I wanted to learn more about telephone communication for a while, and finally I took some time to do that. Here I talk about modern telephony and make an overview of the telephony evolution history. I make a lot of references to “Computer Networks” by Tanenbaum. I don’t want to cover very low-level details related to physical layer data exchange.

Let me start with the Wikipedia definition:

Telephony is the field of technology involving the development, application, and deployment of telecommunication services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is intimately linked to the invention and development of the telephone.

Basics

All communications between telephones rely on Public Switched Telephone Network(PSTN). Now it also serves as Internet communication backend, PSTN is leveraged to provide individuals and organizations with access to the Internet.

The story began with Alexander Graham Bell, who invented the telephone in 1876. At that time this technology had good reception, so Mr. Bell founded Bell Telephone Company, later renamed to AT&T, to provide customers with telephone communication service. It revolved around creating five-level hierarchy PSTN system. The smallest unit located at the system’s edges is the end office used to joint subscribers' communication lines(two copper wires between an end office and a subscriber’s telephone). Such two-wires connection is called local loop.

When a service subscriber makes a call to the subscriber connected to the same end office as him, the switching mechanism of the end office constructs electrical connection between endpoints. In early days of the telephony, human operators conducted switching in end offices manually when a person at one end called another subscriber.

The next stage of telephony hierarchy is toll offices. They provide the same function as end offices. If a customer makes a call to his endpoint, and they both don’t share the same end office, the caller’s end office connects to the toll office their end offices share. This can be depicted with following picture.

Alternatively, if there is not any single toll office providing communication for two phones, intertoll trunks connect separate toll offices.

After AT&T breaking up due to monopoly issues, USA was divided between LATAs (Local Access and Transport Areas). This is similar to Internet’s Autonomous Systems, abbreviated as AS. Autonomous System is a chunk of IP addresses belonging to corresponding ISPs. A LATA covers an area of specific area code.

Antimonopoly court cases also introduced InterXchange Carrier(IXC) to the American telephony system. They have the same functionality as BGP(Border Gateway Protocol) for any AS. In short, BGP’s purpose is to explore adjacent routes from computers located in one particular AS to computers in another. It discovers all gateways between different autonomous systems.

As a reminder, the current Internet infrastracture depends on telephony systems: when ISPs provide their service to customers, they reuse telephone local loops installed to their homes, and supply them with routing equipment(modems, routers, whatever) to transform analog signal to digital.

Analog signal is different from digital signal in the way it is represented and measured. Consider some light signal as a light ray, you have to use measuring device to check light intensity. The outcome value you get from the device can greatly vary from, say, “very dim” to “very bright”. And your device can report a lot of different values, depending on its accuracy. Ideally infinite amount, if you represent the range between “very dim” and “very bright” as a set of real numbers. Any stream of bits is digital signal, whether it is a file on your computer’s disk drive or network traffic. This type of information is measured in finite and discrete units. You can definitely distinguish between differently positioned bytes in a piece of information, and any byte or bit has finite number of states. Shortly, analog signal is something that can have different, or infinite degrees of precision. Most of the times analog information is not anthropogenic, it wasn’t produced by human beings. Digital signal can be measured in definite and discrete values.

The backbone of the telephony systems deploys two switching techniques: packet and circuit switching. Modern systems are beginning to use packet switching with advent of VoIP communication. The type of switching greatly influences information transfer at network layer.

When a person makes telephone call, the telephony system, using circuit switching, preliminary discovers the path all communication nodes(end office, toll office…) use to deliver the data through. This has big advantage: the data traffic will unlikely be affected by congestion. At small expense of time passed to construct the circuit.

The packet switching is different. As soon as your phone discovers the first node to talk with, it sends packets right then to it without constructing any circuit, choosing its peer while you already talk with someone. The same is true for all computers that transmit your traffic, till the destination is reached.

With packet switching, your connection is resilient to network faults. When any communication node goes down, the preceding node just transfers the traffic to the other node. Circuit switching can’t handle such occasions, and it has to terminate connection because it uses a fixed path.

Packet switching is more prone to network congestion, and out-of-order traffic arrival.

Overview of mobile communication standards

Now we can look at mobile phone system. There are 5 generations of mobile phone data transmission, from 1G to 5G. Each one of them can transmit:

  • 1G - analog voice
  • 2G - digital voice
  • 3G - digital voice and other types of digital data
  • 4G - even more digital data due to higher transfer rate
  • 5G - mission critical data exchange, data exchanged between large number of interconnected devices

4G also abandoned circuit switching, it can use IPv6 addresses, and features femtocell. This is the same thing as ISP routers but this one is used for home mobile phones communication over TCP/IP.

