Evolution of Switching

As noted, the first switch was a switching matrix (board) operated by a human. The 1890s saw the introduction of the first automatic step-by-step systems, which responded to rotary dial pulses from 1 to 10 (that is, digits 1 through 9 to 0). Cross-bar switches, which could set up a connection within a second, appeared in late 1930s. Step-by-step and cross-bar switches are examples of space-division switches; later, this technology evolved into that of time-division. A large step in switching development was made in the late 1960s as a consequence of the computer revolution. At that time computers were used for address translation and line selection. By 1980, stored program control as a real-time application running on a general-purpose computer coupled with a switch had become a norm.

At about the same time, a revolution in switching took place. Owing to the availability of digital transmission, it became possible to transmit voice in digital format. As the consequence, the switches went digital. For the detailed treatment of the subject, we recommend Bellamy (2000), but we are going to discuss it here because it is at the heart of the matter as far as the IP telephony is concerned. In a nutshell, the switching processes end-to-end voice in these four steps:

1. A device scans in a round-robin fashion all active incoming trunks and samples the analog signal at a rate of 8000 times a second. The sampled signal is passed to the coder part of the coder/decoder device called a pulse-code modulation (PCM) codec, which outputs an 8-bit string encoding the value of the electric amplitude at the moment of the sample
   

2. Output strings are fed into a frame whose length equals 8 times the number of active input lines. This frame is then passed to the time slot interchanger, which builds the output frame by reordering the original frame according to the connection table. For example, if input trunk number 3 is connected to output trunk number 5, then the contents of the 3rd byte of the input frame are inserted into the 5th byte of the output frame. (There is a limitation on the number of lines a time slot interchanger can support, which is determined solely by the speed at which it can perform, so the state of the art of computer architecture and microelectronics is constantly applied to building time slot interchangers. The line limitation is otherwise dealt with by cascading the devices into multistage units.)

3. On outgoing digital trunk groups, the 8-bit slots are multiplexed into a transmission carrier according to its respective standard. (We will address transmission carriers in a moment.) Conversely, a digital switch accepts the incoming transmission frames from a transmission carrier and switches them as described in the previous step.

4. At the destination switch, the decoder part of the codec translates the 8-bit strings coming on the input trunk back into electrical signals.

Note that we assumed that digital switches were toll offices (we called both incoming and outgoing circuits trunks). Indeed, initially only the toll switches on the top of the hierarchy went digital, but then digital telephony moved quickly down the hierarchy, and in the 1980s it migrated to the central offices and even PBXs. Furthermore, it has been moving to the local loop by means of the ISDN and digital subscriber line (DSL) technologies addressed further in this part.

The availability of digital transmission and switching has immediately resulted in much higher quality of voice, especially in cases where the parties to a call are separated by a long distance (information loss requires the presence of multiple regenerators, whose cumulative effect is significant distortion of analog signal, but the digital signals are fairly easy to restore—0s and 1s are typically represented by a continuum of analog values, so a relatively small change has no immediate, and therefore no cumulative, effect).

We conclude this section by listing the transmission carriers and formats. The T1 carrier multiplexes 24-voice channels represented by 8-bit samples into a 193-bit frame. (The extra bit is used as a framing code by alternating between 0 and 1.) With data rates of 8000 bits per second, the T1 frames are issued every 125 ms. The T1 data rate in the United States is thus 1.544 Mbps. (Incidentally, another carrier, called E1, which is used predominantly outside of the United States, carries thirty-two 8-bit samples in its frame.)

T1 carriers can be further multiplexed bit by bit into higher-order carriers, with extra bits added each time for synchronization:

• Four T1 frames are multiplexed into a T2 frame (rate: 6.312 Mbps)

• Six T2 frames are multiplexed into a T3 frame (rate: 44.736 Mbps)

• Six T3 frames are multiplexed into a T4 frame (rate: 274.176 Mbps)

The ever increasing power of resulting pipes is depicted in Figure 1

Figure 1: The T-carrier multiplexing nomenclature.

