Disaster Recovery

(see also Diversity & Avoidance)

Disaster Recovery is optional, and it does cost the customer.  It is normally accomplished through the use of redundant circuits, which can either ride the same path, or travel along two diverse paths.  

Private Line (T1, T3, etc) backup Methods

Private Lines, also called Dedicated Lines, or Point-to-Point circuits - are on and fully available 24x7.  Their bandwidth can be fully used at any given instant, or not at all.  They are priced using a flat-rate fee.  They are not usage-based.  Therefore, installing a second, backup private line is very expensive, especially considering that it may NEVER be used.  

Shadow Circuit - some companies offer what they call a shadow circuit, which is a second private line, that is not to be used except as a backup - therefore the vendor gives the customer a price break on it.  However, most vendors do not offer this, and even if they do, the price break is minimal since they themselves have to pay the LEC full access charges (whish is the majority of the cost for a private line and has to be paid at both ends).  The more common offering is a shadow PVS with Frame Relay or ATM, since this costs the provider basically nothing. 

Most Common PL Backup (ISDN BRI backup) - due to the high cost of an additional private line, many customers opt for a usage-based backup circuit, such as ISDN.  ISDN BRI circuits (2B+1D) come in two flavors - with 1 or 2 B (Bearer) channels for 64k or 128k, and each has a single 16k D channel for control and signaling.  

NOTE: We will not discuss PRI ISDN (same bandwidth as a T1, 23B+1D) with 23 64k B channels and 1 64k D channel) because they are "always ON" and their fees are not usage-based.  

The BRI circuits do carry a monthly fee, but it is minimal, since the service places a call only when needed.  Each call carries a fee based on how many minutes it remains connected for (same as your own residential voice phone billing - but more expensive since it carries data).  Click HERE for a document by AHK, detailing how a T1 private line can be backed up by a series of four, 128k BRI ISDN circuits.  The four BRI's are BONDed (Bandwidth ON Demand) together into one 512 k stream of data.

What about DSL or Cable?  DSL and Cable are primarily Internet access methods and they do not support Private Lines !!  Although it would be possible to convert these access protocols to emulate private lines, there are simply not enough customers for this, and therefore the telco's (the LEC's) do not offer it.

Automatic Backup vs Manual Backup 

The connecting device for a private line is a CSU/DSU, or in some cases just a CSU (T1 voice circuit between two PBX's, for example).  Automatic backup requires that the access device can sense when the primary private line goes down, and place the backup into service.

Manual - for manual transfer to a backup circuit, the customer would need to either use an A/B switch to transfer the connection to the secondary circuit, or physically swap the plugs into the CSU/DSU.  Of course, the customer would prefer to be notified in case of an outage, and the provider does not always call them immediately, since they rarely have an audible alarm - most NOC's (Network Operation Centers) have lights that show outages, and the operators do not keep their eyes glued on the lights 24x7.  Actually, the customer usually notices the outage first.  

Carrier Auto-Sensing with Manual Cutover - there are units that montitor the circuit for the presence of a carrier signal.  It will notify the customer of a downed circuit - they run SNMP, which causes a trap signal, and results in either a notification on a workstation or a page can be sent via an RJ-11 telephone port.  

For example, the following Adtran TSUe canm perform such a service - and if the operator is paged they will rush to the facility and manually cutover to the backup.

Carrier Auto-Sensing with Automatic Cutover to a Backup Private Line - (for automatic transfer to the backup ckt) - there are CSU/DSU units available that have two ports (one for a primary and one for a secondary circuit), that can sense when the primary circuit has gone down.  Due to high costs of the service, they are few and hard to find.  The service requires two private lines  . . . one of them dormant - hence it is a very costly solution since PL fees are not usage-based.  When this happens, they wait a certain amount of time, and then transfer to the backup circuit.  The reason they wait, usually about 15 seconds, is because the circuit may have just gone down briefly, and there is no need to go to the backup.

Carrier Auto-Sensing with Automatic ISDN Dialout Cutover  - (for automatic transfer to a dialup ISDN BRI or group of bonded BRI's).  This is the most popular private line backup solution. In many cases the customer purchases a single 128k BRI to backup a full T1.  True, the data rate is drastically reduced.  However, the philosophy is that they do not expect to be down for long, and they feel that they need at least a minimum connection to be available during the outage.  The service requires one private lines  . . . and a normally disconnected BRI line (or multiple BRI's).  When the primary PL goes down, the CSU/DSU waits a certain amount of time, and then places a call via ISDN to the remote site.  It uses the BRI connection, and continually checks the primary circuit until it comes back up - and then disconnects the BRI call and uses the primary circuit once again.  Again, the reason for the wait, usually about 15 seconds, is because the circuit may have just gone down briefly, and there is no need to go to the backup.

 

Switched (Layer 2) - Frame Relay, ATM Backup Methods

Not to be confused with switched voice circuits, FR and ATM operate using an access circuit, and PVC's (Permanent Virtual Circuits).  Customers can have a backup PVC provisioned, but it will not protect them in case of an access circuit outage.  

For true disaster recovery with these services, a secondary access circuit AND a secondary PVC (shadow PVC) will need to be provisioned.  Some FR and ATM networks have their own internal network management functions that will automatically reroute traffic, and redefine the PVC path if needed - therefore the most important backup consideration is the access loop.

TCP/IP (Layer 3) Backup Methods

The TCP/IP protocol includes routing protocols that support backup failover.  For telecom TCP/IP cirsuits are typically used to connect remote LAN's together (such as Ethernet networks), or for access to the Internet.

There are numerous ways to provide backup for TCP/IP networks.  Typically, a second backup circuit is provisioned either on a second router, or a second port on the same router.  The two circuits can then load share (send/receive data using both circuits) or the secondary can be dormant during normal operation, and will swing into service if the primary goes down.  Here are some of the most popular techniques used: