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High Availability Networking with DCB SR Async Frads

Merchant's Tire is a retail and wholesale tire distributor headquartered in Manassas, VA, servicing Virginia and the surrounding states. They run accounting and inventory applications on Sun servers, with dedicated frame relay links to their primary stores. These links are mission-critical and outages are not acceptable. The frame relay links use the DCB SR frame relay asynchronous frad multiplexers to connect the terminals and printers in each store to the host location.


Headquarters

One of the Remote Stores
A three tier approach was taken to provide two levels of communications link backup. First, each site is connected through two dedicated paths. Although the local loop at the remote stores is a single frame relay line, the MC I frame relay network connects it to two different T1 feeds to the host site. One of those T1 feeds through a fiber optic system, the other is through an alternate copper connection and the two trunks take different physical routes between the host computer site and the MC I network.. As discussed below, the system automatically switches sites from a failed link to the alternate when needed. A final backup is provided by analog dial-up.

This high-reliability system consists of equipment provided by Data Comm for Business and used with an Athena frame relay switch. First, some information on the equipment they used...

Frame Relay Switch

The Athena frame relay switch (at the Manassas, VA headquarters) supports up to 64 ports of RS-232 or V.35, with speeds up to 2 Mbps/port. It sustains up to 6000 packets per second, far in excess of the applications demand.The two fractional T1 trunks to the MCI frame relay network, each running at 256 Kbps fractional T1 are connected to the Athena on V.35 interface ports using DCB FT-2 fractional T1 CSU/DSU's.

Async FRAD Multiplexers

The Asynchronous FRAD multiplexers are DCB SR models that are capable of automatic switch over to a second DLCI when they detect a failure of their assigned primary DLCI. This is possible because of the use of Default and Alternate DLCI's set in the DCB SR's.

The picture at right shows some of the host site equipment racks.


Host site equipment

How It Works

Each of the MCI T1 trunks supports eight to ten SR Asynchronous FRAD multiplexers. Since Merchants has route diversity into the MCI network, where one fractional T1 trunk is fiber, the other fractional T1 trunk is copper, half of the SR's use the copper fractional T1 as their default DLCI and half of the SR's use fiber fractional T1 as their default DLCI. Each group uses the opposite T1 trunk as its backup.

Merchants configured two DLCIs in each SR. One of the DLCIs is the default, the second is the Alternate. If the SR is unable to communicate with the remote site SR on the Default DLCI, the SR automatically begins to communicate over the Alternate DLCI. The Athena switch then routes the SR data out the alternate T1 frame relay trunk.

The remote SR's also switch from the default to the alternate DLCI's (on the same 56 Kbps link) when the default DLCI loses communications to the host end SR. The SR's automatically seek the alternate trunk for backup with no manual intervention.

Since Merchants uses MC I as their frame relay service provider, all frame relay equipment used, (the SR asynchronous frad multiplexers and the Athena frame relay switch) have been certified at the MC I developers' lab in Dallas, Texas.

Dial Backup

Merchants also has at least 10 trunks with modems for dial backup in the event that both the default and alternate trunks go out of service. The busiest stores will be backed up via dial lines. Although the dial backup modems are not as fast as is available at 9600 bp s synchronous, they are very reliable.

Dial backup requires no change in the remote SR link management, since the Athena trunk connected to the modem is set to perform the required network link management. On dial backup, all that needs to be done is to make a voice call to the remote store, have store personnel change the A/B switch to the backup position, and turn on the back up modem. Backup dialing is initiated from the host site to the remote stores. At the host Athena switch, each modem trunk has two DLCI's assigned, one for the DLCI on the copper trunk, the second DLCI for the fiber trunk.

When initiating dial backup, the Athena must be programmed temporarily for the dial up route, since the Athena allows only one alternate trunk which is used for the alternate fractional T1 trunk. The dial link is a second alternate trunk. The Athena has scripting capabilities that make the dial back up process easy, taking just a minute or two.

Merchants uses an intelligent mix of multiplexers,routers and the internet to communicate with their large number of retail stores. For the higher volume stores, multiplexers and terminals are used. For some low volume stores, dial up through the Internet is adequate.

Merchants' Information Services personnel are highly trained and very technically competent. They know their way around multiplexers, T1, Frame Relay, routers and the Internet. They selected technology that best fits each application. Multiplexers, terminals and printers are still the best thing going when simplicity, speed, and cost effectiveness are desired in a WAN based transaction processing system such as point of sale use.

This system provides extremely reliable communications in a retail point-of-sale application that requires maximum reliability with the economy of frame relay links.



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