Every generation of mobile phone communication is built around the notion of cell - specified wide area designated by your phone carrier. Every cell is appointed to its own radio frequencies set, which must not be the same for adjacent cells, note this might be a bit different for 4G and 5G because I’m too lazy to check everything. This comes as advantage for cellular systems because this is why they can dedicate more data streams for mobile communication while deploying the same frequencies. Again, this is true as long as adjacent cells don’t use the same frequencies.

For 1G and 2G, every cell has a base station or BSC(Base Station Controller) all phones in nearby area connect to(practically, there can be more than one base station). All base stations connect to the MSC(Mobile Switching Center). There can be multiple first-level MSCs connecting to second-level MSCs in regions with higher communication density. MSCs communicate with telephone end offices.

Cellular networks, AMPS(Advanced Mobile Phone System) in case of 1G, use 800+ duplex radio channels divided between four categories:

  1. Control channels used to control the network
  2. Paging channels which alert phone users of incoming calls
  3. Access channels used to initiate phone calls and choose channel for this
  4. Data channels which transmit voice and data.

With advent of 2G, new standards and systems for mobile communication emerged. One of the most popular 2G system is D-AMPS, as you can see by its name is just a Digital remake of AMPS, and GSM.

At the time of its introduction, GSM was only Europe-based communication system made as an attempt to create the universal 2G standard. Later it stretched over the world. GSM is mainly an AMPS clone, but anyway it has some modifications. In GSM network any phone must contain a SIM(Subscriber Identity Module) card to make calls. SIM cards make it available to authenticate a user to the network and encrypt data communications. GSM introduces Home Location Register(HLR) and Visitor Location Register(VLR). The first one is database that registers last known location of each mobile phone, the second one stores information about current users within an MSC.

There are 2 popular industry standards for 3G: UMTS(Universal Mobile Telecommunications System) aka WCDMA(Wideband CDMA), and CDMA2000. CDMA is just a name to physical layer multiplexing method. 2G and 3G networks are nearly identical.

Now follows the more accurate description of UMTS/GSM. These networks consist of 3 parts: Mobile Station(MS), Access Network(AN), and Core Network(CN). In fact mobile station is just a handset any subscriber uses to connect to mobile phone network. A mobile station communicates with AN. I already described AN internals above, though in 3G the terms are different. CN is divided in two parts, one serves circuit switched communication, which is used for telephone calls. The other part is used for packet switched data transfering which gets redirected to Internet.

RNC stands for Radio Network Controller, RNC transmits data from base stations(NodeB in 3G terminology) connected to it to according subpart of Core Network. Core network can be connected to Internet and PSTN or other networks.

4G deploys new data transmission methods at physical layer. OFDMA(Orthogonal Frequency Division Multiple Access) is one of them. For this time, 4G-based networks are dramatically different from 3G design, mostly due to complete turnover to packet-based switching favored over circuit-based option. They didn’t change the network architecture only, but also all definitions from pre-3G are purged and replaced for others. LTE is one of a few known 4G networks.

The LTE network is called EPS(Evolved Packet System). It is composed from E-UTRAN, the subsystem similar to Access Network, and EPC(Evolved Packet Core) which is the same as Core Network. UE is User Equipment, eNodeB is a new term for base station. In LTE network, eNBs are not connected to higher hierarchy entities, instead they are interconnected and send and receive data to EPC on their own. MME(Mobile Management Entity) fullfils functions of user authentication, storing information about current subscribers within an MSC, like VLR did, and selection of most appropriate S-GW for each user based on its location. HSS(Home Subscriber Service) keeps records of information on all subscribers. S-GW is the routing gateway between E-UTRAN and EPC. P-GW(Packet Data Network Gateway) is the gateway connecting to an outside world network, such as Internet. PCRF(Policy Control and Charging Rules Function) is software element of policy enforcement and flow-based subscriber charging rules. It can be used to support different QoS(Quality of Service) levels and data stream managing in real-time.

5G utilizes higher frequency waves spectrum to achieve greater data transfer rates. It makes use of 5G NR(New Radio), which is a new standard for radio transmission in 5G networks. 5G NR operates in 2 following modes - non-standalone(NSA) mode and standalone(SA) mode. As of the end of 2020, there is no standard for new 5G Core Network. Until the time comes when it’s published, existing 5G prototype networks can use only NSA mode. That is, all such systems depend on existing 4G networks to transmit certain kind of traffic, before they get fully replaced by their 5G counterparts. If this happens, such networks will operate in SA mode with no dependency on 4G systems.