Techniky spojovaní v komunikačních systémech

Transkript

Techniky spojovaní
v komunikačních systémech
přednáška na FEKT VUT v Brně v předmětu Telekomunikační a informační systémy
Doc. Ing. Miroslav Vozňák, Ph.D.
[email protected]
27.11.2013
Tato přednáška je podporována projektem č. CZ.1.07/2.2.00/28.0062 "Společné aktivity
VUT a VŠB-TUO při vytváření obsahu a náplně odborných akreditovaných kurzů ICT
Outline
• telephony from history toward future
• classification of individual generations, principles of switching
• problem of complexity in space switching
• principles of time switching
• telephony as an application (VoIP)
• bandwith requirements
Katedra telekomunikační techniky
VŠB-Technická univerzita Ostrava
17. listopadu 15/2172, 708 33 Ostrava
http://kat440.vsb.cz
http://liptel.vsb.cz
2
From history
• instead teleph. exchange we use expression Communication
System/Server nowadays
• modern telephony is result more than 100 years of evolution
• differences between narrow-band, wide-band and ultra-band
telephony
• amazing story about telephone invention, actors: A.G. Bell,
E. Gray and A. Meucci
3
Generations of switching systems
• 1876, Bell gained unbelievable publicity at exposition in
Philadelphia
• 1878, the first teleph. exchange in Connecticut,with manual
switching in switchboard
5
Generations of switching systems
1st generation, Strowgers Switch
2nd generation, Crossbar Switch
3rd generation, SPC (Stored Programme Control)
4th generation, Time switch (TDM)
5th generation, Softswitch (packed based
switching in IP Multimedia Subsystems)
6
Strowger switch
• 1891, grave digger A. Strowger inveted
electromechanical step-by-step switch, Strowger
switch, included in 1st generation
- Step-by-Step (stepping selector)
7
Crossbar (Space switching)
The crossbar switch is an electromechanical switch with a matrix
of vertical and horizontal bars and simpler motions that worked
more reliably then Strowger switch
The crossbar switch is an assembly
of individual switches between multiple
inputs and multiple outputs. The switches
are arranged in a matrix.
Switching in junctions
- ferred switch in beginning
- lately semiconductors
x1
y1
x2
y2
Ferreed switch
8
SPC
SPC, Stored-program Control
CPU, digital control but with space switching
9
Complexity of space switching
Issue of complexity in switching matrix, usually 3-stages, N
inputs, N/n elements in 1st and in 3rd stages, k elements in 2nd
stage, crosspoints C(n)
10
Compexity in space switching
Clos non-blocking condition
crosspoints from previous slide
and expressed with Clos condition
min. complexity, derivative is zero
then for n >> 1
for C(min)
simplified
11
Example: a design of non-blocking switch with
minimal complexity for 512 inputs, N=512
then N/n elements 512/16=32
32 elements in 1st stage
Every element will be equipped with 16 inputs
Clos condition says: k=2*16-1=31
31 elements in 2nd stage
Compexity - overall number of crosspoints C=63488
12
Time Switch
Time switching, key condition for 4th gen.
time switch is able
to change Timeslot
of individual bits
(mostly 8 bits word)
13
Time Switch
Tr - switch
Input
Output
type of
T-switch
Synchronous writing
Asynchronous
reading
Tr
Asynchronous writing
Synchronous reading
Tw
Asynchronous writing
Asynchronous
reading
Twr
Synchronous writing
Synchronous reading
Does not
make sense
14
Softswitch

IP telephony, VoIP – different approach,
“ telephony is an application in IP “
which application protocols are needed?
 signalling: mostly SIP (Session Initiation
Protokol), RFC 3261
 media: RTP (Real-Time Protocol), RFC 3550,
or SRTP RFC 3711, ZRTP RFC 6189
15
Softswitch
open-source project Asterisk
“I was so excited the first time I got
a phone call delivered through
my PC using my own software.”
Mark Spencer
16
Real Time Protocol

TCP vs. UDP, transport layer

RTP is based on UDP

+ timestamps and sequence numbers
17
Real Time Protocol


payload type identifies the media type
RFC 1889 and 3550 (Transport Protocol for
Real-Time Applications)
18
Audio from Sender to Receiver






audio signal, coding
packetizing
Transmission
timing impairments - > jitter
de-jitter buffer, elimination
and decompression
19
Audio from Sender to Receiver
increasing bandwith requirements caused by
overhead
Take into account in your design
• G.711
cca 90 kbps (64kbps)
• GSM FR
cca 40 kbps (13 kbps)
• G.729
cca 35 kbps (8 kbps)
• G.723.1
cca 25 kbps (5.3 kbps)
Security techniques bring next overhad as well
20
Bandwith Requirements
• packetizing
t [s]
......... timing
PS [b]
......... payload size
CR [bps] ......... codec rate
PS
t=
CR
SAL [b] .......... size at application layer
HRTP [b] .......... RTP header SAL  H RTP  PS
21
Bandwith requirements
SF [b]
HJ [b]
.......... frame
.......... headers in OSI model
3
SF  SAL   H j
M
j 1
.......... number of concurrent calls
3


 H RTP   H j 
j 1

BWM  M  C R   1 


P
S




22
23
Cipher Block Chaining
• the block size is affected by Cipher algorithm
24
TLS
• BS [b] represents the cipher block size
• STLS [b] instead of SAL
S
S T LS  C 0   AL
 BS

  BS

 x   m in  n   x  n 
• Ceiling function of 'x' gives the smallest
integer greater than or equal to 'x'.
25
IPsec
• AH , Authentication Header Protocol
• ESP , Encapsulating Security Payload Protocol
IPsec in tunnel mode ESP+AH
26
TLS vs. IPsec
codec
∆t
[ms]
Bandwith
Bandwith
Bandwith
without VPN
OpenVPN
IPsec
[kbps]
[kbps]
[kbps]
G.711
20
90.4
106.64
117.6
G.729
20
34.4
83.6
60
G.723.1 / 6,3
30
24
50.68
41.07
G.723.1 / 5,3
30
22.93
50.4
40
27
28
Thank you for your attention
Q&A
Tato přednáška je podporována projektem č. CZ.1.07/2.2.00/28.0062 "Společné aktivity
VUT a VŠB-TUO při vytváření obsahu a náplně odborných akreditovaných kurzů ICT

Techniky spojovaní v komunikačních systémech

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