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New Member

Bandwidth Calculations & General WAN Help

Hello all,

I'm fairly new to working on WAN links, i've brought up a few T1's, and worked with some frame relay circuits, but really, am limited in my expertise in this area. I've got a few questions, on learning more about WAN links in general.

Currently we have a T1 in place for a remote office, and in total there are around 25 - 30 people working there. Generally, how many "users" does a T1 support. The reason I'm asking is because they are currently having issues where the line is very very slow, when there are a few people there doing normal day to day tasks such as checking email, and browsing the web.

There have been talks of upgrading the line to a DS3, not just to support expansion, but for data mirroring to this remote site. How many "users" would a DS3 be able to support?

I've never brought up a DS3 before, are there any things that I'll need to know in advance, configs, and such for our routers. We will be purchasing 2 new 3845's w/ NM/T3 cards in them to support the WAN link..

Lastly, are there any recommendations to books I can purchase in regards to planning, and upgrading lines, I really would like to know more about the "WAN" aspect of networking.

Thanks in advance for any help -


Re: Bandwidth Calculations & General WAN Help

The fundamentals of T-1 are that it has 24 timeslots of 64kBps channels. So at one given time you can have a max. of 24 users performing 64kBps of data troughput at any given time.. However for your 25 to 30 users it is fine unless they are hammering the T-1 with traffic (You have to assume that all of them are not transmitting traffic at the same time). You need to look at the line utilization off that T-1 on any given business day during peak hours. This will tell you the load on the T-1 and if you should consider an additional T-1 or up to a Ds-3. The price of a T-3 today is favorable in that if you require 4 to 5 T-1's the cost benefits you to order a DS-3. 4 to 5 T-1's will cost the same price as a DS-3 (other factors will imply). But that is useally the case... A DS-3 is 45MBps and you can look at it as 28 T-1's worth of bandwidth. If you are going to replicate data across this WAN link then you will need a DS-3. A T-1 will not be enough bandwidth for you. Bringing up a DS-3 is not hard to do. There are 2 types of framing. M-13 or C-bit. C-Bit is an unchannelized line and the line acts like one large pipe. M-13 breaks the line down to 28 T-1's. However you can find the correct WAN modules for the 3845 routers you have. As for any books or literature, I cannot recommend anything in particular. Experience is the best form of learning....Good Luck...

New Member

Re: Bandwidth Calculations & General WAN Help

Understood. I do know the line speeds, and fundamentals of T-1's, but really I deal more w/ the voice side of things w/ Voice T's, and voice PRI's.

Is there any tools that you can recommend to measure link utilization? Is there anything in the Cisco IOS software to help gauge the usage of the link?

Before ordering our routers, I just want to make sure that I'm getting the correct card to support the DS3. What was recommended to me was the NM-1T3/E3, but do I have to speak w/ our soon to be carrier about what type of framing they are using, or will the card support either?

Thanks again for the help :)


Re: Bandwidth Calculations & General WAN Help

You can enable Route Cache Flow on the interface and use a NetFlow Analyzer. There are some on the market that are cheap. However a NetFlow Analyzer will take the stats from the Cisco device which has NetFlow enabled on it and give you stats on link utilization as well as applications on that link, etc.. The quick, cheap and easy way is to use the SHOW INTERFACE command and look at the Load on the router link. It is stated as a ratio (255/255). I have attached a cheatsheet that looks at that ratio and provides to you a link utilization for the link type you are using. It comes in handy.. The 1T3/E3 module will work with both types of framing from your carrier.

New Member

Re: Bandwidth Calculations & General WAN Help

Again, thanks so much. After doing a show interface, it shows reliability 255/255, txload 44/255, rxload 19/255.

Which would I use to reference against the spreadsheet?



Re: Bandwidth Calculations & General WAN Help

If the stats were taken from the T-1 link then you use the column for T-1. Find the value of the load in the first column. The second column tells you the percentage of the link you are using and the column for the T-1 link tells you the amount of data the link has on it. Remember that the SHOW INT command shows you the link utilization from a 5min period (default). If you want to find out a more realistic view then use the Load-Interval 30 command under the interface. It will tally the stats every 30 secs. That will present to you a more realistic utilization of the link....Good Luck...

Super Bronze

Re: Bandwidth Calculations & General WAN Help

Without getting into all the flavors of WAN links, it helps to keep in mind, vs. typical LANs, that you're often working with much less bandwidth (usually due to cost), 100 to 1,000 times less isn't uncommon; your working with much more latency, 10 to 50 times more isn't uncommon; and often users consider LAN performance the network norm. The last point is why even if you install DS3s, user's might still consider the line very, very slow (with luck maybe without one or two verys).

Instead of simple rules for how many users might be supported per "X" bandwidth, which often works well on LANs, for WANs, it can be of benefit to analyze what you're users are doing, and various ways to satisfy that need (with attendant costs).

For instance, you mention users are browsing the web, if a large proportion is Internet, instead of clogging a dedicated corporate link with that traffic, a local inexpensive Internet connection (e.g. DSL, cable) might be of benefit.

You also mention users checking mail. Perhaps a local mail server might be of benefit.

For other WAN traffic, perhaps a WAN accelerator WAFS type product might be of benefit.

For some other services, perhaps you could replicate some servers.

For some, or all, of above, and especially perhaps with the upcoming data mirroring, a sophisticated QoS model could be of benefit.

The goal is to find the best combination that supports your network requirements at the least CAPEX/OPEX costs. Often with WAN, more bandwidth alone might not meet this criteria, as it often does on the LAN. (Don't misunderstand, there are times WANs require bandwidth increases, but often there are other methods, at less cost, to provide the same or better benefits.)

New Member

Re: Bandwidth Calculations & General WAN Help

Understood, currently we are working on upgrading the entire network infrastructure, from the cabling, to the core equipment used for connectivity.

Lets just say this site is still using 10mb hubs, and "thin net". So, we are currently in the middle of getting single mode fiber run to replace the actual wiring, then the L2 from 10 to 100 mbs. Then we'll be looking at the performace upgrades, and make our decisions based on what we find.