IEEE 802.3-2008 CSMA/CD标准：1Gbps基带网络介绍与物理层规格

802.3-2008

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802.3-2008标准，正式名称为"IEEE Standard for Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications"，是关于局域网和城域网中特定要求的部分，特别是在1000兆比特（Gigabit Ethernet）的实现上。该标准于2008年发布，第三版修订于2010年6月22日，主要关注1000兆比特基带网络（1000Mb/s baseband network）的设计与规范。第34节介绍了1000兆比特以太网的基础，重点在于它如何扩展了ISO/IEC 8802-3（即基本的CSMA/CD介质访问控制协议）以支持1000兆比特的数据传输速率。在这一扩展中，Gigabit Ethernet 采用了一种增强版的ISO/IEC 8802-3 MAC层接口，通过媒体独立接口（Media Independent Interface, MII）连接到物理层子层，如1000 BASE-LX（长距离光纤）、1000 BASE-SX（短距离多模光纤）、1000 BASE-CX（近程单模光纤）以及1000 BASE-T（双绞线）等不同的传输媒介。在设计上，Gigabit Ethernet 的CSMA/CD机制保留了原有的介质访问控制方法，但提升了数据传输速度。这种协议允许设备在发送数据前检测信道是否空闲，以避免冲突（collision），从而提高了数据传输的效率和可靠性。图34-1展示了Gigabit Ethernet、其扩展的ISO/IEC 8802-3 MAC层以及与OSI参考模型的关系，强调了各层之间的协调作用。总结来说，802.3-2008标准为1000兆比特以太网规定了统一的接口标准和物理层规范，确保了不同设备间的兼容性和高效通信，对于现代网络基础设施的构建和发展具有重要意义。了解和遵循这个标准对于设计、部署和维护高速以太网网络至关重要。

IEEE

Std 802.3-2008 REVISION OF IEEE Std 802.3:

35.2.2.8 RX_ER (receive error)

RX_ER is driven by the PHY and shall transition synchronously with respect to RX_CLK. When RX_DV is

asserted, RX_ER shall be asserted for one or more RX_CLK periods to indicate to the Reconciliation

sublayer that an error (e.g., a coding error, or another error that the PHY is capable of detecting that may

otherwise be undetectable at the MAC sublayer) was detected somewhere in the frame presently being trans-

ferred from the PHY to the Reconciliation sublayer.

The effect of RX_ER on the Reconciliation sublayer is defined in 35.2.1.5. Figure 35–11 shows the behavior

of RX_ER during the reception of a frame with errors. Two independent error cases are illustrated. When

RX_DV is asserted, assertion of RX_ER indicates an error within the data octets of a frame. An error within

carrier extension is indicated by driving the appropriate value on RXD<7:0> while keeping RX_ER

asserted.

Assertion of RX_ER when RX_DV is de-asserted with specific RXD values indicates the decode of carrier

extension by the PHY. The transition from RX_DV asserted and RX_ER de-asserted to RX_DV de-asserted

and RX_ER asserted with RXD specifying Carrier Extend shall result in the Reconciliation sublayer indicat-

ing EXTEND INPUT_UNITs to the MAC. Figure 35–9 shows the behavior of RX_DV and RX_ER during

frame reception with carrier extension.

While RX_DV is de-asserted, the PHY may provide a False Carrier indication by asserting the RX_ER sig-

nal for at least one cycle of the RX_CLK while driving the appropriate value onto RXD<7:0>, as defined in

Table 35–2. See 36.2.5.2.3 for a description of the conditions under which a PHY will provide a False Car-

rier indication. Figure 35–12 shows the behavior of RX_ER, RX_DV and RXD<7:0> during a False Carrier

indication.

Figure 35–11—Two examples of reception with error

CRS

RX_CLK

RX_DV

RXD<7:0>

RX_ER

ETXDN1F

SFD

preamble

Figure 35–12—False Carrier indication

RX_CLK

RX_DV

RXD<7:0>

RX_ER

XX XX XX XX XX XX XX XXXXXX XX XX XX XXXX

IEEE

CSMA/CD Std 802.3-2008

receive path is indicated by the de-assertion of both RX_DV and RX_ER or the de-assertion of the RX_DV

signal with an RXD<7:0> value of 00 hexadecimal. On the transmit path the absence of data activity is indi-

cated by the de-assertion of both TX_EN and TX_ER.

Between frames within a burst, the inter-frame period is signalled as Carrier Extend on the GMII. As shown

in Figure 35–7, this is done by asserting TX_ER with the appropriate encoding of TXD<7:0> simultaneous

with the de-assertion of TX_EN on the transmit path; and, as shown in Figure 35–10, by asserting RX_ER

with the appropriate encoding of RXD<7:0> simultaneous with the de-assertion of RX_DV on the receive

path.

Within a burst, the MAC interFrameSpacing parameter defined in Clause 4, is measured from the de-asser-

tion of the TX_EN signal to the assertion of the TX_EN signal, and between bursts measured from the de-

assertion of the CRS signal to the assertion of the CRS signal.

35.2.3.2 Preamble <preamble> and start of frame delimiter <sfd>

35.2.3.2.1 Transmit case

The preamble <preamble> begins a frame transmission. The bit value of the preamble field at the GMII is

unchanged from that specified in 4.2.5 and when generated by a MAC shall consist of 7 octets with the fol-

lowing bit values:

10101010 10101010 10101010 10101010 10101010 10101010 10101010

The SFD (Start Frame Delimiter) <sfd> indicates the start of a frame and immediately follows the preamble.

The bit value of the SFD at the GMII is unchanged from that specified in 4.2.6, and is the following bit

sequence:

10101011

The preamble and SFD are shown above with their bits ordered for serial transmission from left to right. As

shown, the leftmost bit of each octet is the LSB of the octet and the rightmost bit of each octet is the MSB of

the octet.

The preamble and SFD shall be transmitted through the GMII as octets starting from the assertion of TX_EN.

35.2.3.2.2 Receive case

The conditions for assertion of RX_DV are defined in 35.2.2.6. The operation of 1000 Mb/s PHYs can result

in shrinkage of the preamble between transmission at the source GMII and reception at the destination

GMII. Table 35–3 depicts the case where no preamble bytes are conveyed across the GMII. This case may

not be possible with a specific PHY, but illustrates the minimum preamble with which MAC shall be able to

operate. Table 35–4 depicts the case where the entire preamble is conveyed across the GMII.

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IEEE 802.3-2008 CSMA/CD标准：1Gbps基带网络介绍与物理层规格

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