IEEE P802.3br-D3.0-2016 pdf download

IEEE P802.3br-D3.0-2016 pdf download.Draft Standard for Ethernet Amendment: Specification and Management Parame-ters for Interspersing Express Traffic.
99.1 Introduction
This clause specifies an optional MAC Merge sublayer for use with a pair of full-duplex MACs and a single PHY operating at 100 Mb/s or higher on a point-to-point link. The two MACs are:
a preemptahie MAC (pMAC) which carries the preemptahie traffic and
an express MAC (eMAC) which carries the express traffic.
The MAC Merge sublayer supports interspersing express traffic with preemptable traffic. This is achieved by using a MAC Merge sublayer to attach an express Media Access Control (MAC) and a preemptable MAC to a single Physical Signaling Sublayer (PLS) service. The MAC Merge sublayer supports two ways to stop transmission of preemptable traffic in the presence of express traffic:
the MAC Merge sublayer may preempt (interrupt) preemptable traffic being currently transmitted, and the MAC Merge sublayer may prevent starting the transmission of preemptahie traffic.
This clause also specifies a MAC Merge Service Interface (MMSI) providing a primitive that either stops or resumes transmission of preemptable traffic, minimizing the latency for express traffic. For example, when the MAC Client supports scheduled traffic as defined in IEEE Std X02. lQ3, transmission of preemptable packets can be held before express traffic is scheduled to be transmitted.
When preemption capability is active, the MAC Merge sublayer allows frames provided over the express MAC service interface (express traffic) or the MMSI service primitives to interrupt transmission of preemptable frames provided over the preemptable MAC service interface (preemptable traffic).
When preemption capability is inactive, the MAC Merge sublayer does not preempt transmission of preemptable packet even if express traffic becomes available. If the MAC Merge sublayer is idle (i.e. at least an interpacket gap has elapsed since the end of transmission of a prior packet) and an express packet becomes available, the MAC Merge sublayer transmits the express packet. Otherwise, the MAC Merge sublayer transmits any presented preemptahie packets.
Preemption capability is most useful at lower operating speeds. For example, the duration of a 2000 octet packet on a 100 Mb/s link is 160 us and on a 1 Gb/s link is 16 us. This is an upper bound on the additional delay before a MAC Client can send an Express frame when preemption capability is not used. At higher operating speeds this additional delay gets smaller in proportion to the speed, reducing the advantage of the preemption mechanism.
Figure 99—I shows the relationship of MAC Merge sublayer to the other sublayers and the ISO/IEC Open Systems Interconnection (OS!) reference model. The MAC Merge sublayer has two clients that are MAC sublayer instances: the eMAC and the pMAC.
Figure 99—2 shows the service interfaces of the MAC Merge sublayer and its associated MACs and Reconciliation Sublayer.
The hold req parameter takes one of two values, HOLD or RELEASE. The value of hold req=HOLD causes the MAC Merge sublayer to stop transmission of preemptable traffic.
The value of hold_req=RELEASE allows the MAC Merge sublayer to resume transmission of preemptable traffic.
99.2.2.1.2 When generated
The generation of this primitive is out of scope of this standard.
99.2.2.1.3 Effect of receipt
Receipt of the primitive with the value HOLD causes preemption if the current conditions allow preemption and prevents starting transmission of pMAC packets until this primitive is received with the value RELEASE.
The receipt of this primitive with the value RELEASE allows MAC Merge sublayer to transmit packets from the pMAC when the eMAC does not have a packet to transmit.
99.3 MAC Merge Packet (mPacket)
When the preemption capability is active, MAC Merge sublayer sends rnPackets to the Reconciliation sublayer (RS). An rnPacket contains one of the following:
— a complete express packet,
– a complete preemptable packet
– an initial fragment of a preemptable packet, or
– a continuation fragment of a preemptable packet.