Transcript 10_ISDN network functions

Communication
Systems
10th lecture
Chair of Communication Systems
Department of Applied Sciences
University of Freiburg
2008
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Communication Systems
Last lecture – introduction to telephony networks
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Telephony networks rather old communication infrastructure
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Invention of the telephone in the late 19th century
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First manually switched, later on mechanical automatically switched
networks
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In-band dial signaling first with pulses later on with DTMF (to handle
other media than copper wire too)
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Completely other concepts to handle standardization, protocol and
definition of interfaces (than in the IP world)
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Computerized switching centers and the introduction of ISDN in the
1980s
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Communication Systems
Last lecture – introduction to telephony networks
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ISDN – Integrated Services Digital Network
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First fully digital telephony network
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PCM for voice digitization
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BRI offers two B channels (64kbit/s for either voice or data
communication) and a separate D channel for out of band dial and
control signaling
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D channel are defined in the 3 lower OSI layers – physical interface
with different encodings, e.g. 4B3T
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DLL represented by LADP protocol
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DSS1 is handling call setup, signaling, call destruction, gives
information on caller ID, ...
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Communication Systems
plan for this lecture
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Signaling in large scale digital networks
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Last lecture – signaling between terminal endpoints (TE) and
the local switching center
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Primary Rate Interface
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But how is a call setup and routed globally
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Signaling system #7, MTP, SCCP, ISUP
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QSIG for inter-connecting PBX
Introduction to mobile telephony networks and especially
GSM
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Communication Systems
Primary Rate Interface - PRI
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Talked on ISDN Basic Rate Interface (BRI) last lecture
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Enough for average household or small office
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But insufficient to serve larger enterprises and organizations
Primary Rate Interface (PRI) handles large scale connectivity
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ITU-T specifications G.703, G.704, G.705
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includes 30 B channels (each B channel 64kbit/s), a full rate D
channel at 64kbit/s and a framing/synchronization pattern (64kbit/s)
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Channels could be bundled, so called H channels: H0 384kbit/s, H11
1534kbit/s and H12 1920kbit/s
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Communication Systems
PRI – physical specification, interfaces
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Brutto bandwidth sums up to 2.048kbit/s connection
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Copper wire connections allow up to 250m without refresh, for longer
distances often fiber optics is used
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All connections are unidirectional, so no channel separation is
needed as was in BRI
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Name of the interface from switching center: UK2 , for fiber optics:
UG2, user interface is named S2M
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Different channels transmitted in TDM (Time Division Multiplexing)
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International E1 (European ISDN standard) systems use HDB3
(High Density Bipolar 3) line coding
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Communication Systems
PRI - HDB3 line coding
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Two kinds of transmission media used to transmit E1 signals
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Coaxial cable (2,37V peak base)
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Twisted-pair cable (3V peak base)
HDB3 Line Coding is similar to AMI coding
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To avoid co-current flow strings of 4 zeroes are replaced with one of
four bipolar violation codes, example for 8 consecutive zeros
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Communication Systems
Network signaling
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Of course there are options to bundle more than 30 channels into
a site connection – other media, e.g. fibre optics is used then
(underlying transport technology is often ATM – Asynchronous
Transfer Mode)
In BRI and PRI connections a separate channel is used for
signaling D / DSS1 multiplexed into the same physical connection
In modern large scale telephony networks signaling and real
connections are completely independent
Connectivity between switching centers is handled by a
specialized signaling system – Signal System Nr. 7 (SS7)
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Communication Systems
independent networks for signaling and connection
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Signaling layer consists of Signaling Points (SP) and Signaling
Transfer Points (STP)
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Communication Systems
line switching and signaling network
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Signaling layer is a virtual layer ontop the connection layer
network (coupling network)
SP's are the direct involved switching centers of a connection,
STP just route signaling information
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End points of a connection are the the end switching centers where
the subscribers are connected to
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Every switching center should be connected at least to two STP for
backup
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Thus route optimizations and fall-back routes implemented
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Signaling data itself is transported in usual bearer channels (B
channel) of the connection layer of the switching centers
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Communication Systems
line switching and signaling network
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Signaling network uses a network indicator to distinguish different
networks
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SP use a Signaling Point Code of 14bit and could be associated with
