Telephone network and access network

For PSTN ( Public Switched Telephone Network ) is the system that allows them to interconnect between fixed line phones in every (?) house, from which evolved all telecommunications developments. While there we will develop the ' analysis of the methods of switching and routing with which it has identified the path taken by a communication between the input and the output from the transport network , on the contrary we will discuss in the next sections of the methods of multiplexing with which the individual communications terminating at the interfaces user / network are aggregated by the local exchanges to use the same connection of junction inside the transport network . The access network is the most relevant part of the same and expensive, and consists of the pair that reaches the home telephone jack. On this well-known services are available in their entirety as

POTS ( Plain Old Telephone Service )
ISDN ( Integrated Service Network and Data )
ADSL ( Asimmetric Digital Subscriber Loop ) - which in fact uses the PSTN just as a conduit to reach an IP network

In addition to these, are now covered in the access network also other types of connection, such as

Optical access - such as FTTH ( Fiber To The Home ), and allows

to interconnect a set of more numerous connections POTS already multiplexed together, as in the case of a large corporate switchboard, overlapping in order to access an ISDN-PRI
to interconnect to an IP network at a speed greater than that allowed by ADSL technology

radio access - as in the case where a mobile operator checks to a mobile phone also a number of fixed network, which is valid in a limited range in the vicinity of their residence, and that in fact replaces the twisted pair, with the coverage area of a given cell

Other types of deals instead can not be considered for access to the network, even if realized by exploiting both the access network that the transport, as in the case of

CDN ( Direct Numerical Circuit ) - provides direct and continuous connectivity with another (very specific) user interface / network, and therefore fails the switching component
VPN ( Virtual Private Network ) - as above, with the difference that in this case is based on the connectivity of a communication packet rather than circuit

Multiplexing in time division

Multiplexing more direct communications to the same destination, so that they share the same transmission medium, can achieve the advantages of

groped to occupy all the bandwidth made available by the transmission medium
maximize the percentage of use of the transmission medium, in the case of sources not continuously active
simplify management and maintenance of long distance connections, as these are fewer in number

We approach the study of multiplexing techniques, studying first at the differences between the problems associated with a network for data transmission, and those more typical of the telecommunications world:

The data package (8.5.1). Multiplexing scheme (8.5.2.1). statistical multiplexing .
Switching network circuit (6.1), ( see below ). Time multiplexing numerical (6.2). Plot PCM (6.2.1).

At this point, we explicitly noted that a result of the theory of traffic shows how the adoption of a strategy oriented to the delay, ie a pattern of statistical multiplexing in which there are queues, improve significantly the efficiency of use of the medium itself.

Reporting in the transport network

After discussing the user signaling that occurs in POTS and ISDN, we note explicitly that communications between the central transit adopt other solutions, suitable to describe the state of all the communications that come multiplexed connections of the junction, and which are routed on multiple outgoing links.

Associated Channel (and Common Channel) Signalling - or CAS and CCS (6.2.2), ( see below ); Synchronization of central (6.2.3).
Architecture of the telephone network .
Signalling System No. 7

Multiplexing hierarchies

Gradually the transport network is interconnected via a central hierarchical level, associated with larger geographic areas of influence, the links joining carry a number of increasingly high tax obtained by grouping together all the conversations at once directed toward the same destination . Whereas at the same time also the problems associated with having to play in the central switching function, there confronts the problem of finding efficient methods to group together more tributaries, also at different speeds, also doing so that the operation of insertion / removal of a single tax is relatively easy.

Asynchronous and plesiochronous multiplexing network (6.3), bit stuffing (6.3.1).
PDH
SDH

POTS

The basic telephone service is to connect audio banda in the telephone channel with a remote user terminal, and reporting (referred to as a user signaling) required to establish the connection. When the local exchange to make the phone ring, send pair on an alternating voltage that activates the alarm. When the telephone handset is lifted, the phone closes a switch that determines the flow of current in the subscriber loop , indicating the response from the called party. Inside the phone there is a particular transformer four-port, in capable of separating the input signal from the output shaft, so as to send the first all'altoprlante, and to send to the second one of the microphone. to dial the number, until the 80 were in use the dial discs , which, by opening and closing the switch, determined a pulsed waveform, in which the number of pulses corresponded to the entered digit. This mechanism is directly related to the presence, in the telephone exchanges of the first generation of stepper motors that drive the derminavano of central switches. The rotary dial was then superseded by the current numeric keypad DTMF ( Dual Tone Multi Frequency ) in which each key is associated with two frequencies that identify the number (or the * and #) key depressed, as described by the figure in the lower left. Conversely, the user signaling in the central management -> user takes place by means of a code based on the audible tones, whose durations are described in the figure at the bottom right.

Report user
User -> Central	Central -> User

The result of the signaling messages of user is exemplified in the figure, in which it is shown how each conversation is actually composed of three phases are essential:

formation of the connection ( call setup ), in which they are carried out the functions of addressing, and are reserved by the network the necessary resources to the communication
HOLD ( hold ), during which the resources involved are used exclusively by the parties in conversation
exit ( release ) in which the resources committed are released

The transition from telephony analogy to the numeric, wherein the speech signal is sampled and quantized as PCM, has not in fact altered the presence of these three stages.

ISDN

The Integrated Service Data Network is a mode of access number to the telephone network, defined by a set of standards available from the ' ITU . ISDN in the A / D conversion is done at the user, who can connect to the same bus (ISDN S interface a four-wire, using a line code AMI), several devices numeric, or also analog, interposing a device for the latter said terminal adapter (TA).

Access to the network by the device NT ( Network Termination ) connected to the twisted pair, corresponds to the U interface , on which is transmitted a four-level signal known as 2B1Q , which are standardized for the two different transmission rate. In the mode of so-calledBasic Rate (BRI, Basic Rate Interface ), you provide a link numerical banda base to 144 kbps, which is placed a frame structure that houses two voice channels (B1 and B2, from Bearer , with PCM data) at 64 kbps, in which the transmission takes place without interruption, and a data channel (D) to 16 kbps, in which the transmission takes place in packet mode, and in which there are the informationof signaling , such as the protocol Q.931 . In the mode of Primary Access (PRI, Primary Rate Interface ), intended for connecting PBXs, you have 30 B channels (voice) at 64kbps, and a D channel (data) signal at 64 kbps.

Since the ISDN preserves the binary stream sent on channel B end-to-end network, on those same channels can also be sent information any voice at all, but rather numerical natively, provided that the recipient share the same mode of interpretation of the bits Incoming. Lard this possibility, for example, are defined the first video telephony standard H.320 .

Statistical multiplexing

In the case of the transmission packet data, is not necessary to engage the transmission medium in an exclusive way, but the transmission can take place in mode sporadic, and the data sent at irregular intervals. This reason, together with the variable size of the individual communications, leads to divide the communication in autonomous units referred to as packet data . The transmission of packets can be multiplexed in the form (ie, sharing the same transmission medium between multiple communications) so statistical , or without reserving resources at exactly this or that tributary. In fact, in this case, the multiplexer is limited to insert the packets received in appropriate queues , from which picks them up for them to be transmitted in sequence. The presence of queues, involves

the occurrence of an unpredictable and variable delay
the possibility that the queue is full, and the incoming packet is discarded

Conversely, if each packet carries with it the information necessary for its delivery, the transmission grid does not need a special setup stage of routing: the case of packet switching in datagram , each packet is a special case itself.

Circuit Switching

The public telephone network ( PSTN ), is historically based on the principles of circuit switching , and on the time division multiplexing with frame structure . In the early days of telephony, ie in the era of mobile crank, with the receiver on the wall The communication was based on creating a true own electrical circuit through the action of a human operator, physically linking together the ends of different users. In the case where multiple telephone operators in cascade, the call is routed through multiple exchanges. Since then, the term circuit switched identifies the case in which

phase is required setup prior to the actual communication, in which the resources are reserved;
in the setup stage is also determined the ' routing of the call within the network, which remains the same for the entire duration of the same;
transmission resources are engaged exclusively for the duration of the conversation.

Things are not changed much (from a conceptual point of view) with the advent of the telephone number: in this case, more voice signals are sampled and quantized in a synchronous manner, and the result (numeric) ismultiplexed in a plot PCM , in which it is reserved for a time interval for each of the tributary flows.

The signaling in telephone networks

In a telephone exchange the governing bodies process the signaling information in order to control the switching devices , so that the communications in transit being routed towards the output port that faces the seam correct. Thus, in the case ofchannel-associated signaling , the signaling information are re-entered in the output frame which carries the communication routed, so they can be used also in the subsequent central transit . In the case of signaling to the common channel instead, the signaling messages traveling in a dissociated from the corresponding communications, and are transmitted on a signaling network separate from that on which travel communications, with a transmission mode packet .

