04
May
14

Ethernet Hubs

Today switched ethernet networks are the norm, but before the price of switches decreased hubs were the main means of linking many nodes together. I have two old school hubs in my collection, one from my early home network and one I rescued from scrap during my time working in IT support.

The main difference between a hub and a switch is basically how smart they are. A hub is an inherently dumb device that distributes packets of data to every listening node. About the only function it does perform is checking for packet collisions on the network. Collisions happen basically when two nodes (any device using the Ethernet) try to talk on the same link at the same time. This corrupts the data being sent and requires the nodes to resend their data. Because a hub effectively links all the nodes together on one link collisions can be a serious issue. Typically the data rates for hubs are 10Mbit/s as they were displaced by switching technology as speeds increased.

Other forms of Ethernet such as the coaxial type that was commonly found had one link as well, but had less collision problems. This was because the node could look at the cable directly and see if anyone was using it and then wait until it was free to transmit. It is still possible to get collisions, but less likely and usually improved the throughput.

Switches are a completely different beast, they are effectively a node within the network with a multitude of links coming out. A switch has a CPU that basically inspects each packet and sends it only to the ethernet port that it is destined for. Because each node connected to the switch has its own and separate link the possibility of collisions is either significantly reduced or eliminated depending on whether the link is full-duplex or half. With every node talking all at once a switch has to be very good at inspecting and routing packets quickly, so that is usually all done with specialised hardware designed to do it at ethernet speeds. Of course it can be overloaded and depending on the hardware this causes something called packet loss. Switches can have data rates up to 1Gbit/s at the moment.

I only have one switch that isn’t so interesting to look at and is currently in service. So I’ll only show the hubs I own.

Skymaster Hub

Here I have a Skymaster 5 port 10Mbit/s ethernet hub. Our home network was originally based on thin coax which we chose because it was the cheaper option. The 50 ohm cable was cheap and we didn’t need to buy a hub which at the time was still expensive as they weren’t a common item. But at some point I got a new machine which no longer had the BNC connector required so I needed this hub to connect to the network. It also proved useful over the years at uni when we ran small LAN parties at a friend’s house. Although the data throughput wasn’t great for file transfers it worked well for games of the time.

The original home network was created when me and my older brother were home from Uni on summer holidays with both of our machines, so we networked them with our Dads machine so we could play Doom, Age of Empires and other network games. With only two machines we usually just used a null modem cable.

Skymaster internals

Looking inside the hub doesn’t reveal very much. There is a single SMT chip doing the heavy lifting with the ethernet transceivers located around it. Note the one larger transceiver is for the uplink port. This port would be used to link a small network into a larger one. I’m not sure if there is much difference but perhaps the larger transceiver also supported the BNC connector and longer cables.

Magnum 3000 Stackable Hub

This is a Magnum 3000 Stackable hub made by Garrett Communications in 1996.  It has 12 ports and other models in the same series had 24, but if you needed more ports for a large network you could connect a number of these together to for a much larger network. I wonder how effective this could have been with many nodes, there could have been collision problems if too many nodes were on the same hub.

Rear panel

Here we see the back of the unit, it has an extra port here that can optionally be switched, some DIP switches for configuration, a serial port and the two connecters used for stacking the devices. The serial port indicated that this one has the SNMP module fitted.

Internal Overview

Here is an overview of the internals, I noted that there was room for another board on the right. This makes me think that the 24 port version was largely identical with the addition of a secondary board. On top the SNMP module is dominant in frame so I had to disassemble more to see the main board.

SNMP Module

The SNMP module removed from the unit. Interestingly it seems to be using a 386sx processor for doing most of the work on this board. It basically looks like a small specialised PC. Whilst the CPU is rated for 25Mhz it seems from the oscillator that it is only being run at 20Mhz.

SNMP Module up close

A close-up reveals the oscillator at 40Mhz (usually divided by 2 for the CPU clock) and a Chips and Technologies chipset. The SIMMS installed only have 3 chips each, meaning they were probably quite small.

Hub main board

The hubs main board with extra modules removed looks bare in comparison. It is dominated by a single chip in the middle but also has a ROM and small micro controller towards the top of the board.

National Semiconductor Chip

A close-up of the main chip, it is a National Semiconductor part designed in 1992, so this design/type of hub may have been in manufacture for quite some time.

Ethernet Tranceivers

A close-up of the ethernet transceiver chips all lined up in a row for the front of the unit. I believe the 74 series logic is to enable the partitioning feature of this hub. The partitioning feature basically has the hub automatically disconnect a node which is “jabbering”. Basically this means the hub will disconnect a node which is causing a disproportionate amount of collisions.

LED Board

This small board simply carries the indicator LEDs for the front panel. This module would likely also appear twice in the 24 port version. Everyone likes to see the blinky lights somewhere on their equipment and these fit the bill.

That’s all the photos I have for today. I think I need to get better lighting or a better camera as the quality of some of the shots seems lower than they should be. Oddly some lines that should be straight are slightly curved, I don’t know what caused that.

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