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Chapter 4 Circuit-Switching Networks

Multiplexing SONET Transport Networks Circuit Switches The Network Signaling Traffic and Overload Control in Telephone Networks Cellular Telephone Networks Circuit Switching Networks z End-to-end dedicated circuits between clients z Client can be a person or equipment (router or switch) z Circuit can take different forms z Dedicated path for the transfer of electrical current z Dedicated time slots for transfer of voice samples z Dedicated frames for transfer of Nx51.84 Mbps signals z Dedicated wavelengths for transfer of optical signals z Circuit switching networks require: z & switching of circuits z Signaling & control for establishing circuits z These are the subjects covered in this chapter How a network grows

(a) A switch provides the network to a cluster of users, e.g. a telephone switch connects a local community

Network Access network

(b) A multiplexer connects two access networks, e.g. a high speed line connects two switches A Network Keeps Growing

1* a b 2 a b 4 (a) Metropolitan network A 3 A viewed as Network A of A Access Subnetworks c d c d

Network of Metropolitan (b) National network viewed Access as Network of Regional Subnetworks Subnetworks (including A)

A

zVery high- speed lines α

Network of Regional National & Subnetworks International Chapter 4 Circuit-Switching Networks

Multiplexing Multiplexing z Multiplexing involves the sharing of a transmission channel (resource) by several connections or information flows z Channel = 1 wire, 1 , or 1 frequency band z Significant economies of scale can be achieved by combining many signals into one z Fewer wires/pole; fiber replaces thousands of cables z Implicit or explicit information is required to demultiplex the information flows.

(a) (b) Shared A A A Channel A

B B B MUX MUX B

C C C C Frequency-Division Multiplexing z Channel divided into frequency slots

A f 0 Wu (a) Individual signals occupy B z Guard bands f required W Hz 0 u Wu z AM or FM stations C f 0 W z TV stations in u air or cable (b) Combined z fits into telephone channel A B C systems f 0 W Time-Division Multiplexing z High-speed digital channel divided into time slots

… A1 A2 t 0T 3T 6T z Framing B … required (a) Each signal 1 B2 t transmits 1 unit 0T 3T 6T z Telephone every 3T digital seconds … transmission C1 C2 t z Digital 0T 3T 6T transmission in (b) Combined backbone signal transmits A B C … network A1 B1 C1 2 2 2 t 1 unit every T 0T 1T 2T 3T 4T 5T 6T seconds T-Carrier System z Digital telephone system uses TDM. z PCM voice channel is basic unit for TDM z 1 channel = 8 bits/sample x 8000 samples/sec. = 64 kbps z T-1 carrier carries 1 (DS-1) that combines 24 voice channels into a digital stream:

1 1

2 MUX MUX 2 22 23 24 b . . . b 1 2 . . . 24 . . . 24 Frame 24 Framing bit = 8000 frames/sec. x (1 + 8 x 24) bits/frame = 1.544 Mbps North American Digital Multiplexing Hierarchy

1 . DS1 signal, 1.544Mbps . Mux 24

1 DS2 signal, 6.312Mbps 24 DS0 . 4 DS1 . Mux 4 1 . DS3 signal, 44.736Mpbs 7 DS2 . Mux 7 1 . z DS0, 64 Kbps channel 6 DS3 . Mux 6 z DS1, 1.544 Mbps channel z DS2, 6.312 Mbps channel DS4 signal z DS3, 44.736 Mbps channel 274.176Mbps z DS4, 274.176 Mbps channel CCITT Digital Hierarchy

z CCITT digital hierarchy based on 30 PCM channels

1 . 2.048 Mbps . Mux 30 1 8.448 Mbps 64 Kbps . . Mux 4 1 . 34.368 Mpbs . Mux 139.264 Mbps 1 z E1, 2.048 Mbps channel . . Mux z E2, 8.448 Mbps channel 4 z E3, 34.368 Mbps channel z E4, 139.264 Mbps channel Clock Synch & Bit Slips z Digital streams cannot be kept perfectly synchronized z Bit slips can occur in multiplexers Slow clock results in late bit arrival and bit slip

MUX t

514 3 2 1 5 4 3 2 Pulse Stuffing z Pulse Stuffing: synchronization to avoid data loss due to slips z Output rate > R1+R2 z i.e. DS2, 6.312Mbps=4x1.544Mbps + 136 Kbps z Pulse stuffing format z Fixed-length master frames with each channel allowed to stuff or not to stuff a single bit in the master frame. z Redundant stuffing specifications z signaling or specification bits (other than data bits) are distributed across a master frame.

