yog-gigabit ethernet.ppt

7/22/2019 YOG-Gigabit ethernet.ppt

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Gigabit Ethernet

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Ethernet, Fast Ethernet, and

Gigabit Ethernet

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Ethernets

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Collision Domains Should Be

Small

The packets sent by the hosts on the same collision domain maycollide with each other.

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2 Pairs of Wires in a UTP Cable Are

Always Used

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No matter whether the device is a hub or a switch, 2 pairs of wires

in a UTP cable are always used to connect a host to the device.

2 pairs


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Ethernet

MAC flow

in Half-Duplex

Mode

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Switching Between Full and Half-

Duplex Modes Is Simple

No matter whether full or half-duplex mode is used,2 pairs of wires in a UTP cable are always used toconnect a host to a hub/switch port.

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Full-Duplex Ethernet Is No Longer

Ethernet

Ethernet is characterized by itsCSMA/CD MAC protocol whenoperating in half-duplex mode.

In full-duplex mode, because CSMA/CDis disabled and not used, actually full-

duplex Ethernet is no longer Ethernet. This is especially true for Gigabit Ethernet

because Gigabit Ethernet mostly operate infull-duplex mode.

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Transmitted Bit Order

Little Endian: The least significant bit in a byte istransmitted first.

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Transmitted Byte Order

The bytes of a multi-byte field are sent from firstbyte to last byte, with each byte sent in LittleEndian bit order.

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Ethernet Address Format

Every vendor (e.g., 3COM) is assigned a vendor block code.Therefore, every globally administered address is globally unique.

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IEEE 802.3 Ethernet Frame Format

If length isless than orequal to 1500,

it representsthe length ofdata.Otherwise, it

represents thehigher-levelprotocol type.

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Fast Ethernet

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Encoding Method

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Flow Controls Needed When

Ethernets Switches Are Used

When a hub is used to connect all sending hosts(half-duplex mode), Ethernets CSMA/CD MACalgorithm is an effective flow/congestion control(backoff) and retransmission (up to 16 times)method.

However, when a switch is used and full-duplexmode is used, each host now has its owncollision domain and CSMA/CD no longer works.Frames now may be lost due to buffer overflow

inside the switch. Hosts no longer know this type of collision.

Congestion control cannot automatically be performed.

Also, retransmission cannot automatically be performed.

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Backpressure Can Be Used In Half-

Duplex Mode

If a switch uses half-duplex mode to connect to

hosts, although each host has its own collision

domain, we can use some methods to ask

sending hosts to slow down their sending rates: Force collisions with incoming frames

Make it appear as if the channel is busy

If a switch uses full-duplex mode, the above two

methods wont work. We need an explicit flowcontrol for switched full-duplex Ethernet switches.

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IEEE 802.3x Flow Control

Introduced for Fast and Gigabit Ethernets. Introduce special MAC-layer control packets

(PAUSE packets) to enable or disable frametransmission.

Thus a more generic MAC framework is definedand an entity called (MAC Control) is introduced.

MAC Control is responsible for generating,sending, receiving, and performing the PAUSEoperation.

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IEEE 802.3x Architecture

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MAC Control Frame Format

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PAUSE Function

Implement a simple stop-start flow controlscheme.

If a device wants to temporarily inhibitincoming frames, it sends a PAUSE frame to

the full-duplex partner. This PAUSE framecontains a parameter indicating the length oftime the partner should wait before sendingmore frames.

If the device wants to cancel the timer at itspartner which is set up by its previous PAUSEframe, the device can send another PAUSEframe that contains a parameter of zero time.

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PAUSE Frame Implementation

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PAUSE frames have

higher priority over

normal data frames.


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Must Stop Transmission in 512

Bit Time

After receiving a

PAUSE frame, the

receiver has 512 bit

time to decode it.

No more new

transmission is

allowed after this512 bit time period.

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Buffer Thresholds to Control When to

Issue PAUSE Frames

The buffer space

(above the high

water mark and

below the lowwater mark) should

be larger than link

RTT * link BW.

Otherwise, framesmay be dropped or

link utilization may

not be 100%.

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Input-Buffered Switches Assumed for

IEEE 802.3x

IEEE 802.3x is a link-level flow controlscheme.

It uses the bufferoccupancy level of aninput port to determinewhether to inhibitframes from incoming.

Unfortunately, thisscheme does not workfor output-bufferedswitch. (Why? becausethere will be no queuein input ports.)

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IEEE 802.3x May Cause Low Link

Utilization IEEE 802.3x is a link-level flow control

applied to input-buffered switches.

If the input-buffered switch has the HOL

blocking problem, then using IEEE 802.3xwill not further lower the link utilization. (It is

already low .)

However, if the input port uses virtual output

queuing (one queue for each output port) toavoid the HOL blocking problem, using

IEEE 802.3x may further lower the link

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Low Link Utilization Example

After receiving the PAUSE frame, switch A cannot send any moreframe to switch B. However, sending P2 should be allowedbecause P2 can be immediately forwarded when it arrives at switchB.

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P1, P1,P1, P1

Output port 1 is busy or blocked all the time.

Output port 2 is idle.