Architecture of the telephone network

Signalling System No. 7

The Signaling System # 7 (SS7) is a set of telephone signaling protocols common channel, used to control the majority of telephone calls in the PSTN world, which in this case takes the name of Intelligent Network (IN). In addition to managing the phases of establishment and reduction of the call, allows other services such as redirection, pre-paid cards, SMS, toll free, conferencing, call back on busy ...

SS7 is described by the series of recommendations ITU-T Q.700 , whose members are also regional variations described by other regulatory agencies. The SS7 messages are transferred via digital connections between signaling entities, housed in telephone exchanges, designated by the terms of

Service switching points (SSPs), which terminates the signaling of user, and sends a query to the SCP to determine how to handle the service request;
Signal Transfer Points (STPs), which routes SS7 messages between the various entities of the IN;
Service Control Point (SCPS), which queries a Service Data Point (SDP), which in turn holds a database that identifies the geographic number to which the call should be forwarded. Alternatively, the SCP can determine the ripoduzione prerecorded message, or request additional input from the caller, according to ' Intelligent Network Application Protocol (INAP) , which operates on top of the Transaction Capabilities Application Part (TCAP) of SS7 protocol stack.

In addition to the entities that take part in the architecture, SS7 is also defined in terms of the protocol hierarchy that describes the layering of functions necessary to accomplish the required serizi. The simple exchange of messages between entities is based on a packet-switched network, and is based on the procedures referred to collectively as the Message Transfer Part (MTP), responsible for the reliable delivery of SS7 messages between the communicating parties. The functions of MTP are stratified on three levels, from bottom to top, dealing with aspects of transmission between the entities, the error handling in order to guarantee reliable communication, and routing the messages between entities.

Above the MTP can operate several procolli indicated as User Part, such as the Signalling Connection Control Part (SCCP), which enriches the functionality of the network, offering ulteriri addressing capability, and a service-oriented rather than allowed to package; through SCCP application processes can operate based on the Transaction Capabilities Application Part (TCAP).

Other examples of User Part are the Telephone User Part (TUP) and ISDN User Part (ISUP). TUP was the first to UP essre defined, and provides support through the provision of PSTN SS7 network. Currently it is almost everywhere replaced by ISUP, which offers other services, such as caller identification, and that can communicate with the MTP also by means of SCCP.

If the interconnection network between the entities of the IN is an IP network, then have to consider the additional protocols referred to as SIGTRAN .

Plesiochronous Digital Hierarchy

(Taken from Wikipedia ) Different countries have different speeds defined for the levels of gerachia multiplexing PDH, and even though these are comparable with one another, if it makes the interoperability problem, as reported in the following table, in which are indicated codes classification of different cases:

Level	North America		Europe		Japan
Level	Mbit / s	Classification	Mbit / s	Classification	Mbit / s	Classification
1	1.544	(T1)	2.048	(E1)	1.544	(J1)
2	6.312	(T2)	8.448	(E2)	6.312	(J2)
3	44.736	(T3)	34.368	(E3)	32.064	(J3)
4	274.176	(T4)	139.264	(E4)	97.728	(J4)
5			564.992	(E5)	397.200	(J5)

Add and Drop Multiplexer (ADM)

The operation to add and remove is a basic function in a multiplexing system, and consists of inserting and / or extract a single tribuario from the multiplexed stream. The main disadvantage of a PDH system is that to extract a single tributary from a flow multiplexed to the upper hierarchy, you must perform an inverse operation to that of multiplexing or demultiplexing the entire flow, including other tributaries (loops), and then re-multiplex all again.

This feature greatly limits the flexibility in network configurations that can be obtained with this technology, and for the tributaries passing involves the addition of a delay time due to the additional operations of multiplexing and demultiplexing. In practice, today they are used only flows E1, E3 and E4, which are those most likely to be transported in the synchronous hierarchy SDH multiplexing tributaries directly sixteen 2 Mbit / s within a single stream 34 Mbit / s.