Muxing of equal-rate signals Pulse stuffing requires perfect synch Wavelength-Division Multiplexing z Optical fiber link carries several wavelengths z From few (4-8) to many (64-160) wavelengths per fiber z Imagine prism combining different colors into single beam z Each wavelength carries a high-speed stream z Each wavelength can carry different format signal z e.g. 1 Gbps, 2.5 Gbps, or 10 Gbps

Optical Optical λ1 MUX deMUX λ1

λ2 λ λ 1 λ2. λm 2

Optical fiber

λm λm Example: WDM with 16 wavelengths

30 dB 1540 nm 1550 nm 1560 nm Typical U.S. Optical Long-Haul Network Chapter 4 Circuit-Switching Networks

SONET SONET: Overview

z Synchronous Optical NETwork z North American TDM standard for optical fiber communications z 8000 frames/sec. (Tframe = 125 μsec) z compatible with North American digital hierarchy z SDH (Synchronous Digital Hierarchy) elsewhere z Needs to carry E1 and E3 signals z Compatible with SONET at higher speeds z Greatly simplifies multiplexing in network backbone z OA&M support to facilitate network management z Protection & restoration SONET simplifies multiplexing

Pre-SONET multiplexing: Pulse stuffing required demultiplexing all channels

MUX DEMUX MUX DEMUX

Remove Insert tributary tributary SONET Add-Drop Multiplexing: Allows taking individual channels in and out without full demultiplexing

MUX ADM DEMUX

Remove Insert tributary tributary SONET Specifications z Defines electrical & optical signal interfaces z Electrical z Multiplexing, Regeneration performed in electrical domain z STS – Synchronous Transport Signals defined z Very short range (e.g., within a switch) z Optical z Transmission carried out in optical domain z Optical & receiver z OC – Optical Carrier SONET & SDH Hierarchy

SONET Electrical Optical Signal Bit Rate (Mbps) SDH Signal Electrical Signal

STS-1 OC-1 51.84 N/A STS-3 OC-3 155.52 STM-1 STS-9 OC-9 466.56 STM-3 STS-12 OC-12 622.08 STM-4 STS-18 OC-18 933.12 STM-6 STS-24 OC-24 1244.16 STM-8 STS-36 OC-36 1866.24 STM-12 STS-48 OC-48 2488.32 STM-16 STS-192 OC-192 9953.28 STM-64 STS: Synchronous OC: Optical Channel STM: Synchronous Transport Signal Transfer Module SONET Multiplexing

DS1 Low-speed DS2 mapping E1 function STS-1 51.84 Mbps

Medium DS3 speed STS-1 44.736 mapping OC-n function STS-n . . . E/O . . . STS-3c Scrambler MUX E4 High- STS-1 speed STS-1 mapping STS-1 139.264 function STS-3c STS-1 High- STS-1 ATM or speed STS-1 POS mapping function SONET Equipment z By Functionality z ADMs: dropping & inserting tributaries z Regenerators: digital signal regeneration z Cross-Connects: interconnecting SONET streams z By Signaling between elements z Section Terminating Equipment (STE): span of fiber between adjacent devices, e.g. regenerators z Line Terminating Equipment (LTE): span between adjacent multiplexers, encompasses multiple sections z Path Terminating Equipment (PTE): span between SONET terminals at end of network, encompasses multiple lines Section, Line, & Path in SONET

PTE PTE LTE LTE STE STE STE SONET SONET terminal terminal MUX Reg Reg Reg MUX

Section Section Section Section STS Line STS-1 Path

STE = Section Terminating Equipment, e.g., a repeater/regenerator LTE = Line Terminating Equipment, e.g., a STS-1 to STS-3 multiplexer PTE = Path Terminating Equipment, e.g., an STS-1 multiplexer

z Often, PTE and LTE equipment are the same z Difference is based on function and location z PTE is at the ends, e.g., STS-1 multiplexer. z LTE in the middle, e.g., STS-3 to STS-1 multiplexer. Section, Line, & Path Layers in SONET