PAUSE

P1 P2

P2P2

Switch B Switch A


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Auto-Negotiation Eases Configuration

Starting from Fast Ethernet For UTP cables

Auto negotiation about duplex mode (half orfull), maximum speed (10 or 100 Mbps), and

flow control support.Avoid configuration headache.

The same RJ45 jack can support 10 or 100Mbpsand half or full duplex mode, how do you know

which setting you should use when you plug yourRJ45 into a hub or a switchs port?

So, the best setting that you should choose is auto-negotiation.

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Auto-Negotiation Message Format

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Auto-Negotiation Message Transfer

Because link speed is one of the parameter thatneeds to negotiate, the auto-negotiation messagecannot be transferred at a chosen speed (e.g., 10or 100 Mbps). Instead, it is transmitted at apredetermined clock rate.

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Gigabit Ethernet (IEEE 802.3z)

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Gigabit Ethernet Architecture

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Encoding Method

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1000BASE-X Physical Media

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Shortwave Laser

Longwave laser

Fibers


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Ethernet Technology Migration

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Ethernet Technology Migration

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Minimum Ethernet Frame Length

Needs to be Increased

When link speed increases from 100 Mbpsto 1000 Mbps, for half-duplex CSMA/CD towork well (to be able to detect collision), we

can: Reduce the network size by a factor of 10 to

only 20 meter Not acceptable

Or increase the minimum Ethernet frame lengthby a factor of 10 Network efficiency will decrease.

Higher layer protocols needs to be modified.

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Carrier Extension

Gigabit Ethernet does not want to modifyapplication programs. We want application programs to still use 64 bytes

as the minimum frame size. Ideally, they should notneed to know whether they are using Ethernet,Fast Ethernet, or Gigabit Ethernet.

However, to be able to detect collision, theminimum frame size must be enlarged from64 bytes to 512 bytes (4096 bits).

Carrier extensionis introduced to insertpadding at the MAC layerso that everytransmitted frame is at least 512 bytes.

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Carrier Extension

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Frame Bursting

Although carrier extension solves thecollision detection problem, the linkutilization can be very low.

If every transmitted frame is a minimum-sized frame, because in every transmitted512 bytes, there are only 64 useful bytes.The effective link utilization is only 64/512 =

12.5% Frame burstingis introduced to improve

link utilization.

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Frame Bursting

If the source host has many small frames to send, it can sendthem in a burst after a MAC arbitration.

Except for the first frame, other frames sent in a burst need notbe carrier-extended.

If the first frame is larger than 512 bytes, it need not be carrier-extended.

The burst can be as long as 8192 bytes.

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Why Doing Carrier Extension for the

First Frame in a Burst?

First, if there is no carrier extension, when collisionhappens, the sending host may have sent out manysmall frames. If the sending host resends all framesthat have been sent, two problems may occur:

They may already be removed from the buffer. The receiver may receive duplicate frames.

Second, if there is carrier extension and collision isdetected in this period, the sending host knows that

only the first frame should be retransmitted. Third, if there is carrier extension and there is no

collision detected in this period, the sending hostcan be assured that no collision will happen in the

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Gigabit Ethernet Mostly Operates in

Full-Duplex Mode

Although half-duplex (hub) Gigabit Ethernets MACperformance is academically interesting,commercially they are rarely used. Under 1000 Mbps high speed, CSMA/CDs performance

is bad when the number of participating hosts increases. Why not using full-duplex mode (switch) to get much

more bandwidth at slightly increased cost?

Nowadays, almost all Gigabit products areswitches. Since CSMA/CD is totally disabled, Gigabit Ethernet

actually no longer is Ethernet. The sending host just pumps its frames into the cable as

fast as it can.

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Physical Layer Architecture

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Various Line Coding Methods

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Gigabit

Ethernet

uses NRZ.

Manchester

consumes

too much

Bandwidth.

Gigabit

Ethernet

cannot afford

to use it.


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1000BASE-X Uses 8B/10B Block Code

Unlike Manchester encoding, whose baud rate is200% of the data rate, in 8B/10B, the baud rateis only 125% of the data rate. Supporting 1000 Mbps is already difficult for some

media such as UTP. Asking UTP to support 2000 Mbps

baud rate is too much. In 8B/10B, an 8-bit byte is encoded into a 10 bit

code word. Provide sufficient signal transitions for clock recovery at

the receiver

Allow easily detecting single or multi-bit errors The number of one and zero bits transmitted are almost

the same (DC balance).

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8B/10B Block Coding

No more 4 consecutive ones or zeros will be generated in a 10-bit code.

The imbalance between the number of ones and zeros in a 10-bit code is at most 1.

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1000BASE-X Physical Layer

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1000BASE-X Connectors

One jack is for transmission. The other jack is for reception.

To connect two hosts together, you need to cross-over thetransmission and reception jacks by yourself.

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1000BASE-T over UTP

Product is now available. Use four (all) pairs of wires in an UTP

cable to together support 1000 Mbps.

The quality level of UTP cables should be

at least CAT 5E or CAT 6. The cost of a 1000BASE-T network

adapter is much cheaper than that of a1000BASE-X network adapter.

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