Switching Technology
•
•
•
•
•
•
Circuit switching
Structure and interfaces of telephone exchanges
Call control
Maintenance functions and fault-tolerance
The principle of circuit switching
Packet switching and Challenges in switching
technology
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
1
ITU-T’s definition of switching:
The establishing, on-demand, of an individual
connection from a desired inlet to a desired
outlet within a set of inlets and outlets for as
long as is required for the transfer of
information.
An inlet represents a line, such as a channel (or a flow line,
entering an SDL/GR macro call).
Source: (Blue book Fascicle I.3 Definitions).
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
2
By setting up connections on demand networks are utilized efficiently
Subscriber/user/client
- speech
- data
Without
- video
Subscriber/user/client
- speech
- data
- video
switching technology all subscribers
would need permanent connections to all others.
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
3
Subscriber lines and/or subscribers must
have unique addresses in networks
• PSTN/ISDN/GSM - E.164 conformant phone
numbers
– fixed length or variable length numbers
• IP -addresses in the Internet
– IPv4 addresses --> IPv6 - addresses
• In ATM - networks: NSAP - addresses
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
4
There are many communication
modes
communication mode
connection-oriented
Circuit switching
PSTN
GSM
ISDN
Connection less
Packet switching
X.25 Global addresses
+ logical channel id’s
on each hop
Packet switching
Internet
Global addresses
ATM ?
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
5
Connection set-up is needed in connectionoriented communication, so that the network can
change from global addresses to local ones
Set-up message [ B’s global address]
Response [ hop specific logical link/channel id]
B -subscriber
A -subscriber
Connection state
E.g. speech state
Control messaging which enables setting up
and tearing down connections is called signaling
in telecommunications networks.
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
Unique on a hop or link
- time slot (PSTN…)
- logical channel (x.25)
- ...
6
Connections can be set up also with
network management commands
• Telephone networks are used to build up leased links
• Frame relay is a switching technology for variable
length packets that replaces leased lines - e.g. linking
LANs
• Also ATM can be used for establishing semipermanent virtual connections.
Nodes of the network need not support signaling
in this operation model.
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
7
Life cycles of product generations
Optic switching ?
Broadband switch ?
IP + Servers ?
Digital exchange
?
SPC, analog field
Coordinate exchange
Direct controlled
1950
1960
1970
1980
1990
2000
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
2010
2020
8
Basic structure of a digital exchange
Subscriber
interface
Subscriber lines/
or radio links
Switching
Matrix
Control system
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
Junctionlineinterface
junction lines
9
Types of subscriber access in a local exchange
Directly connected
to an exchange
- analog and
- ISDN subscriber
lines
subscriber multiplexer
V2
(1:1)
2/8/34M
channel associated subscriber signaling
length of subscriber
lines usually< 1km
V5.1
(1:1)
2/8/34M
message based subscriber signaling
V5.2
(concentration: N:1)
message based subscriber signaling
RSS
V2
interface
Junction line interface
subscriber
module
Switching
Matrix
V5.1
interface
V5.2
interface
proprietary signaling
Access network
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
Local Exchange
10
Functions of the subscriber interface
module in a fixed network
•
•
•
•
•
•
power feed of subscriber lines
call detection (on-hook/off-hook)
receiving dialed digits
A/D conversion
traffic concentration
subscriber line testing and line status
management
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
11
The function of the switching matrix is to
connect the incoming and outgoing time slots
Incoming lines
Switching Matrix
Outgoing lines
incoming
time slots
Line = 2Mbit/s link
Time slot = 64kbit/s link
incoming pcm,
incoming tsl
outgoing pcm,
outgoing tsl
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
12
The switching matrix is based on
time switches and space switches
• A switching matrix can be extended without
disturbing existing connections.
• Extension step can be e.g. 64 PCMs.
• Matrixes can be single-stage or multistage.
• Due to advances in technology compared to
single connection bandwidth, the trend is
towards single-stage matrixes.
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
13
Time switch - serial writes - addressed reads
incoming frame buffer
1 2 3
outgoing frame buffer
1 2 3
n
1
2
3
Switching
memory
rotating
write addresses
x
n
Clock
Time
slot
counter
n
read
addresses
1
2
3(x)
Control
memory
n
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
14
Space switch - an example
output
input
1
1
2
n
&
2
&
m
&
Control memory
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
15
Most often, control systems in
Exchanges are distributed
Control can be
Centralized
Hierarchical
Singleprocessor
Multiprocessor
Distributed
Non-hierarchical
Singleprocessor
Multiprocessor
Some
functions
centralized
All
functions
distributed
Pragmatic
distribution
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
16
An example of pragmatic distribution is the
Finnish DX200 -exchange
Division of load
by connections/lines
and the signaling
system type.
