaters, simulating the half-duplex coax-cable, which
is known as "CSMA/CD in a box". Logically, nothing has changed, we have still
one single collision and broadcast domain.
Note that the Ethernet topology became star-shaped.

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History: Bridges
Store and forwarding according destination MAC
address
Separated collision domains
Improved network performance
Still one broadcast domain
Three collision
domains in this
example !
Bridges were invented for performance reasons. It seemed to be impractical that
each additional station reduces the average per-station bandwidth by 1/n. On the
other hand the benefit of sharing a medium for communication should be still
maintained (which was expressed by Metcalfe's law).
Bridges are store and forwarding devices (introducing significant delay) that can
filter traffic based on the destination MAC addresses to avoid unnecessary
flooding of frames to certain segments. Thus, bridges segment the LAN into
several collision domains. Broadcasts are still forwarded to allow layer 3
connectivity (ARP etc), so the bridged network is still a single broadcast domain.

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History: Switches
Switch = Multiport Bridges with HW acceleration
Full duplex Collision-free Ethernet No CSMA/CD
necessary anymore
Different data rates at the same time supported
Autonegotiation
VLAN splits LAN into several broadcast domains
10Mbit/s
100 Mbit/s
100 Mbit/s
1000 Mbit/s
Collision-free
plug & play
scalable Ethernet !
Several vendors built advanced bridges, which are partly or fully implemented in
hardware. The introduced latency could be dramatically lowered and
furthermore other features were introduced, for example full duplex
communication on twisted pair cables, different frame rates on different ports,
special forwarding techniques (e,g, cut through or fragment free), Content
Addressable Memory (CAM) tables, and much more. Of course marketing rules
demand for another designation for this machine: the switch was born.
Suddenly, a collision free plug and play Ethernet was available. Simply use
twisted pair cabling only and enable autonegotiation to automatically determine
the line speed on each port (of course manual configurations would also do). This
way, switched Ethernet become very scalable.
Furthermore, Virtual LANs (VLANs) were invented to split the LAN into several
broadcast domains. VLANs improve security, utilization, and allows for logical
borders between workgroups.

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Today
No collisions no distance limitations !
Gigabit Ethernet becomes WAN
technology !
Over 100 km link span already
Combine several links to "Etherchannels"
Acts as single link from the spanning-tree view
•Cisco: Port Aggregation Protocol (PAgP)
•IEEE 802.1ad: Link Aggregation Control Protocol
(LACP)
1Gbit/s or even 10 Gbit/s long reach connection !!!
Today, Gigabit and even 10 Gigabit Ethernet is available. Only twisted pair and
more and more fiber cables are used between switches, allowing full duplex
collision-free connections. Since collisions cannot occur anymore, there is no
need for a collision window anymore! From this it follows, that there is virtually
no distance limit between each two Ethernet devices.
Recent experiments demonstrated the interconnection of two Ethernet Switches
over a span of more than 100 km! Thus Ethernet became a WAN technology!
Today, many carriers use Ethernet instead of ATM/SONET/SDH or other rather
expensive technologies. GE and 10GE is relatively cheap and much simpler to
deploy. Furthermore it easily integrates into existing low-rate Ethernet
environments, allowing a homogeneous interconnection between multiple
Ethernet LAN sites. Basically, the deployment is plug and play.
If the link speed is still too slow, so-called "Etherchannels" can be configured
between each two switches by combining several ports to one logical connection.
Note that it is not possible to deploy parallel connections between two switches
without an Etherchannel configuration because the Spanning Tree Protocol (STP)
would cut off all redundant links.
Depending on the vendor, up to eight ports can be combined to constitute one
"Etherchannel".

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What About Gigabit Hubs?
Would limit network diameter to 20-
25 meters (Gigabit Ethernet)
Solutions
Frame Bursting
Carrier Extension
No GE-Hubs available on the market
today forget it!
No CSMA/CD defined for 10GE (!)
Remember: Hubs simulate a half-duplex coaxial cable inside, hence limiting the
total network diameter. For Gigabit Ethernet this limitation would be about 25
meters, which is rather impracticable for professional usage. Although some
countermeasures had been specified in the standard, such as frame bursting and
carrier extension, no vendor developed an GE hub as for today. Thus: Forget GE
Hubs!
The 10 GE specification does neither consider copper connections nor hubs. 10
GE can only run over fiber.
At this point please remember the initial idea in the mid 1970s: Bus, CSMA/CD,
short distances, no network nodes.
Today: Structured cabling (point-to-point or star), never CSMA/CD, WAN
capabilities, sophisticated switching devices in between.

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MAC Control Frames
Additional functionality easily integrated
Currently only Pause-Frame supported
preamble FCSMAC-ctrl parametersMAC-ctrl opcode8808hSADA
8bytes 6 6 2 2 44 4
Always 64 bytes
MAC-ctrl opcode ........... Defines function of control frame
MAC-ctrl parameters .... control parameter data (always filled up to 44 bytes)
Different data rates between switches (and different performance levels) often
lead to congestion conditions, full buffers, and frame drops. Traditional Ethernet
flow control was only supported on half-duplex links by enforcing collisions to
occur and hereby triggering the truncated exponential backoff algorithm. Just let
a collision occur and the aggressive sender will be silent for a while.
A much finer method is to send some dummy frames just before the backoff
timer allows sending. This way the other station never comes to send again.
Both methods are considered as ugly and only work on half duplex lines.
Therefore the MAC Control frames were specified, allowing for active flow
control. Now the receiver sends this special frame, notifying the sender to be
silent for N slot times.
The MAC Control frame originates in a new Ethernet layer—the MAC Control
Layer—and will support also other functionalities, but currently only the "Pause"
frame has been specified.

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Auto Negotiation
Enables each two Ethernet devices to
exchange information about their
capabilities
Signal rate, CSMA/CD, half- or full-duplex
Using Link-Integrity-Test-Pulse-Sequence
Normal-Link-Pulse (NLP) technique is used
in 10BaseT to check the link state (green LED)
10 Mbit/s LAN devices send every 16.8 ms a
100ns lasting NLP, no signal on the wire
means disconnected
Several Ethernet operating modes had been defined, which are incompatible to
each other, including different data rates (10, 100, 1000 Mbit/s), half or full
duplex operation, MAC control frames capabilities, etc.
Original Ethernet utilized so-called Normal Link Pulses (NLPs) to verify layer 2
connectivity. NLPs are single pulses which must be received periodically
between regular frames. If NLPs are received, the green LED on the NIC is
turned on.
Newer Ethernet cards realize auto negotiation by sending a sequence of NLPs,
which is called a Fast Link Pulse (FLP) sequence.