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In 1000BaseT the same pairs of wires (all four pair) are used both for transmission and reception. In this gigabit mode how is it decided which of the link partners will send FLPs first. Further what happens if both the link partners send FLPs at the same time?

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The link pulses are always sent on the transmit pair for 10BASE-T/100BASE-TX even on a 1000BASE-T port.

This creates a problem only when two ports with the same pinout - MDI vs MDI-X - are connected to each other, but the practically standard Auto MDI-X algorithm (more or less) alternates the transmit and receive pairs, so eventually the pulses will find the right path and the link can be brought up.

1000BASE-T also needs to figure out which pairs are connected to each other (straight cable, two-pair crossover cable, four-pair crossover cable). This is done in the physical coding sublayer (PCS).

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  • In 1000BaseT simultaneous transmission occurs in both directions by using echo cancellation. In this is case both the link partners are transmitting over the same pair. – Shreyas S Aug 31 '17 at 11:35
  • @ShreyasS Simultaneous transmission on all pair happens once the link is established. The pulses are required to establish the link and work in the very same logic as with 100BASE-TX. It explicitly uses the same fashion defined in Clause 28 for 100BASE-T. – Zac67 Aug 31 '17 at 16:55
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From Gigabit Ethernet Auto-Negotiation, By Rich Hernandez (Issue 1 2001):

The Auto-Negotiation standard allows devices based on several Ethernet standards, from 10BaseT to 1000BaseT, to coexist in the network by mitigating the risks of network disruption arising from incompatible technologies. This capability helps ensure a smooth migration path from Ethernet to Fast Ethernet and Gigabit Ethernet. This article provides an in-depth explanation of auto-negotiation and its functioning and also discusses special cases that may be encountered.

Today a number of technologies, such as 10BaseT, 100BaseTX, and 1000BaseT, use the same RJ-45 connector, creating the potential for connecting electrically incompatible components together and causing network disruption. In addition, with the advent of Gigabit Ethernet over copper, three-speed devices now support 10 Mbps, 100 Mbps, and 1000 Mbps operation. The Institute of Electrical and Electronics Engineers (IEEE®) developed a method known as auto-negotiation to eliminate the possibility of dissimilar technologies interfering with each other.

Gigabit transceivers at the physical layer (PHY) of the Open Systems Interconnection (OSI) model use auto-negotiation to advertise the following modes of operation: 1000BaseT in full or half duplex, 100BaseTX in full or half duplex, and 10BaseT in full or half duplex. Although auto- negotiation can be disabled for 100BaseTX or 10BaseT connectivity, it is always required for normal 1000BaseT operation.

Auto-negotiation enables an easy upgrade path to gigabit speeds by future proofing the server network connectivity with a three-speed network interface card (NIC) or LAN on motherboard (LOM). A server connected to a Fast Ethernet switch or hub can easily be upgraded to Gigabit Ethernet by connecting the NIC to a Gigabit Ethernet switch. If both the NIC and the switch are set to auto-negotiate, the interface will be automatically configured to run at 1000 Mbps.

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