Ethernet is commonly implemented in a shared hub/switch environment where if one station broadcasts a frame then all devices must synchronize to the digital signal to extract the data from the physical wire.  The connection is between physical medium, and all the devices that share this need to listen to each frame as they are considered to be on the same collision domain.  The downside of this is that only one device can transmit at each time plus all devices need to synchronize and extract all the data.

If two devices try to transmit at the same time, and this is very possible – the a collision will occur.  Many years ago, in 1984 to be precise, the IEEE Ethernet Committee released a method of dealing with this situation.  It’s a protocol called the Carrier Sense Multiple Access with Collision Detect protocol or CSMA/CD for short.  The function of this protocol is to tell all stations to listen for devices trying to transmit and to stop and wait if they detect any activity.  The length of the wait is predetermined by the protocol and will vary randomly, the idea is that when the collision is detected it won’t be repeated.

It’s important to remember that Ethernet, uses a bus topology.   This means that whenever a device transmits then the signal must run from one end of the segment to the other.   It also defines that a baseband technology should be used which means that when a station does transmit it is allowed to use all potential bandwidth on the wire.  There is no allowance for other devices to utilise the potential available bandwidth.

Over the years the original IEEE 802.3 standards have been updated but here are the initial settings:

  • 10Base2: 10 Mbps, baseband technology up to 185 meters in cable length.  Also known as thinnet capable of supporting up to 30 workstations in one segment.  Not often seen now.
  • 10base5: 10 Mbps, baseband technology allows up to 500 meters in length. Known as thicknet.
  • 10BaseT: 10Mbps using category 3 twisted pair cables. Here every device must connect directly into a network hub or switch.   This also means that there can only be one device per network segment.

Both the speeds and topologies have changed greatly over the years, and of course 10Mbps is no longer adequate for most applications.  In fact most networks will run on gigabit switches in order to facilitate the increasing demands of network enabled applications.    Remember allowing access to the internet means that bandwidth requirements will rocket even if you allow for places like the BBC blocking VPN access (article here).

Each of the 802.3 standards defines an Attachment Unit Interface (AUI) that allows one bit at a time transfer using the data link media access method to the Physical layer.  This means that the physical layer becomes adaptable and can support any emerging or newer technologies which operate in a different way.  There is one exception though and it is a notable one, the AUI interface cannot support 100Mbs Ethernet for one specific reason – it cannot cope with the high frequencies involved.   Obviously this is the case for even faster technologies too like Gigabit Ethernet.

John Smith

Author and Network VPN Blogger.

No Comments Networks, Protocols, VPN

Leave a Reply

Your email address will not be published. Required fields are marked *