up to four networks
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This allows changeovers between different networks
For international connections special ISTP (International Signal
Transfer Points) are operated
SS7 is specified in ITU Q.700 recommendation
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Resembles the OSI 7-layer model because mostly packet orientated
operation
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Major distinction of protocol levels is made into transport and
application part
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Communication Systems
Signaling System 7
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SS7 does not
conform exactly
to OSI
Primarily made
not for direct
user data
exchange but
telephony
network specific
information
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Communication Systems
Signaling System 7 – layer 1
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Layer 1 defines the (physical) access to the coupling network
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Protocol name: MTP (Message Transfer Part) specified in in ITU
G.701-710
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Defines a bi-directional signaling link
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Uses the standard 64kbit/s connections
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In PRI normally channel 16 is defined for that purpose
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Communication Systems
Signaling System 7 – layer 2
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Layer 2 main task is frame synchronization (more tasks compared
to OSI data link layer)
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Distinction of blocks (signal units) through flags
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Flag consists of 8 bits: 0111 1110
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Securing of block ordering via numbering
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Transparent transmission
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Flow and congestion control
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Error detection with the help of checksums in every signal unit
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Error correction through retransmission and acknowledgements –
preventive cyclic retransmission and ARQ
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Communication Systems
Signaling System 7 – layer 2
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There are three different message types
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Message Signaling Unit (MSU) is meant for transmission of standard
signaling information
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Link Status Signaling Unit (LSSU) transmits detailed information on
the current status of the signaling link
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Fill-in Signaling Unit (FISU) – synchronization if no signaling
information is transmitted - kind of “keep-alive”
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Communication Systems
Signaling System 7 – layer 3
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Layer 3 composed of MTP and SCCP
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Decides if a message is to be routed or not
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General routing decisions for signaling messages
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Distribution of messages to application layer (different types of
user parts)
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Network management and monitoring
Four types of messages
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MTP management information
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Messages for the Telephone User Part (TUP), ISDN User Part
(ISUP)
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SCCP (Signaling Connection Control Part) messages
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Communication Systems
Signaling System 7 – layer 3
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All messages contain Destination Point Code (DPC) and
Origination Point Code (OPC) (comparable to addresses in
TCP/IP)
Routing in SS7 follows these principles
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Minimal paths, pass minimum number of SP, STP
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If more then one link, distribute load equally
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Every signaling information should take the same path
The Signaling Link Selection (SLS) is for load balancing
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All messages with same SLS are sent through the same
channel
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Communication Systems
Signaling System 7 – layer 3, SCCP part
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SCCP is an extension to MTP for OSI conformance, defined in
Q.711-716
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Transports ISUP and TCAP messages
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End-to-end routing (MTP handles the hop-to-hop routing)
Extends the Signaling Point Code for GT (Global Title) addressing
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GT is worldwide unique for international routing of signaling
information
SCCP defines four protocol classes:
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class 0: connection less transmission, segmentation (max. 16),
distribution of messages over several signaling links
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Communication Systems
Signaling System 7 – SCCP protocol classed
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SCCP defines four protocol classes:
class 1: connection less, keeps sequence of messages, same
SLS code (no load distribution)
class 3: simple connection orientated transmission, end-to-end,
flow control
class 4: connection orientated, option to bypass flow control,
end-to-end connection
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There are several message types defined for the different classes
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Communication Systems
Signaling System 7 – layer 4-7, application layer protocols
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Transaction Capabilities Application Part (TCAP) defined for the
services of the “Intelligent Network” (IN, advanced network for
management, configuration and mobile telephony services)
Message exchange through structured or unstructured dialogs for
several actions within network
TUP and DUP are the old style User Parts for non ISDN telephony
and data
The ISDN User Part (ISUP) handles the ISDN signaling stuff
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End-to-end, so that intermediate switching centers do not have
to decode signaling information
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Communication Systems
Signaling System 7 – ISUP
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ISUP is used for:
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Setup and destruction of B channel connections
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Handling of signaling of the several ISDN characteristics
(call deflection, call on hold, conference, ID signaling, ...)