Synchronous Digital Hierarchy

Instead of PDH, the ' SDH has only a variant (in North America), referred to as SONET ( Synchronous Optical Network ), whose levels are labeled STS or OC in the case in which we refer to the corresponding optical signal, and that interoperates enough well with SDH. The following table lists the speeds associated with different levels of the hierarchy of multiplexing SDH / SONET:

SONET Frame Format	SDH level and Frame Format	Payload bandwidth (kbit / s)	Line Rate (kbit / s)
STS-1		48.960	51.840
STS-3	STM-1	150.336	155.520
STS-12	STM-4	601.344	622.080
STS-24	STM-8	1,202,688	1,244,160
STS-48	STM-16	2,405,376	2,488,320
STS-96	STM-32	4,810,752	4,976,640
STS-192	STM-64	9,621,504	9,953,280
STS-768	STM-256	38,486,016	39,813,120
STS-1536	STM-512	76,972,032	79,626,120

The structural difference compared to PDH, SDH tributaries is that all use the same clock, hence the ability to add and remove a single tax without affecting the flow in which it is immersed, as exemplified in the following figure.

SDH was established in order to allow the transport of data of different origin (PCM telephone, ISDN, Ethernet and IP packets, ATM cells), as shown in the figure, which is stacked the various processes that the tributaries must undergo in order to be placed flow SDH. SDH is based on a frame structure of a duration of 125 usec, during which are transmitted in a byte interleaved a sequence of octets from different tributaries at 64kbps who share the same timing source, so that each tributary can be inserted / removed simply by writing / reading always at the same point (with the same phase) an octet each plot. The lowest level of the hierarchy is referred to as STM-1 ( Synchronous Transport Module ), and operates at a speed of 155.52 Mbit / s , equivalent to the multiplexing of 270 flows PDH E1. STM-1 is in fact characterized by a texture composed from 2430 octets, of which 81 signaling and 2349 data, namely using a signaling octet every 30 total, almost as is the case for the flow PDH E1 (in which there is an interval signaling, 16 °, each 31 octets). Octets signaling, however, are now grouped in groups of nine, followed by 29 * 9 = 261 bytes of data, and the result is traditionally represented incolonnando the 9 sub-sequences of 270 octets as in the figure, thus representing a plot as a matrix of 9 rows by 270 columns. The first 9 columns are called overhead of the plot, while the data is indicated as a payload (or payload). The overhead contains signaling information closely related to the process of multiplexing, that is designed to carry out functions of OAM ( Operation, Administration, Maintenance ), which are now associated with a nesting of the sections of transmission: Path ,Multiplexing and Regeneration . In fact, the path (Path) made from a single tributary, winds through a single input multiplexer and demultiplexer a single output, but each Add and drop multiplexer (or switch) met, a new section is called multiplexing. Similarly, for each repeater regenerative met, there is defined a new section of regeneration. For each of these sections, is defined an OH specific for the operations associated with OAM. Since an ADM is also regenerator, and that the output of the tributary input devices are also ADM, it is determined the functional stratification for signaling depicted by side, where it is shown that the overhead associated with the outer sections to be stacked one on the inner sections. But unlike its encapsulation formats packet transmission, in this case the three types of Overhead (Path POH , Multiplex Section MSOH , Regenerator Section RSOH ) are inserted into the STM-1 frame at different points, as shown by the following figure . In the first three rows of the overhead of the STM-1 RSOH is placed, which is written from the devices of regeneration, and then read and re-written next to each regenerator, in particular, to the first row are the flags that allow to acquire the timing of the plot. In the last five lines oh, we find the MSOH, written, read and re-created by multiplexing devices.The POH is placed inside the payload, and return from this brief. OH in the fourth line of the weft, we find a pointer ( Pointer AU ), which specifies the start position of the payload (now called AU, or Administrative Unit ) within the frame structure. The presence of this pointer is derived from the desire to minimize the use of buffers and the introduction of delays in delivery; therefore the data to be transmitted are placed in the NON-frame structure at the beginning of the same, but to the first octet can at the time of availability of the data. Thus, it is more than the normal case where the AU beginning in the middle of a frame, and terms in the middle of the subsequent frame. The pair AU AU and then takes the name of Pointer Administrative Unit Group (AUG). Filling the AU with the reporting requirements, the case (see figure) following a series of steps, which first saw the creation of a data structure called container , to which is added the POH to obtain a Virtual Container, from which the AU, after the addition of the pointer. Now note that not necessarily the AU must be filled from a single tributary, on the contrary, the multiplexing is used to precisely accommodate different! For this purpose, more VC at low speed can be in turn multiplexed mode byte interleaved, to produce an intermediate data structure indicated TU ( Tributary Unit ), which in turn can be inserted together with other TU, within the VC higher order than not we explore this topic, which requires a good dose of patience to be thoroughly investigated, and we simply insert a diagram showing the possibilities of combining different tributaries, according to ETSI spcifiche.