he signaling portion of the network is packet-switched while the part that carries customer data is circuit-switched. Out-of-band signaling can be extended all the way to the customer's premise, but only if a special path is provided for its transit. If it is not, the carrier must use a portion of the line for signaling. The customer, who is able to transmit only 56 Kbps of data, pays the price. Out-of-band signaling was first introduced by AT&T in 1976, when long distance competition forced it to become more efficient. Even in the '70s, AT&T's network was shared by millions of calls on any given day. Slow set-up times, and bandwidth wasted on failed calls, necessitated additional circuits. To streamline the process, AT&T devised a way to signal network status on a call-by-call basis. In order to identify available facilities, AT&T developed several huge databases of circuit inventories. These were accessed using a system known as Common Channel Interoffice Signaling (CCIS). CCIS circuit status requests were replicated on databases across a separate, analog packet-switched network, operating at speeds between 2.4 and 4.8 Kbps. Multiple routing-request packets streamed along the common highway, one after another. In 1980, the CCITT approved an updated out-of-band signaling system. Known as Signaling System 7 (SS7), it also relies on a separate, packet-switched network. SS7 differs from SS6 in that it is digital and operates at either 56 or 64 Kbps. Call set-up times are significantly reduced because the network is SO fast. The SS7 protocol has gradually developed over the last decade to become the "nervous system" of the PSTN. Almost all modern carriers have migrated the trunk side of their networks to SS7 but the line side (the part that provides local access service to customers) sometimes uses bit-robbed signaling. Central offices must be retrofitted to accommodate clear channel service, and upgrades can be expensive. Thus, bit-robbed (switched 56 and T-1) service is sometimes all that is available to customers