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Connection of different logical connections, e.g. when
passing network borders (international connections)
ISUP consists of a header, required parameters of fixed
length, required parameters of variable length and optional
part
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Communication Systems
Signaling System 7 – ISUP
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Header contains
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Routing label for all signaling messages needed by
participating switching centers (OPC, DPC, ...)
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Circuit Identification Code for addressing of a certain B
channel used
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Message type defines the kind of signaling and message
format
Construction of body (the parameters are the message) differs
dependent of the message type
End octet: 0000 0000
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Communication Systems
Signaling in private branch exchanges
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The telephony providers often do not handle internal
communication of (large) firms, organizations, ...
They use private branch exchanges (PBX) under their own
jurisdiction
Digital PBX are connected to the public network most
commonly via ISDN BRI or PRI interfaces
Internally they use not SS7 but DSS1 (D channel protocol) in
small scale exchanges and QSIG in large scale PBX
Q.931 protocol was intended to be used for signaling within
PBX but every manufacturer created his own protocol (there
is much money in the market and thus much interest that a
customer does not uses different equipment)
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Communication Systems
Signaling in private branch exchanges
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QSIG was specified by the ECMA (European Computer
Manufacturers Association)
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Q in the name refers to the Q reference point in the PBXs
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At layer 1&2 QSIG is identical to the DSS1 EURO ISDN
protocol
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The layer 3 is split into three sub-layers: Basic Call (BC),
Generic Function (GF) Protocol and QSIG Procedures for
Supplementary Services (SS)
BC implements ISDN standard functionality, GF should allow
the inter-connect of devices of different vendors, SS allows
for transparent services extensions (automatic call
completion, display of tolls, ...)
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Communication Systems
mobile telephony networks - history
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With the development of digital telephony networks and
defined signaling standard between switching centers the
preconditions for digital mobile telephony were met
First mobile phone networks started around end of 1940s in
the US
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St. Louis, Missouri, single cell system
“A-Netz” in Germany operated from 1957 to 1977
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Analogous network in the frequency range between 156MHz and
174MHz
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15 manually switched channels (in the final version over 100
wireless sector areas with together over 300 channels)
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Communication Systems
mobile telephony networks – German A-Netz
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50 kHz distance between
channels
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Use of frequency
modulation
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10.500 participants
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Mostly used in cars
(size of equipment!!)
