Telecommunication Switching Network
Tuhina Samanta
BEIT, 6th Semester
IT-602: TELECOMMUNICATION SYSTEMS • Introduction, elements of Tele traffic, Erlang's formula. • Switching techniques - Electronics switching, generic switch and Engset formula, SD/TD/STS networks. Hybrid time and space division switching • Telephone networks- Signaling, DTMF techniques, transmission, digital transmission requirements. • Cellular Mobile Telephone System- Cell concepts, architecture, hardware procedures, GSM and CDMA standard, • Data networks, Packet/Circuit Switching, LAN, MAN, WAN, ISDN, Broadband network, ATM concept and functionality. • Services- FAX, Cable TV, Video on demand.
BEIT, 6th Semester
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Telecommunication Switching Network
BEIT, 6th Semester
Basic Terms • Calling subscriber – initiates the call • Called subscriber – call is destined to • Setting up connection to required destination – information sent to serving exchange. – May pass through several exchanges (all exchanges on route).
• Signaling— required to draw attention of the called subscriber BEIT, 6th Semester
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Modes of Communication • Simplex – One way communication involving two entities
• Half-duplex – An entity is capable of both transmitting and receiving but not simultaneously
• Full-duplex – Information transfer takes place in both the directions BEIT, 6th Semester
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A Network with a Point-to-point Link
N entities, Number of links (L) = (n-1) + (n -2) + 1 + 0 = n (n-1)/2 BEIT, 6th Semester
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Communication Network
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A Telecommunication Network Subscriber Lines
Subscriber Lines
SS1
SS2 Trunks Trunks
SS4
Subscriber Lines
SS3 SS= Switching System BEIT, 6th Semester
Subscriber Lines 8
Switching Network Configurations Outgoing Trunks
Incoming Trunks N inlets
M Outlets
Switching Network
Switching Network Subscriber Subscriber inlets outlets Inlet-Outlet Connection
Symmetric Network (M= N)
N Incoming Trunks
Switching Network
M Outgoing Trunks
N subscriber Lines
Switching Network
Nonfolded Network Folded Network BEIT, 6th Semester
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Tele-traffic • Nonblocking network – networks with N subscribers has N/2 simultaneous switching paths
• Blocking Network – number of simultaneous switching paths is less than number of simultaneous conversation that can take place
• Erlang(E) – internationally accepted standard for traffic intensity – A switching resource is said to carry one Erlang of traffic if it is continuously occupied throughout a given period of observation BEIT, 6th Semester
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Evolution of Switching System
Space Switch
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Manual System • Obtain the number to connect from caller • Connect the call of the called line (inserting plug of cord circuit) • Alerts the called customer • Monitoring process called supervision
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Strowger Step-by-Step • Connections are set up in stages called progressive control. • The uniselectors act as concentrator at each customer premise. • The final is the expander.
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Crossbar Switching • Strowger switches require regular maintenance. • Replace manual operated switch by matrix telephone relays called Matrix Crosspoints. • The switch retains a set of contacts at each crosspoint. • Contacts are multiplied together. • Uneconomic as N inlets and N outlets requires N2 relays. BEIT, 6th Semester
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Electronic Switching • Facilities controlled by customers – Call barring, Repeat last call, Reminder calls, Call diversion, Three way calls, Charge advice
• Multiplex System – FDM – TDM
• Space Division (SD) System – Connection is made over a different path in.
• Time Division (TD) System – connection made over the same path but different instant in time.
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Speech Digitization and Transmission • Speech digitization – PCM – DPCM
• Line Coding – Properties – Types of coding
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Time Division Multiplexing • Subdivide the capacity of a digital transmission system into parts in time. • STDM– Synchronous TDM – Time slots are assigned in repetitive manner
• ATDM – Asynchronous TDM – Assign capacity as and when needed
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Four Channel TDM Frame Structure 1
2
3
Frame
4
1
2
3
4
1
Bit Interleaving
4 4 1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4 1 1 1 1
Frame
4-bit word Interleaving BEIT, 6th Semester
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Frame Synchronization • Extra frame bit is added, or some data bit treated as frame bits to ensure resume of new frame. • For T1 channel structure, 24 channels are multiplexed having frame length of 193bits (24 8 + 1)bits. • Average frame acquisition time Ft is important for frame synchronization BEIT, 6th Semester
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Frame Synchronization • Assume Ft uses 1 framing bit per frame with alternating ones and zeros • Ft’ = avg. time required to examine the required number of bits before a framing bit is hit upon • Ft’’ = avg. time required to test and ensure that chosen bit is a frame bit • Ft = Ft’ Ft’’ • This implies bits are chosen one by one and tested sequentially BEIT, 6th Semester
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Frame Synchronization • p = prob. of finding a 1 in a bit position • q = p -1 = prob. of finding a 0 in a bit position • If the first bit observed is a 1, then the probability that a mismatch occurs at the end of, – – – – – –
One frame = p Two frame = q.q = q2 Three frames = q.p.p = qp2 Four frames = q.p.q.q = q3p Five frames = q.p.q.p.p = q2p3 Six frames = q.p.q.p.q.q = q4p2
• Hence Ft’’ = p + 2q2 + 3qp2 + 4q3p + 5q2p3 + 6q4p2 BEIT, 6th Semester
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Frame Synchronization • Assume, p = q • Then, Ft’’ = p/(1-p)2 frame times • Setting p =1/2 , Ft’’ = 2 frame times = 2N bit times • Average number of bits that must be tested before the framing bit is encountered is N/2, i.e.Ft’ = (N/2) • Ft = (2N)(N/2) = N2 bit times BEIT, 6th Semester
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Added Channel Framing • Recent technique use dedicated separate channel for framing • Framing digits are added in a group • CCITT multiplexing standard – 32 channels per frame with one channel carrying framing information, one channel signalling information, and 30 channels for data N2 N • For multibit frame code, Ft L bits 2(2 1)
2
N = length of the frame L = length of the frame code Assumption: 1 and 0 occurs with equal probability BEIT, 6th Semester
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Books 1. B.P. Lathi, “Modern Digital and Analog Communication Systems”, Oxford University Press 2. Thiagarajan Viswanathan, “Telecommunication Switching Systems and Networks”, Prentice Hall India, 2007 3. Taub, Schilling, “Principle of Communication Systems”, Tata Mcgraw Hill 4. J. E. Flood, “Telecommunications, Switching, Traffic and Networks”, Prentice Hall, 1999. BEIT, 6th Semester
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