Question on Routers connected directly to a Frame Relay switch

[SecureBlob]

As per Cisco documentation at  

"You can create Frame Relay connections using one of the following hardware configurations:

1. Routers and access servers connected directly to the Frame Relay switch
2. Routers and access servers connected directly to a channel service unit/digital service unit (CSU/DSU), which then connects to a remote Frame Relay switch"

My question is on point number 1.

• In this configuration, what kind of transmission media is connected between the Router to Frame Relay Switch? 

Here is the link i am referring to.

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/wan_frly/configuration/15-s/wan-frly-15-s-book/wan-cfg-frm-rly.pdf

[Georg Pauwen]

Hello,

they probably refer to routers with DSU/CSU WAN Interface Cards, which means the DSU/CSU is integrated into the router rather than having to connect to an external CSU/DSU device. Check the link below for reference.

https://www.cisco.com/c/en/us/td/docs/routers/access/interfaces/ic/hardware/installation/guide/dsu_wic.html

[SecureBlob]

Thank you. It make sense now. 

I have one more question though. Please have a look at the link below.

https://www.cisco.com/c/en/us/support/docs/wan/frame-relay/14193-frbacktoback.html

It shows how to connect 2 routers back to back to simulate a Frame relay. They do it by configuring one Router as DTE and another one as DSU. To configure the Router as DSU, they will set the clock. If a router does not have a Serial interface it can be added as an additional module. Once that is done a Serial cable is used to connect 2 routers and simulate a Frame Relay by setting Frame Relay as encapsulation protocol on the Routers.

• So in the above case, do you think a WAN card is required?
• Or Router can work like a frame relay switch by just configuring it? 
• Do this kind of back to back connection is used only in test set ups? or are there cases where, a router in customer premise is connected to a frame relay switch using a Serial Cable?
• As per some documentation Serial cable maximum length is some 50 feet. So would there is cases where customer uses just a serial cable to connect to a frame relay switch if the Frame Relay providers Central Office is very near to Customer Premise?

I am very new to this field and trying to learn how these things work in the field. 

[Joseph W. Doherty]

"So in the above case, do you think a WAN card is required?"

If you mean integrated CSU/DSU, I believe no, but if you mean a serial interface, I believe yes.

"Or Router can work like a frame relay switch by just configuring it? "

I don't believe it can fully emulate a FR switch. (I'm thinking of it forwarding FR's frames along a VC path, also setting congestion bit in frame, etc.)

"Do this kind of back to back connection is used only in test set ups? or are there cases where, a router in customer premise is connected to a frame relay switch using a Serial Cable?"

As far as I know, would only be used as a test or lab setup.

"As per some documentation Serial cable maximum length is some 50 feet. So would there is cases where customer uses just a serial cable to connect to a frame relay switch if the Frame Relay providers Central Office is very near to Customer Premise?"

In theory, yes, but in practice, unlikely you'll find a FR provider's FR switch within fifty feet. Normally you had a leased line to the FR network (on both ends).