International Interconnection (Inter Exchange Networks)

IXC backbone carrier facilities primarily use microwave and fiber transmission lines. Microwave systems offer medium capacity of up to several hundred Mbps communications with a range of 20-30 miles between towers. Fiber optic communication systems offer data transmission capacity of over one million Mbps (million million bits per second).

Microwave transmission systems transfer signal energy through an unobstructed medium (no blocking buildings or hills) between two or more points. In 1951, microwave radio transmission systems became the backbone of the telecommunications infrastructure. Microwave systems require a transducer to convert signal energy of one form into electromagnetic energy for transmission. The transducer must also focus the energy (using an antenna dish) so it may launch the energy in the desired direction. Some of the electromagnetic energy that is transmitted by microwave systems is absorbed by the water particles in the air.

Although the extensive deployment of fiber optic cable has removed some of need for microwave radio systems, microwave radio is still used in places that are hard to reach or not cost effectively served by fiber cable such as in developing countries.

Figure 1 shows a terrestrial microwave system-connecting IXC switches in Philadelphia and New York City. The microwave signals are moved between the two switching offices through a series of relay microwave systems located approximately 30 miles apart. Microwave is a line-of-sight technology that must take the earth’s curvature into consideration. Also note that microwave towers are not limited to only facing one or two directions. A single tower can be associated with several other towers by positioning and aiming additional transceiver antennas at other microwave antennas on other towers.


Figure 1: Long Haul Microwave

Fiber optic transmission is the transfer of information (usually in digital form) through the use of light pulses. Fiber optic transmission can be performed through glass fiber or through air. Fiber optic transmission lines are capable of extending up to 1200 km without amplifiers. Each fiber optic strand can carry up to 10 Gbps optical channels and a fiber can have many optical channels (called DWDM). Each fiber cable can have many strands of fiber.

Fiber cable is relatively light, low cost, and can be easily installed in a variety of ways. It does not experience distortion from electrical interference and this allows it to be installed on high voltage power lines or in other places that have high levels of electromagnetic interference.

Figure 2 shows common installations of fiber optic cable. This diagram shows that fiber transmission systems are installed along railway and natural gas pipelines, under water, and along high voltage lines.


Figure 2: IXC Fiber Optical Cable Installation Options

High Speed Switching Systems
IXCs use high speed switching systems to interconnect transmission lines. The key high speed switching system used in IXC networks is asynchronous transfer mode (ATM). ATM is a fast packet switching technology that transports information through the use of small fixed length packets of data (53 byte cells).

The ATM system uses high-speed transmission facilities (155 Mbps/OC-3 and above). OC-3 is the entry-level speed for commercial ATM. Higher speeds (such as OC-192) are used in backbone networks of IXC’s and other specialized service providers. ATM service was developed to allow one communication technology (high-speed packet data) to provide for voice, data and video service in a single offering.

International Interconnection
International interconnection issues include converting transmission line and control signaling formats, transcoding different types of digital voice signals, and rating billing records.

IXC networks must be capable of converting transmission line formats. These include digital signaling standards (e.g., T1 to E1), different optical standards (SONET and SDH), and command signaling protocols differences such as ISDN signaling differences.

Transcoding is the conversion of digital signals from one coding format to another. Transcoding is necessary because the digital signal companding process that is used for encoding/decoding signals is different throughout the world. This companding process increases the dynamic range of a binary signal by assigning different weighted values to each bit of information than is defined by the binary system. The A-law encoding system is an international standard and the uLaw standard is used in the Americas.

IXC systems must be capable of creating billing record in different formats. Billing systems in different countries use different rating systems (e.g., flat rate compared to time usage). It may be necessary for IXCs to receive and pay in different currencies and currency exchange rates for different countries rapidly vary. The payment or receipt of payments for calls routed through the IXC must be settled through clearinghouse companies that have relationships with many IXC, LEC, and PTT operators.