Main problems were the
manual operation, no
automated handover when
moving and limited
possible number of
participants
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Communication Systems
mobile telephony networks - history
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The deployment of German B-Netz started in 1972 and was
the automated successor of the A-Netz
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Main advantage: calls could origine both in the mobile
and the wired network – but for calling a B-Netz
participant you had to know the area (diameter of 27km)
where located
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The old federal republic was split into 150 zones with a
diameter of up to 150km (a zone could host more then one
base station)
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Frequency ranges 148,4 MHz - 149,13MHz and 153,0MHz 153,73MHz, later 157,61 MHz -158,33MHz / 162,2MHz 162,93MHz
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Communication Systems
history – B1/B2 mobile network
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B-Netz, B1/B2 net from 1982
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Offered 38 voice channels up to 1980
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75 channels after reusing of the A-Netz frequencies
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Bandwidth per channel 14kHz, channel distance of 20kHz
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Frequency modulation with a 4kHz frequency deviation
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The network even offered a limited roaming with Austria,
Luxemburg and the Netherlands
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In the beginning 16.000 participants, after the extension
27.000 participants would be the maximum
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Peak usage was in the middle of the 80s: 850 frequency
channels and 158 base stations
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Communication Systems
history of mobile telephony networks – cellular networks
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Main problem of the analogous networks was the limited
number of participants
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Challenge was how to accommodate much more users
within a mobile phone network
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Cellular concepts were developed and tested from the late
seventies: technology advances enable affordable
cellular telephony
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entering of the modern cellular era started 1974-1978 with
first field Trial for Cellular System by AMPS in Chicago
Cellular concepts reuse frequencies and do not try to use the
same frequency over a wide area (to avoid handovers of
moving participants)
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Communication Systems
history of mobile telephony networks – cellular networks
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First generation of digitally switched mobile networks started
in the 1980’s
Several competing standards in different countries
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NMT (Nordic Mobile Telephone) scandinavian standard;
adopted in most of Europe
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First european system (Sweden, 1981)
TACS (Total Access Communication Systems), starts in 1985
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UK standard; A few of Europe, Asia, Japan
AMPS (Advanced Mobile Phone Service)
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US standard
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Communication Systems
history of mobile telephony networks – cellular networks
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Radiocom 2000 (Only in France)
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Analog transmission of voice (no data services) using
frequency modulation
Various bands were defined in different countries, areas:
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NMT: 450 MHz first, 900 MHz later
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TACS: 900 MHz and 1230 bidirectional channels (25KHz
each)
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AMPS: 800 Mhz
Most of the first generation mobile networks are switched of,
but the frequencies still partly in possession of the several
network operators (new services like “Wireless DSL” ...)
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Communication Systems
first generation cellular networks - C-Netz in Germany
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C-Netz in Germany (used too in Portugal and South Africa)
started in 1985 and offered a lot of advantages
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Common prefix: 0161for all participants
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Interruption-less handover between cells
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Scrambling of the analogous radio signal exacerbated the
eavesdropping of connections
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Not only car systems but real portable devices
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“huge" capacity of up to 850.000 participants in Germany
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Frequency range 451-455,74MHz, 461-466,74MHz
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Operated up to the 31th of December 2000
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Communication Systems
cellular networks - planning
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Planning of cellular networks is a complex procedure
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cover the same area with a larger number of base stations
(BS)
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Partitioning of an area into radio cells idealized as
hexagons, the hexagon is a rather good approximation of
a circle
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Frequency channels could not be reused in neighboring cells
because of interference
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modeling: setup of clusters
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Cluster contains: k cells, which use together the complete
frequency range
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k size of the cluster
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Communication Systems
cellular networks - clustering of areas
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Cell radius will
differ in size
depending on
expected density
of users
Real coverage of
a cell is often
different from
idealized mode
Ideal coverage
pattern would
generate no holes
and no cell
superposition
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Communication Systems
cellular networks - planning
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Example of cluster size 3
Generic formula
k = i² + i*j+ j²
Thus cluster sizes of 1, 3,
4, 7, 9, 12, 13, 16, 19, 21, ...