Path Path Line Line Line Line Section Section Section Section Section Section Section Optical Optical Optical Optical Optical Optical Optical

z SONET has four layers z Optical, section, line, path z Each layer is concerned with the integrity of its own signals z Each layer has its own protocols z SONET provides signaling channels for elements within a layer SONET STS Frame z SONET streams carry two types of overhead z Path overhead (POH): z inserted & removed at the ends z Synchronous Payload Envelope (SPE) consisting of Data + POH traverses network as a single unit z Transport Overhead (TOH): z processed at every SONET z TOH occupies a portion of each SONET frame z TOH carries management & link integrity information STS-1 Frame z810x64kbps=51.84 Mbps 810 Octets per frame @ 8000 frames/sec 90 columns

A1 A2 J0 J1 B1 E1 F1 B3 1 D1 D2 D3 C2 Order of 2 transmission H1 H2 H3 G1 9 rows B2 K1 K2 F2 D4 D5 D6 H4 D7 D8 D9 Z3 Special OH octets: D10 D11 D12 Z4 A1, A2 Frame Synch S1 M0/1 E2 N1 B1 Parity on Previous Frame (BER monitoring) 3 Columns of Synchronous Payload Envelope (SPE) J0 Section trace Transport OH 1 column of Path OH + 8 data columns (Connection Alive?) H1, H2, H3 Pointer Action Section Overhead Path Overhead K1, K2 Automatic Protection Line Overhead Data Switching SPE Can Span Consecutive Frames

Pointer First octet Frame 87 Columns k

Synchronous 9 Rows payload envelope

Pointer Last octet Frame k+1 First column is path overhead

z Pointer indicates where SPE begins within a frame z Pointer enables add/drop capability Stuffing in SONET z Consider system with different clocks (faster out than in) z Use buffer (e.g., 8 bit FIFO) to manage difference z Buffer empties eventually z One solution: send “stuff” z Problem: z Need to signal “stuff” to receiver

FIFO 1,000,000 bps 1,000,001 bps Negative & Positive Stuff

Frame Frame k Pointer k Pointer First octet First octet of SPE of SPE

Stuff byte Stuff byte Frame Frame k + 1 Pointer k + 1 Pointer First octet First octet of SPE of SPE

(a) Negative byte stuffing (b) Positive byte stuffing Input faster than output Input is slower than output Send extra byte in H3 to catch up Stuff byte to fill gap Synchronous Multiplexing z Synchronize each incoming STS-1 to local clock z Terminate section & line OH and map incoming SPE into a new STS-1 synchronized to the local clock z This can be done on-the-fly by adjusting the pointer z All STS-1s are synched to local clock so bytes can be interleaved to produce STS-n

STS-1 STS-1 STS-1 STS-1 Map

STS-1 STS-1 STS-1 STS-1 Byte STS-3 Map Interleave STS-1 STS-1 STS-1 STS-1 Map

Incoming Synchronized new STS-1 frames STS-1 frames Octet Interleaving

Order of transmission 1

A1 A2 J0 J1 2 A1 A2 J0 J1 3 B1 E1 F1 B3 A1 A2 J0 J1 B1 E1 F1 B3 D1 D2 D3 C2 B1 E1 F1 B3 D1 D2 D3 C2 H1 H2 H3 G1 D1 D2 D3 C2 H1 H2 H3 G1 H1 B2 K1 K2 F2 H2 H3 G1 B2 K1 K2 F2 D4 D5 D6 H4 B2 K1 K2 F2 D4 D5 D6 H4 D7 D8 D9 Z3 D4 D5 D6 H4 D7 D8 D9 Z3 D10 D11 D12 Z4 D7 D8 D9 Z3 D10 D11 D12 Z4 S1 M0/1 E2 N1 D10 D11 D12 Z4 S1 M0/1 E2 N1 S1 M0/1 E2 N1 Concatenated Payloads

Concatenated Payload OC-Nc z Needed if payloads of interleaved frames are “locked” into a bigger zN x 87 columns unit z Data systems send big blocks of J1 information grouped together, e.g., B3 a router operating at 622 Mbps C2 z SONET/SDH needs to handle G1 these as a single unit F2 z H1,H2,H3 tell us if there is H4 concatenation Z3 z STS-3c has more payload than 3 STS-1s Z4 N1 z STS-Nc payload = Nx780 bytes z OC-3c = 149.760 Mb/s z OC-12c = 599.040 Mb/s z OC-48c = 2.3961 Gb/s (N/3) – 1 87N - (N/3) columns of columns of z OC-192c = 9.5846 Gb/s fixed stuff payload Chapter 4 Circuit-Switching Networks

