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Ethernet, Fast Ethernet, and

Gigabit Ethernet

In this presentation, we will take a closer look

at Ethernet, Fast Ethernet, and Gigabit Ethernets.

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Ethernets

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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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Full-Duplex Ethernet Is No Longer

Ethernet

• Ethernet is characterized by its CSMA/CD

MAC protocol when operating in half-

duplex mode.

• In full-duplex mode, because CSMA/CD is

disabled 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. 12- 7

Transmitted Bit Order

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

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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. (^) 12- 10

IEEE 802.3 Ethernet Frame Format

If length is less

than or equal to

1500, it

represents the

length of data.

Otherwise, it

represents the

higher-level

protocol type.

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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), Ethernet’s CSMA/CD MAC algorithm is an effective flow/congestion control (backoff) and retransmission (up to 16 times) method.
• However, when a switch is used and full-duplex mode is used, each host now has its own collision 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.Docsity.com (^) 12- 14

IEEE 802.3x Flow Control

• Introduced for Fast and Gigabit Ethernets.
• Introduce special MAC-layer control packets (PAUSE packets) to enable or disable frame transmission.
• Thus a more generic MAC framework is defined and an entity called (MAC Control) is introduced.
• MAC Control is responsible for generating, sending, receiving, and performing the PAUSE operation.

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IEEE 802.3x Architecture

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PAUSE Function

• Implement a simple “stop-start” flow control scheme.
• If a device wants to temporarily inhibit incoming frames, it sends a PAUSE frame to the full-duplex partner. This PAUSE frame contains a parameter indicating the length of time the partner should wait before sending more frames.
• If the device wants to cancel the timer at its partner which is set up by its previous PAUSE frame, the device can send another PAUSE frame that contains a parameter of zero time. 12- 19

PAUSE Frame Implementation

PAUSE frames have higher priority over normal data frames.

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