On bus Ethernet, how a connected station detect link connectivity?

Question

On point to point Ethernet, link pulse is used between 2 connected devices to detect link connectivity.

Reference

On bus Ethernet, how a station connected to Ethernet bus detect link connectivity?

Answer 1

In 10BASE5 and 10BASE2, a link relation (between whom?) cannot be resolved at the physical layer because there is no physical (i.e. "inside" the corresponding IEEE 802.3 Physical sublayer) link ("logical connection/association") at all here, despite the presence of a link ("hardwired connection"), typically 1:n as it is a bus, at the medium (i.e. by the coax itself). Therefore, a special protocol operating over the MAC sublayer was designed for, including but not limited to, resolve such a link. It was the Logical Link Control (LLC) protocol forgotten later as TCP/IP became popular.

In other words, in 10BASE5 and 10BASE2 you can state that a local station is linked with a distant station only by a higher layer association activated/supported, e.g. a TCP session established between the stations. At the physical layer itself, a station can observe only packet activity/collision: this is CDMA/CD in its pure "reincarnation".

It seems logical because on a point-to-point Ethernet, a link shows only that a station is already connected to a distant, compatible physical layer, knowing nothing about what is it: a switch, a terminal device, or a test loop, i.e. the (physical) link itself does not make you informed about whose MAC or higher (IP, socket) entities are your counterparts in this topology (virtual bus) too and to resolve are the selected stations linked or not, a higher layer association (identified by at least pair of MAC/IP addresses) is also needed.

Answer 2

Having worked with bus ethernet systems in the past: A PC/workstation connected to one will indeed have no idea whether it is connected or not, you could literally pull the plug (in an AUI System, the connection to the bus, NOT the NIC<->transceiver connection :) ) and it will still think it "has link", and this is what you will also see at an OS level.

Such a station will simply "send what it has to send" without checking whether it is "on the network". Either something will answer, or nothing will answer.

In an extreme scenario, if you connect 50 PCs to a 10base2 ethernet but power them all off, then power one PC on, it is just "as connected" as that PC would be in a 10baseT system if only that PC and the hub/switch it is physically connected were powered on.