Data Transport: Circuit-Switching and Packet-Switching

For many years there has been a migration from traditional voice-centric networks to systems optimized for data.?This does not mean that voice goes away altogether.?What this does mean is that traditional circuit-switched (bearer channel) is becoming less important as voice is increasingly packetized and transported over Internet Protocol (IP).

Circuit Switching

The Public Switched Telecommunications Network (PSTN) that supports Plain Old Telephone Service (POTS) has historically relied upon circuit switching, which entails seizing a telecom resource (or circuit) such as a T-1 channel facility for the duration of a voice/data session, even when there is no voice or data being transmitted.

Traditional PSTN-based circuit switching relies upon Time Division Multiplexing (TDM), which is an in-band signaling methodology in which transmission of information occurs by means of synchronized switches at each respective end of the signal path. This was the case initially with toll-free calling, started in 1967 by AT&T as an alternative to collect calling and to reduce the need for operators.

Through its Inward Wide-Area Telephone Service (IN-WATS), AT&T was the first company to use toll-free lines involving hosted numbers for major companies.??Initial growth of toll-free calling was slow until the creation of remote databases within AT&T's network in the 1980s and leveraging query/response mechanisms and out-of-band signaling known as CCIS6.

By leveraging CCIS6 signaling and remote databases, businesses could have a single nationwide toll-free number instead of requiring different numbers for each state.?Toll free calling is accomplished within the network by administering and mapping toll free numbers (800 numbers) to real, network-routable numbers with a remote database.

While CCIS6 was closely controlled by AT&T, its successor, Signaling System # Seven (SS7), was intended to be widely used by any company in any network. Every call in every network became dependent upon the ISDN User Part (ISUP) portion of SS7 for call set-up and tear-down. Prior to out-of-band signaling, trunk signaling was performed very inefficiently on an "in-band" basis via low bandwidth dedicated trunks.?

In addition to the role of SS7 in trunk signaling, every mobile cellular phone user became dependent on the Transaction Capability Application Part (TCAP) portion of SS7 to facilitate cellular service validation, inter-system mobile roaming and automatic call delivery. SS7 became the vehicle for signaling between switches and databases and other platforms associated with enhanced services enabling the advent of intelligent networking.??

SS7 would also become the "glue" to stick together circuit switched (traditional) networks with packet switched (IP based) networks. Signaling between SS7 and IP networks for voice communications requires protocol conversion and address mapping via Electronic Number (ENUM), which maps an E.164 (regular phone number) to an Internet Protocol (IP) address.

Packet Switching

The World Wide Web uses a form of packet switching to transport data and information between devices. Next generation telecommunications networks also use different forms of packet switching for next generation infrastructure and services.

With packet switching, information is sent in bursts called packets, providing more efficient operation. One example of a packet-switched protocol is Transmission Control Protocol (TCP) and the Internet Protocol (IP), commonly referred to as TCP IP, which is relied upon by IP-based technologies to interconnect network devices on the public Internet and private IP-based networks.

SIP represents a Packet Switched, Protocol relied upon for Signaling and Control in IP-based Networks

Session Initiation Protocol (SIP) is an important example of packet switching used for signaling in support of Voice over Internet Protocol (VoIP) as well as many elements of next generation ICT infrastructure such as IP Multimedia Subsystem.

SIP is principally a mechanism employed to seamlessly create, modify and terminate sessions involving multiple participants. Such sessions could be VoIP calls, multimedia conferences or multicast sessions. SIP can work with any type of media content. SIP is independent of the transport layer and can therefore be used with multiple transport protocol.

Packet Switching vs. Circuit Switching

SS7 routes packets over finite, dedicated routes on private networks that are closely controlled, typically by legacy carriers. Instead of relying upon a vast public "web" of interconnecting facilities and routing equipment, SS7 networks are private and logically self-contained. The private nature of SS7 networks has been a purposeful design consideration for the sake of security and reliability.

Unlike circuit switching, which uses dedicated and finite paths for transmission (from point “A” to point “B”), packet switching takes advantage of a multitude of routing schemes that allow information to be routed over many diverse paths to get from origination to destination. Compared to traditional circuit-switched networks, packet switching offers many advantages including greater economies of scale and cost savings.