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HALF Duplex and FULL Duplex Network Question

Hi there,

I have been called in on site of one client and what I saw amazed me and challenged me at the same time.

I would like to obtain professional opinion on the subject.

My first question is :

Is it possible to achieve no collisions and no packet drops in Half Duplex network if All ports and nodes on the network are set to half duplex ?

My Second question is:

If I change the Blue Link (per diagram) to Half Duplex will this reduce amount of Collisions / Packet drops ?

My Third question is:

Why half duplex was ever developed anyways ?

My Forth question is:

What can be done to improve the network in case half duplex can never be changed to full ?

Network.png

The problem is that Node 1 is NOT a computer. It's a network device that supports ONLY HALF DUPLEX 10 Mbps.

Node 1 needs to communicate with Node 2

Switches report Collisions in Port statistics.

We want to achieve the state where there are NO collisions or reduce them.

If I were to change the Blue Link to HALF Duplex 10 Mbps as other links, Will that improve this ?
We want the network to be slow, we do not want any drops, or collisions.

Can this be achieved and why ?

I really appreciate your help.

Kindest Regards,

Mariusz

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4 REPLIES 4
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Hall of Fame Cisco Employee

HALF Duplex and FULL Duplex Network Question

Hello Mariusz,

A key concept to keep in mind is that a collision domain (i.e. a part of the network where at most a single station can send data, otherwise a collision would ensue) is always limited by a port on a switch, and you are using switches in your topology. Whatever collisions may occur on a switched port, they never propagate to other ports on the switch, therefore, the discussion of the impact of collisions is always limited up to a switch port but never beyond it.

Is it possible to achieve no collisions and no packet drops in Half  Duplex network if All ports and nodes on the network are set to half  duplex ?

Unfortunately, no, this is not possible. In the half duplex mode, you are reverting back to CSMA/CD to know if the other party is sending frames, and if you happen to send frames in the moment the other party starts sending its own frames (because both stations heard a "silence" on the medium immediately before sending), you will end up with having a collision. Only a full-duplex setting on a link provides for collision-free operation of that link.

My Second question is:

If I change the Blue Link (per diagram) to Half Duplex will this reduce amount of Collisions / Packet drops ?

No, it will not. On the contrary, this will lead to the increase of collisions and frame drops on the link between SW1 and SW2.

My Third question is:

Why half duplex was ever developed anyways ?

Because that was the original operating mode of the Ethernet on the coaxial cabling (10Base5, 10Base2) and with Ethernet hubs (10BaseT) where it was technically impossible to communicate in full duplex. Even newer switches that are equipped with 100Mbps and 1Gbps metallic ports must be capable of reverting back to the 10BaseT operation if such a legacy device is ever connected, and this mode would be 10BaseT-Half Duplex.

My Forth question is:

What can be done to improve the network in case half duplex can never be changed to full ?

Sadly, nothing can be done.

It should be noted that collisions are a normal product of half duplex Ethernet operation. Their occurence alone does not necessarily indicate any problems yet - it is just how half duplex Ethernet operates. All coax-based and hub-based Ethernets had collisions. You should get concerned about collisions only if you perceive the network operation to be impaired in any way, i.e. excessive frame and packet losses, delays, data consistency issues etc.

Best regards,

Peter

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VIP Expert

Re: HALF Duplex and FULL Duplex Network Question

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

What you've described is about as good as it gets when your host is limited to 10/half.

There are a couple of things you can do that might decrease your collision rate by a small amount.

As the 10/half host is connected to a switch, if the switch supports VLANs (and you have routing support), moving the 10/half host to its own VLAN/subnet may reduce traffic (multicast and broadcast) seen by the 10/half host.  If you can't migrate the host, if the switch supports IGMP snooping, enabling it will limit the link from seeing undesired multicast traffic.  For both approaches, less traffic seen by the host then reduces your chance of collisions as a collision is when both your host and switch transmit a frame at the same time.

However, as Peter noted, collisions are normal with half duplex, and unless your link is really, really busy (like 70% or more) they generally don't impact your overall transmission rate too much.  This because when there's a collision, both transmitters "randomly" wait to retransmit the same frame again but the wait is rather short.  If the link is really, really busy what can happen is the retransmit might cause another collision, and another new (longer) wait to retransmit and the cycle repeats with either a further delayed transmission or the frame will be dropped if the wait is very excessive.  (At least I recall it works that way.  [Peter please correct me if I remember this wrong.])

(BTW, don't forget a collision is when both transmitters attempt to transmit at the same time, but half duplex Ethernet will first "listen" to see if wire is being used before it transmits.  What this means is traffic may be delayed even when there's no collisions, but again, delay is normally minimal.)

With just two transmitters on the link, your host and your switch, it's less likely there will be multiple collisions for the same frame (except is cases where there's a high rate of duplex data transmission from both transmitters.  I'm guessing whatever this host is, it's unlikely to have high two way transmission rate, concurrently.

The only other thing that might help to improve performance when full duplex isn't supported, and likely your 10/half host doesn't support it, would be running at 100/half.  Reason I mention this, for a while, there were hosts that supported 10 or 100 half, but didn't support 10 or 100 full.

Highlighted
Hall of Fame Cisco Employee

Re: HALF Duplex and FULL Duplex Network Question

Hello Joseph,

An insightful and very informative reply, as always. Thank you!

The idea about reducing unnecessary traffic by putting the device into a separate VLAN and reducing unwanted broadcasts/multicasts is a good idea indeed. I've got to remember that

Regarding the Ethernet collision backoff, you are absolutely correct. After each collision, the colliding stations will cease the transmission of the frame and they will try to send it again after a random period whose length increases exponentially with the number of unsuccessful repetitions. The maximum count of attempts to resend a collided frame in Ethernet is 16.

It would be actually nice to see if the devices Mariusz is concerned about support 100/half. While not technically impossible, I have to admit I have not had the "pleasure" of working with such devices.

Best regards,

Peter

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VIP Expert

Re: HALF Duplex and FULL Duplex Network Question

Disclaimer

The   Author of this posting offers the information contained within this   posting without consideration and with the reader's understanding that   there's no implied or expressed suitability or fitness for any purpose.   Information provided is for informational purposes only and should not   be construed as rendering professional advice of any kind. Usage of  this  posting's information is solely at reader's own risk.

Liability Disclaimer

In   no event shall Author be liable for any damages whatsoever (including,   without limitation, damages for loss of use, data or profit) arising  out  of the use or inability to use the posting's information even if  Author  has been advised of the possibility of such damage.

Posting

Peter thank you for the compliment.

I've worked with 100/half (and Token-Ring too - laugh).  When both 100/half and 10/full were choices, a common question was, which is better?  Of course (assuming you didn't have 100/full), the answer was "it depends".  Often 100/half would perform better than 10/full because of its 10x transmission rate.  Again, "normal" volume of collisions often weren't as detrimental as many believe.  It's a different story for a "high" volume of collisions.

One quirk about 100/half hubs, you were much more restricted in your hub to hub cascade rules vs. 10/half.

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