HP 180 Degree Turn Manuale utente

2005/03/11(C) Herbert Haas

The Ethernet Evolution

The 180 Degree Turn

“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

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

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.

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.

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

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.

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".

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.

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.

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.

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