Because the network connections to ABC’s stores are mission critical, ABC’s Net II has included a back-up network connection to each store.The back-up connection is an integrated services digital network (ISDN). ISDN differs from PSDNin that it is based on circuit switching, which is expensive for data transmission because it create sand holds an open circuit between hosts. Users must pay to keep this ISDN circuit open, even if no data traffic enters the circuit. The maximum connection speed of ISDN is 128 kbps and the cost for each store’s ISDN connection ranges from $1,100 to $3,000 per month depending on the store location.