Transport Networks Transport Networks z Backbone of modern networks z Provide high-speed connections: Typically STS-1 up to OC-192 z Clients: large routers, telephone switches, regional networks z Very high reliability required because of consequences of failure z 1 STS-1 = 783 voice calls; 1 OC-48 = 32000 voice calls;

Telephone Switch

Router Router

Transport Network

Telephone Switch Telephone Switch Router SONET ADM Networks

MUX ADM DEMUX

Remove Insert tributary tributary

z SONET ADMs: the heart of existing transport networks z ADMs interconnected in linear and ring topologies z SONET signaling enables fast restoration (within 50 ms) of transport connections Linear ADM Topology z ADMs connected in linear fashion z Tributaries inserted and dropped to connect clients

1 2 3 4

z Tributaries traverse ADMs transparently z Connections create a logical topology seen by clients z Tributaries from right to left are not shown

2

1 3

4 1+1 Linear Automatic Protection Switching T = Transmitter W = Working line R = Receiver P = Protection line W T R

Bridge Selector

T R P • Simultaneous transmission over diverse routes • Monitoring of signal quality • Fast switching in response to signal degradation • 100% redundant bandwidth 1:1 Linear APS Switch Switch W T R

APS signaling

TR P

• Transmission on working fiber • Signal for switch to protection route in response to signal degradation • Can carry extra (preemptible traffic) on protection line 1:N Linear APS

Switch Switch

W 1 T R

W T ² R … … … W … n … TR

P TR

APS signaling • Transmission on diverse routes; protect for 1 fault • Reverts to original working channel after repair • More bandwidth efficient SONET Rings z ADMs can be connected in ring topology z Clients see logical topology created by tributaries

(a) (b) a a

OC-3n OC-3n

b

b c c OC-3n Three ADMs connected in Logical fully connected physical ring topology topology SONET Ring Options z 2 vs. 4 Fiber Ring Network z Unidirectional vs. bidirectional transmission z Path vs. Link protection z Spatial capacity re-use & bandwidth efficiency z Signalling requirements Two-Fiber Unidirectional Path Switched Ring

Two fibers transmit in opposite directions z Unidirectional z Working traffic flows clockwise z Protection traffic flows counter-clockwise z 1+1 like z Selector at receiver does path protection switching UPSR

1

W

4 2

P

W = Working Paths zNo spatial re-use P = Protection Paths Each path uses 2x bw 3 UPSR path recovery

1

W

4 2

P

W = Working line P = Protection line 3 UPSR Properties z Low complexity z Fast path protection z 2 TX, 2 RX z No spatial re-use; ok for hub traffic pattern z Suitable for lower-speed access networks z Different delay between W and P path Four-Fiber Bidirectional Line Switched Ring z 1 working fiber pair; 1 protection fiber pair z Bidirectional z Working traffic & protection traffic use same route in working pair z 1:N like z Line restoration provided by either: z Restoring a failed span z Switching the line around the ring 4-BLSR 1

Equal W delay

P Standby bandwidth 2 4 is shared

Spatial Reuse

3 BLSR Span Switching 1 W Equal delay

P zSpan Switching 2 4 restores failed line

Fault on working links 3 BLSR Span Switching 1 W Equal delay

P zLine Switching 2 4 restores failed lines

Fault on working and protection links

3 4-BLSR Properties

z High complexity: signalling required z Fast line protection for restricted distance (1200 km) and number of nodes (16) z 4 TX, 4 RX z Spatial re-use; higher bandwidth efficiency z Good for uniform traffic pattern z Suitable for high-speed backbone networks z Multiple simultaneous faults can be handled Backbone Networks consist of Interconnected Rings

