Ethernet ,FastEth,GigE

7/28/2019 Ethernet ,FastEth,GigE

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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 may collide with each other. 12- 3


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2 Pairs of Wires in a UTP Cable Are

Always Used

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 to

connect a host to a hub/switch port. 12- 6


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Full-Duplex Ethernet Is No Longer

Ethernet

Ethernet is characterized by its CSMA/CDMAC protocol when operating in half-

duplex mode.

In full-duplex mode, because CSMA/CD isdisabled and not used, actually full-duplex

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

because Gigabit Ethernet mostly operate in full-duplex mode.

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Transmitted Bit Order

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

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Ethernet Address Format

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

address is globally unique. 12- 10


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IEEE 802.3 Ethernet Frame Format

If length is less

than or equal to

1500, it

represents the

length of data.

Otherwise, it

represents thehigher-level

protocol 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 own collisiondomain and CSMA/CD no longer works. Framesnow may be lost due to buffer overflow inside theswitch.

Hosts no longer know this type of collision.Congestion control cannot automatically be performed.

Also, retransmission cannot automatically be performed. 12- 14


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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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MAC Control Frame Format

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PAUSE Function

Implement a simple stop-start flow controlscheme.

If a device wants to temporarily inhibit incoming

frames, it sends a PAUSE frame to the full-duplexpartner. This PAUSE frame contains a parameterindicating the length of time the partner shouldwait before sending more frames.

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

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PAUSE Frame Implementation

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 this 512bit 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

low water mark)should be larger than

link RTT * link BW.

Otherwise, frames

may be dropped orlink utilization may

not be 100%.

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Input-Buffered Switches Assumed

for IEEE 802.3x

IEEE 802.3x is a link-levelflow control scheme.

It uses the buffer

occupancy level of aninput port to determinewhether to inhibit framesfrom incoming.

Unfortunately, this schemedoes not work for output-buffered switch. (Why?because there will be noqueue in 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 blockingproblem, then using IEEE 802.3x will 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) to avoid

the HOL blocking problem, using IEEE 802.3x

may further lower the link utilization.

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Low Link Utilization Example

After receiving the PAUSE frame, switch A cannot send anymore frame to switch B. However, sending P2 should beallowed because P2 can be immediately forwarded when it

arrives at switch B.

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 or full),maximum speed (10 or 100 Mbps), and flow

control support. Avoid configuration headache.

The same RJ45 jack can support 10 or 100Mbps andhalf or full duplex mode, how do you know which

setting you should use when you plug your RJ45 into ahub 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 that needsto negotiate, the auto-negotiation message cannot betransferred at a chosen speed (e.g., 10 or 100 Mbps).Instead, it is transmitted at a predetermined clock

rate. 12- 28


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Gigabit Ethernet Architecture

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Encoding Method

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1000BASE-X Physical Media

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 Mbps to1000 Mbps, for half-duplex CSMA/CD to workwell (to be able to detect collision), we can:

Reduce the network size by a factor of 10 to only 20meter

Not acceptable

Or increase the minimum Ethernet frame length bya 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 asthe minimum frame size. Ideally, they should not needto know whether they are using Ethernet, Fast Ethernet,or Gigabit Ethernet.

However, to be able to detect collision, theminimum frame size must be enlarged from 64

bytes to 512 bytes (4096 bits).

Carrier extension is introduced to insert padding atthe MAC layerso that every transmitted frame isat least 512 bytes.

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Carrier Extension

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Frame Bursting

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

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

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

The burst can be as long as 8192 bytes.12- 39


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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 many small

frames. If the sending host resends all frames that 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 is

detected in this period, the sending host knows that onlythe first frame should be retransmitted.

Third, if there is carrier extension and there is no collision

detected in this period, the sending host can be assured

that no collision will happen in the future. 12- 40


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Gigabit Ethernet Mostly Operates in

Full-Duplex Mode

Although half-duplex (hub) Gigabit EthernetsMAC performance is academically interesting,commercially they are rarely used.

Under 1000 Mbps high speed, CSMA/CDs performanceis bad when the number of participating hosts increases.

Why not using full-duplex mode (switch) to get muchmore bandwidth at slightly increased cost?

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

actually no longer is Ethernet.

The sending host just pumps its frames into the cable asfast as it can.

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Various Line Coding Methods

Gigabit

Ethernet

uses NRZ.

Manchester

consumes

too much

Bandwidth.

Gigabit

Ethernet

cannot afford

to use it.12- 43


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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 rate isonly 125% of the data rate.

Supporting 1000 Mbps is already difficult for some

media such as UTP. Asking UTP to support 2000 Mbpsbaud rate is too much.

In 8B/10B, an 8-bit byte is encoded into a 10 bitcode word.

Provide sufficient signal transitions for clock recoveryat the receiver

Allow easily detecting single or multi-bit errors

The number of one and zero bits transmitted are almostthe same (DC balance). 12- 44


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8B/10B Block Coding

No more 4 consecutive ones or zeros will be generated in a10-bit code.

The imbalance between the number of ones and zeros in a

10-bit code is at most 1.12- 45


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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 ismuch cheaper than that of a 1000BASE-Xnetwork adapter.

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