Signaling system specific
Matrix
control unit
control processor
Centralized
function
Internal bus (or ATM switch)
Databases:
subscriber- and routing
information
Statistics and
charging
operation and
maintenance
Grouping and division by functions, replicated functions if necessary.
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
17
Main functions of the control system
are call control and charging
Service switching(SSF)
Incoming call
control
Outgoing call
control
incoming signaling
outgoing signaling
Resource management
Number analysis
gathering charging
information
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
18
Principles of Functional distribution in
DX200 systems
• The load is partitioned by dividing the incoming and
outgoing line-space either statically or dynamically
– e.g. replaceable 2N or N+ 1 -replication concerning signaling
and call control functions
• Partitioning the load dynamically for a task at a time
– N+1 load partitioning
• Replicating stateless services to several processors
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
19
Exchanges are fault tolerant
Equipment
and
software
Supervision
Active
Alarm
handling
Spare
Recovery
fault
effect
elimination
Fault
location
• Maintenance software manages the states of devices and
programs in the exchange
• The critical equipment has been doubled (2N replication)
• Switchovers from active to spare without disturbing calls
Separated
(out-of-use)
Testing
• Fault tolerance must be taken into account always in all
software development
• Unavailability requirement < 2…3 min/year
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
20
80% of exchange design is software
development
• The size of a software load is between 3 … 10 million
code lines.
• A country adaptation for PSTN typically demands a
work effort of ca.50 man-years.
• Challenges in software design are generated by:
–
–
–
–
real-time call control
variety and nation specificity in signaling, services and features
fault tolerance
maintainability and extensibility of the software and the system
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
21
The junction line interface of an
exchange
• Main functions are:
– adaptation of internal and external transmission
– supervision of the condition of junction lines
• E.g. external format is 2M PCM, internal 4Mbit/s
TTL-logic format (in DX200)
• direct SDH interfaces (e.g. 155Mbit/s ) coming to
digital exchanges (available in some?)
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
22
Circuit switched networks are utilized
also in packet transmission
PSTN/ISDN
GSM
Modem
pole
POP
• long connection set-up time
• inefficient use of resources
(the call is on whether or not any packets
are transmitted)
• Restricted speed (33,6 kbit/s …)
Internet
POP - Point of presence of an Internet Service
Provider
• Internet calls are longer than normal
calls. Due to this fact, dimensioning of
exchanges is not the best possible.
• Because of increasing traffic switching
matrixes and routes must be enlarged.
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
23
ATM-switch
VCI
VPI
ATM cell
Outgoing ATM lines
incoming ATM lines
ATM switching matrix
virtual connections
Incoming
line,
Incoming
path,
Incoming
connection
Outgoing
line,
Outgoing
path,
Outgoing
connection
virtual paths
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
24
Structure of a Packet Switch
Outgoing lines
Incoming lines
Outgoing
buffer
Incoming
buffer
Outgoing
buffer
Controller
Outgoing
buffer
Outgoing
buffer
A problem: Constructing fast enough controllers is difficult!
An example: IP-routers.
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
25
Combined router+ATM-switch
incoming lines
Outgoing lines
Controller
Incoming
ports
Outgoing
port
Outgoing
port
ATMswitching
matrix
Outgoing
port
Outgoing
port
• The controller includes functions of routers and ATM-switch controllers.
• Packet Forwarding functions are in the Incoming ports -block.
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
26
The latest development of switching
technology
• Integration of router and ATM-switch.
Avoiding Layer-3 per-packet forwarding operations.
– loose connection-orientation (flows) in routers
– setting up flows adaptively based on traffic = without an
explicit set-up request
– topology-based label connections
• Gigabit and Terabit routers (Hw packet switching)
• IP voice ==> All Services IP networks(?)
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
27
Challenges of Switching Technology
• Packet switching
– IP calls and their interworking and Interoperability
with PSTN
– Quality of Service in packet networks (IntServ,
DiffServ, Multi Protocol Label Switching)
• Third generation mobile telecommunications
systems
• More open software development environment
S38.118 Principles in Telecommunications Technology s2000 RKa, (translation A.Paju)
28