Several independent companies have installed or operate international transmission lines. These international circuits may be leased to IXCs or to independent corporations. Companies that operate these international transmission lines are often called international carriers (IC’s) or international record carriers (IRC’s).

Figure 3 shows an IXC network that has many international interconnections. This diagram shows that various transmission systems are used for interconnection. There are several high capacity switching points in these networks with redundant links between them. Some of the interconnection lines are operated by satellite and transoceanic cable/fiber carrier services provided IC/IRCs.


Figure 3: International IXC Interconnections

Switching Systems

Switching systems connect two (or more) points together. These connections can be physically connected (mechanical switch) or connected logically (through software).

The first telephone systems performed the mechanical switching of calls by human operators. The operators interconnected telephone lines by manually connecting cables at switchboards. These switchboards contained many wires that had plugs and the switchboard had many sockets for the plugs. To interconnect telephone calls at long distances, one operator would have to call other operators to setup the call. Setting up calls could be a complex process and this process got more complex as many more telephones were installed.

Switching systems have evolved many times over the past 100 years. The types of switching systems that are still in common use today include crossbar, time slot interchange (TSI), and packet switching.

Crossbar
Crossbar switches used mechanical arms to physically connect to wires (or busses) together. These mechanical arms (“Crossbars”) connect horizontal and vertical bars together to connect input and output lines together. Magnets are used to open and close the crossbar switch contacts.

Figure below shows a crossbar switching system. In this example, there is a matrix of lines (busses) where each input line can be connected to any output line. When a connection needs to be made, a mechanical switch connects one of the busses with the other busses. The disadvantage of this system is that the number of mechanical switches for connecting each input port to an output port exponentially increases with the number of ports that require connection. For example, a switch with 10 inputs and 10 output lines requires 100 switches. A switch that has 20 inputs and 20 outputs requires 400 switches.


Crossbar Switching

Time Slot Interchange (TSI)
Time slot interchange (TSI) switching is a process of connecting incoming and outgoing digital lines together through the use of temporary memory locations. In the late 1960’s, mechanical crossbar switching systems began to change to TSI digital switching systems. A computer controls the assignment of these temporary locations so that a portion of an incoming line can be stored in temporary memory and retrieved for insertion to an outgoing line.

Figure below shows a TSI switching system. This diagram shows a simplified matrix switching system. Each input line (port) is connected to a multiplexer. The multiplexer places data from each port in time sequence (time slot) on a communications line (e.g., a T1 or E1 line). This time multiplexed signal is supplied to a matrix switching assembly. The matrix switching assembly core has two memory parts: a section that holds the pulse coded modulation (PCM) data and Control Memory - CRAM that holds switching addresses data.


Time Slot Interchange (TSI) Switching

The time slots (voice channels) from the incoming multiplexed sent through switch S1 to be sequentially stored in the PCM data memory. The data is later retrieved by switch S2 and placed on a specific time slot on an outgoing line. The outgoing multiplexed line is supplied to a de-multiplexer so each time slot is routed to an output port.

Packet Switches
Packet transmission is a mode of data transmission that divides messages or data into small increments (packets) that can be routed through a network. When the packets arrive at their destination, they are reassembled in the proper order to recreate the original message or data.

Packet switching can be connection based or connectionless. For connection based switching, a path through the network is established during call initiation and packets are continuously routed through the same path. For connectionless switching, each packet is given a destination address and the switching points in the network (switching nodes) assist in routing the packet to its destination.

Figure below shows two types of packet switching in a communications system. Diagram (a) shows that connection based packet switching sets up a communication circuit prior to transmitting packets that contain data. Diagram (b) shows connectionless packet switching. Connectionless packet switching requires intelligent switching nodes (routers) that can decode the destination address and select the forwarding route based on the results of the lookup in the routing table. This diagram shows that packets of data arrive at the switch. The routing switch extracts the destination address and possibly the type of message.


Packet Switches