2 - ITU

CHAPTER 2 2. DIGITAL NETWORKS AND SERVICES 1. Public Switched Telephone Network (PSTN) The Public Switched Telephone Network (PSTN) is the basis of today's
telecommunications. It provides the users a transport medium for signals in
the frequency range of 300 Hz up to 3.4 kHz where the signals are encoded
with a technique called Pulse Code Modulation (PCM). This technique encodes
samples of the signal, taken at regular intervals, into a digital code.
This scheme has been well standardized, so that the sampling rate is 8 kHz
and the sampling value is encoded in 8 bits. Each conversation therefore
results in a continuous bit stream of 64 kbit/s which is multiplexed in the
network into higher data rate carriers. Although the basic building block
of the digital hierarchy is 64 kbit/s standards have diverged, so different
data rates for the multiplexed signals exist. In general the PSTN is a circuit switched network. This means, that an end-
to-end circuit is established and maintained between the communicating
entities for the duration of a 'call'. This assures minimum delay and
constant quality of service throughout the duration of the call. [pic] Figure 2.1.1 General Structure of PSTN In general the PSTN is structured as depicted in figure 2.1.1. The access
network has to provide the physical communication link between the
communication endpoint and the local exchange. In the local exchange (LEX)
all users related functions are located. The switches of the toll exchange
level (TEX) are introduced to have the possibility for structuring and
optimizingoptimising the network according to traffic measurements and user
behaviorbehaviour. The objective of a switching network design is primarily to minimisze the
cost of ownership for the network. The main principles applied to decrease
the cost of ownership are: · small number of switching systems and exchange sites, · elimination of small trunk groups between the exchanges and the · introduction of centralized network element management. This network design becomes possible because of the application of: · large digital host exchanges in combination with · advanced access network technologies providing remote subscriber line access and · cost-effective transmission technologies.
2.1.1 Access Networks
Different solutions exist to connect users to a local exchange. Users can
either be connected directly (via copper cables) to a local exchange. This
solution can be taken into account if the distance between user location
and exchange is in a range of up to 14 km (depending on the cable physics). Or, for other type of access or remote locations access network nodes can
be deployed. These access network nodes typically concentrate the traffic
from the remote location towards the host on transmission links. Access
network nodes can be connected to the hosts through channel banks (feeding
of the analogue interfaces of the switch by the access network node) or
through system specific multiplexing interfaces. Some switches and access networks also support system independent ETSI V5.1
and V5.2 interfaces based on 2Mbit/s transmission links. In the case of system independent access networks a correlation between the
subscriber line management of the host and the access network has to be
implemented. This is not necessary if the access node is a remote part of
the host. To support different topology choices or physical access media, access
network equipment is available for radio, copper, coax and fiberfibre
infrastructures. 2.1.2 Local Exchange Level A local network based on today's technology typically serves between 80 000
...120 000 lines. Because of the capacity limitations of the digital
switching systems existing some years ago, this required at least three
switching systems per local network. These may be multiple combined
local/transit switching systems at regional exchange sites and/or further
local exchange sites. The exchange sites of a local network may be meshed
when justified by the traffic volume. For the provisioning of widely deployed IN services in the network the SSP
function should be integrated in each local exchange. Basic functions of the local exchange are:
Conversion of analogue to digital signals and vice versa
Feeding of access lines
Collecting of call data records
Line measurements
Routing Additional functions of the exchange are:
Provisioning of Services (e.g. centrex, call forwarding, advice of
charge) and others. 2.1.3 Toll Exchange Level Depending on the size of the network, the toll exchange level can be split
into two parts as shown in figure 2.1.2. The highest hierarchical level of the network is the wide area network. In
the wide area each exchange site is dedicated to a particular regional
network. This means that the wide area exchange collects the inter-regional
(or international) traffic originating from its regional network and routes
it towards the destination wide area exchange. For incoming transit traffic
destined to its regional network, the wide area exchange distributes the
traffic to the appropriate destination regional exchanges. The wide area network is a fully meshed transit network. Dynamic Non-
Hierarchical Routing (DNHR) can be applied in order to achieve a robust
high-performance backbone. Each wide area exchange consists of two switching systems for safety and
capacity. The exchanges of the regional network are dual homed, i.e. connected to
both switching systems of their wide area exchange. Each regional network is dedicated to a wide area exchange site. The inter-
regional (or international) traffic originating and terminating in a region
is collected and distributed over a (partial) star network topology. The regional network carries the intra-regional traffic. For this purpose
the regional exchanges within each region are partially meshed depending on
the actual traffic volume between the exchange sites. Eventually the
regional networks can apply the same DNHR routing as the wide area network.
This enables a single network planning process to be applied for all
transit sub networks. In medium and small networks the regional network can be dropped. In this
case the wide area network is responsible for the provisioning of functions
described for the regional network. [pic]
Figure 2.1.2 Structuring of Transit Network
2.1.4 Signalling For signalling purposes in the network the Common Channel Signalling No.7
protocol is used. The No. 7 network needs to consider the following different signalling
types: a) Circuit-related link-by-link TUP (Telephone User Part)
signalling for call control b) End-to-end signalling for non-circuit related services
(e.g. call completion to busy subscriber) between exchanges c) End-to-end-INAP (Intelligent Network Application Part)
signalling to/from central servers d) Signalling to/from other networks These signalling types differ with respect to their traffic characteristic
(volume, source and destination, delay constraints) and routing and
management requirements. The No. 7 network basically consists of two levels. The intra-network circuit-related link-by-link TUP signalling traffic (type
a)) is handled by a No. 7 network level strongly related to the switching
network structure. This means that signalling can be mostly associated with
the trunk groups between the exchanges (associated mode). Only in cases
when the signalling traffic figures do not justify a direct link (although
the user traffic justifies a direct trunk group) the quasi-associated mode
(signalling via another exchange) is applied. The quasi-associated mode is
also applied for alternate routes, i.e. these routes are used when the
associated links are overloaded or faulty. The second level of the No. 7 network is the backbone consisting of stand-
alone No. 7 nodes in No. 7 terminology referred to as Signalling Transfer
Points (STP) or Signalling Relay Points (SRP). These No. 7 nodes handle the
traffic types b), c) and d). The STP/SRPs represent packet switches which
merely route signalling messages but do not include application functions. The STP/SRPs allow for a central management of the No. 7 traffic types
implying stronger requirements with respect to routing and monitoring. The
SPR function of the No. 7 nodes support routing based on global title
translation. This is applied e.g. in order to address the SCPs serving a
particular group of IN service customers. The SPR/STP are central sites for
signalling interconnection from where the signalling links to other
networks emerge. Therefore these sites host additional No. 7 equipment
allowing a close monitoring of the interconnection signalling. By this
measure the operator can protect his own network e.g. from potential fault
propagation originating from other networks.
2.2 INTEGRATED SERVICES DIGITAL NETWORK (ISDN) ISDN (Integrated Services Digital Network) refers to both a set of digital
transmission standards and a network infrastructure that allows digital
transmission over existing telephone wiring, as provided by public network
service providers. ITU-T defines ISDN as "a network, evolved from the
telephony network, that provides end-to-end digital connectivity to support
a wide range of services, including voice and non-voice, to which users
have a limited set of multiple user interfaces." The demand for ISDN first has emerged in the mid-1970s when international
telecommunications usage began to push existing analoganalogue networks to
their limits. Advanced applications involving voice, data, and image
transmission have required higher speeds, better performance, integrated
m