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2Mbps line delivers over 400kb/sec downloads!

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2Mbps line delivers over 400kb/sec downloads!

First I need to set the record straight. In an earlier thread I suggested that my new 2Mbps line was under-performing. Since boosting the wi-fi signal by using a spare Apple Airport base station as a bridge, I have been able to determine that the two wireless computers on my LAN are making full use of the line speed - and then some! My problem seems to have been caused by the measly 10baseT on-board ethernet in my wired PowerMac which physically can't support anything like 2Mbps.

Meanwhile, with their signal boosted, my wireless boys - one with an Apple G4 Powerbook + Airport card and the other with a Win XP desktop + Netgear wireless PCI card - have been having great fun and at one point this evening showed me that they were simultaneously downloading different large files from different servers, with each reporting a download speed of over 200kb/sec. Even allowing for the vagiaries of software speed calculations, this apparent combined speed of over 400kb/sec seems well in excess of the theoretical maximum speed of the line. Could it possibly be correct?

Simon
9 REPLIES
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2Mbps line delivers over 400kb/sec downloads!

Hi pr100

Firstly may I point out the 10baseT is a 10Mbps (Mega bits per second)connection hence the "10" in it, as a 100baseT is a 100Mbps connection, so it is more than able to handle your 2Mbps line going down it. Also you may sometime get burst speeds of over 400Kbps (Kilo bits per second), but I doubt that the speed lasted all that long! 230-240Kbps is the norm for your average to 2Mbps line!


Kind regards


AthlonGod
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2Mbps line delivers over 400kb/sec downloads!

The network may be 10Mbps, but it's well-known that Ethernet , being a CSMA/CD type network, will not achieve anything like that in practice, although 2Mbps does seem extremely low. The figure I had in mind was something like 50%-70% of the available bandwidth, but one reference I found quoted 40%:

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CSMA/CD article[/url]"]Performance of CSMA / CD

It is simple to calculate the performance of a CSMA/CD network where only one node attempts to transmit at any time. In this case, the node may saturate the network and near 100% utilisation of the network may be achieved, providing almost 10 Mbps of throughput on a 10 Mbps LAN.

However, when two or more nodes attempt to transmit at the same time, the performance of Ethernet is less predictable (and not covered by this course). The fall in utilisation and throughput occurs because some bandwidth is wasted by collisions and back-off delays. In practice, a busy shared 10 Mbps Ethernet network will typically supply 2-4 Mbps of throughput to the nodes connected to it.

As the level of utilisation of the network increases, particularly if there are many nodes competing to share the bandwidth, an overload condition may occur. In this case, the throughput of Ethernet LANs reduces very considerably, and much of the capacity is wasted by the CSMA/CD algorithm, and very little is available for sending useful data. This is the reason why a shared Ethernet LAN should not connect more than 1024 computers. Many Engineers use a threshold of 40% Utilisation to determine if a LAN is overloaded. A LAN with a higher utilisation will observe a high collision rate, and likely a very variable transmission time (due to back off). Separating the LAN in to two or more collision domains using bridges or switches would likely provide a significant benefit (assuming appropriate positioning of the bridges or switches).

Shared networks may also be constructed using Fast Ethernet, operating at 100 Mbps. Since fast Ethernet always uses fibre or twisted pair, a hub or switch is always required.

[My emphasis]


A home network with a handful of nodes would not normally be considered a "busy network", so it does raise the question of what protocols are in use on the network? Is a lot of unnecessary traffic being generated?

On the face of it, it does seem unlikely that a 10Mbps Ethernet should not be able to keep pace with your 2Mbps/256kbps ADSL connection.
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2Mbps line delivers over 400kb/sec downloads!

my 100Mbps Ethernet will give me 60% (about 7MB/sec) bandwidth if I'm lucky. my 22Mbps wifi is even worse, giving me 30% (less than 1MB/sec).
it makes transferring files between computers interesting...
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2Mbps line delivers over 400kb/sec downloads!

I can a see where you guys are coming from, but that type of collision rate is now manly found only on network hubs, not switches. Many routers have switches built in; it's very rare for you to find a router with a hub in now. Also the use of switches MOSTLY stops collisions, so they are a thing of the past!

Link

[Moderators note (by Thomas): Display text added to link, as it was disrupting the page layout a bit.]
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2Mbps line delivers over 400kb/sec downloads!

A fair point, which, in fact was also made in the article I quoted, though I didn't choose to highlight the particular sentence:

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Separating the LAN in to two or more collision domains using bridges or switches would likely provide a significant benefit (assuming appropriate positioning of the bridges or switches).


A switch, as you say, maximises the number of collision domains, and therefore minimises the possibility of collisions. However, it's not capable of eliminating collisions -- as you say "MOSTLY stops collisions". What happens if two PCs on the network both want to send to the Internet at the same time? The router's the common factor, and if PC-a is sending to it, PC-b cannot do so at the same instant. For data coming in to the network from the router, it's obviously a different matter, for at any instant the data can only be for PC-a or PC-b (the issue then is: is PC-a or PC-b already receiving from someone else?).
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2Mbps line delivers over 400kb/sec downloads!

Yes, however, even on a 10mb LAN, colisions will be so few maybe non existant.
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2Mbps line delivers over 400kb/sec downloads!

I'm not really disagreeing with that (and with a switch, of course, it's potentially a 20Mbps link, since full duplex is possible), but it does depend on what's going on -- has someone activated some protocol that does a lot of broadcasts; is there a duff network card leaking frames onto the network, for example? In other words, although the 10Mbps network should be able to cope with a 2Mbps ADSL connection, if there's other activity (which may not be achieving anything useful) already on the network, then 2Mbps may max it out.
Lorian
Grafter
Posts: 699
Registered: 31-07-2007

2Mbps line delivers over 400kb/sec downloads!

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A switch, as you say, maximises the number of collision domains, and therefore minimises the possibility of collisions. However, it's not capable of eliminating collisions -- as you say "MOSTLY stops collisions".

Depending on the switch It can remove the collisions completely. You need a swich that supports full duplex connections and a store-and-foward architecture. Then all the attached devices (assuming they are full duplex capable too) can transmit and receive at the same time. CSMA/CD is not used in this scenario.

CSMA/CD was originally invented to be used on a shared medium - Coax. Nowadays with Twisted Pair (cat5) connections TX and RX have their own separate wires.

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What happens if two PCs on the network both want to send to the Internet at the same time?

Store and forward on the switch is used to concurrently collect the packets from the two transmitting stations and send them to the router sequentially, so there is no cable contention to the router. The switch is acting as a buffer.

The only time you'll get an issue with this is when you attach one or more devices that are much slower, half duplex or 10mbs. Then your switch needs to suppot 802.3x flow control or you'll likely get data stalls. Although TCP/IP throttlling is supposed to cope wth this it often does not, even on very expensive switches.

My £22 8 port switch works well at 100Mb full duplex. Well worth an upgrade if throughput is an issue.

Jc.
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2Mbps line delivers over 400kb/sec downloads!

I thought someone might mention "store and forward", which is why I referred to not being able to access the destination device at the same "instant". Obviously, this is a better solution than having a collision, and then both parties having to send again.