are possible
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With decreasing of
cluster size the
capacity of the
network increases
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But the interference
will increase (trade-off)
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Communication Systems
cellular networks of second generation
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General formula for reuse distance: D=RD =*Rsqrt(3K)
∗ 3K
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Valid for hexagonal geometry
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D = reuse distance
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R = cell radius
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q = D/R = frequency reuse factor
For the example k=3, the reuse factor is 3, for k=12 is 6
Frequency reuse implies that remote cells interfere with
tagged one
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Co-Channel Interference (CCI)
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Sum of interference from remote cells
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For computation see literature
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Communication Systems
cellular networks of second generation
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Four systems in use today
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Global System for Mobile Comm. (GSM)
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Digital AMPS (D-AMPS), US
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Code Division Multiple Access (IS-95) – Qualcomm,US
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Personal Digital Cellular (PDC), Japan
GSM by far the dominant one
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Originally pan-european
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Deployed worldwide in around 200 countries (even in
countries without any working administration), more then
500 mobile operators
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By now available in US too
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Communication Systems
cellular networks of second generation – GSM history
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1982 the “Groupe Speciale Mobile” was founded by the CEPT
(Conference Europeene de Postes et Telecommunication) to
develop a common standard for european mobile networks of
the second generation
With the increasing popularity of the GSM standard worldwide, the
name was changed to Global System for Mobile Communication
1987 introduction of the transfer method which is still in use today
(with sligth modifications)
1991 first testbed networks started in 5 European countries
1992 the two so called D-Netze in Germany started (“D” as the
successor of the C-Netz)
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Communication Systems
cellular networks of second generation – GSM history
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Other than in the monopoly wired world in many countries
started competing providers of the new services
First variant of GSM operated in the 900MHz (890-915MHz for
uplink and 935-960MHz for downlink) frequency band
available most countries in Africa, Asia, (partly the US),
Europe, Australia
1993 one Million participants in Germany
1993 a secondary frequency band of 1800MHz (DCS1800) was
defined (1710-1785MHz up, 1805-1880 MHz down)
1995 completion of so called Phase2 of GSM with the
introduction of new features like fax transfer, extended cell
choosing mechanism, ...
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Communication Systems
GSM – services and applications
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As GSM was developed on the core of IN (Intelligent Network)
and ISDN, so it inherited a list of features like
– Tele- and bearer services like voice and data (SMS, MMS,
Internet, Fax…) services
– “comfort” services like call deviation and deflection, caller ID,
automatic call back on busy, blocking of numbers (outgoing and
incoming)
– Additional services like telephone answering machines
(“Mailbox”, several information desks on Hotels, itineraries,
traffic congestion, ...)
– Location based services, e.g. for emergency calls, area
dependent tolls (O2 “homezone” and other similar offers), area
dependent tourist information, navigation, tracking of containers
and fright trucks
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Communication Systems
GSM – system
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Communication Systems
GSM – subsystem hierarchy
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A mobile switching center (MSC) not much different from a normal
switching center handles several location areas
MSC region -> N Areas -> M BSC -> K BTS -> I MS
with: N < M < K << I
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Communication Systems
GSM – subsystem hierarchy
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A Location Area (LA) is covered by some BTS (Base Transceiver
Station) which are managed by some fewer BSC (Base Station
Controllers)
Thus a provider of about 15 Million subscribers (which are using
Mobile Stations (MS, simply the mobile phone equipped with a
SIM card) will have to setup up to 30.000 radio cells
The cells are covered by up to 12.000 BTS, which are
operated by some hundred BSC, which are handled by up to
50 MSC
The MSC uses the services of a Visitor Location Register
(VLR) holding copies of user data from the Home Location
Register (HLR)
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Communication Systems
GSM – subsystem hierarchy
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The Home Location Registers operated by each provider is
the place where subscriber information is kept
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Which services (voice, data, fax, roaming, ...) the user is
subscribed to
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Data is copied temporarily to VLR where the MS of a user is
registered
The Authentication Center keeps the user access and
authorization information
The Equipment Identification Center keeps track of mobile
equipment (unique serial of Mobile Terminals (MT) – the MS
without the SIM)
The mobile network is supervised by the Operation and
Maintenance Center
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Communication Systems
ISDN, IN, GSM literature – next lecture
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E. Pehl, Digitale und analoge Datenübertragung (german textbook)
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ISDN (from a seminar held by the professorship)
http://www.ks.uni-freiburg.de/download/papers/telsemWS05/ISDN/seminararbeitBorstHartges.pdf
http://www.ks.uni-freiburg.de/download/papers/telsemWS05/ISDN-erweitert/seminarbThurner.pdf
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GSM
http://www.ks.uni-freiburg.de/download/papers/telsemWS05/G2-GSM/HA_GSM2_Mohry_1.pdf
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Next lecture is 21th June (Friday, here), next practical course 24th
(Tuesday, 2pm sharp at the computer center)
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