Regional UPSR Metro OC-12 ring ring Interoffice rings

BLSR OC-48, OC-192

UPSR or BLSR OC-12, OC-48 The Problem with Rings

z Managing bandwidth can be complex z Increasing transmission rate in one span affects all equipment in the ring z Introducing WDM means stacking SONET ADMs to build parallel rings z Distance limitations on ring size implies many rings need to be traversed in long distance z End-to-end protection requires ring- mechanisms Managing 1 ring is simple; Managing many rings is very complex Mesh Topology Networks using SONET Cross-Connects z Cross-Connects are nxn switches z Interconnects SONET streams z More flexible and efficient than rings z Need mesh protection & restoration

Router B C A

D Router F Router

G E

Router From SONET to WDM

SONET WDM z combines multiple SPEs z combines multiple wavelengths into a into high speed digital common fiber stream z Optical ADMs can be built to insert and z ADMs and drop wavelengths in same manner as crossconnects in SONET ADMS interconnected to form z Optical crossconnects can also be built networks z All-optical backbone networks will z SPE paths between provide end-to-end wavelength clients from logical connections topology z Protection schemes for recovering z High reliability through from failures are being developed to protection switching provide high reliability in all-optical networks Optical Switching … Optical … fiber switch … … DeMUX

MUX Output Input … WDM

… Wavelength cross-connect … WDM … WDM WDM …

Dropped Added wavelengths wavelengths Chapter 4 Circuit-Switching Networks

Circuit Switches Network: Links & switches

z Circuit consists of dedicated resources in sequence of links & switches across network z Circuit switch connects input links to output links

zSwitch zNetwork Control

Link Switch 1 1 2 2 User n 3 Connection 3 of inputs User n – 1 …

to outputs … User 1

N N Circuit Switch Types z Space-Division switches z Provide separate physical connection between inputs and outputs z Crossbar switches z Multistage switches z Time-Division switches z Time-slot interchange technique z Time-space-time switches z Hybrids combine Time & Space switching Crossbar Space Switch z N x N array of crosspoints 1 z Connect an input to an output by closing 2

a crosspoint … z Nonblocking: Any N input can connect to idle output … 1 2 N –1 N z Complexity: N2 crosspoints Multistage Space Switch z Large switch built from multiple stages of small switches z The n inputs to a first-stage switch share k paths through intermediate crossbar switches z Larger k (more intermediate switches) means more paths to output z In 1950s, Clos asked, “How many intermediate switches required to make switch nonblocking?” 2(N/n)nk + k (N/n)2 crosspoints

n×k N/n × N/n k×n 1 1 1 n×k k×n 2 N 2 N/n × N/n N inputs n×k 2 k×n outputs 3 3 … … …

n×k k×n N/n N/n N/n × N/n k Clos Non-Blocking Condition: k=2n-1 z Request connection from last input to input switch j to last output in output switch m z Worst Case: All other inputs have seized top n-1 middle switches AND all other outputs have seized next n-1 middle switches z If k=2n-1, there is another path left to connect desired input to desired output

nxk N/n x N/n kxn 1 1 1

… n-1 busy N/n x N/n Desired nxk n-1 kxn Desired input j m output n-1 N/n x N/n n+1 busy …… … # internal links = N/n x N/n 2x # external links 2n-2 nxk N/n N/n x N/n kxn Free path 2n-1 Free path N/n Minimum Complexity Clos Switch

C(n) = number of crosspoints in Clos switch

= 2Nk + k( N )2 = 2N(2n –1)+(2n – 1)( N )2 n n Differentiate with respect to n:

2 2 2 N 0 = δ C = 4N –2N + 2N ≈ 4N –2N ==> n ≈ √ δn n2 n3 n2 2

The minimized number of crosspoints is then: N2 N 1/2 1.5 C* = (2N + N /2 )(2( 2 ) –1) ≈ 4N √ 2N = 4 √ 2N This is lower than N2 for large N Example: Clos Switch Design z Circa 2002, Mindspeed offered a Crossbar chip with the following specs: z 144 inputs x 144 outputs, 3.125 Gbps/line 8x16 144×144 16x8 1 1 z Aggregate Crossbar chip throughput: 1 1152 450 Gbps 8x16 16x8 2 2

144x144 outputs z Clos Nonblocking Design for 1152x1152 inputs 8x16 2 16x8 switch 3 3 1152

z …

N=1152, n=8, k=16 … … z N/n=144 8x16 switches in first stage 8x16 16x8 z 16 144x144 in centre stage 144 N/n z 144 16x8 in third stage 144x144 16 z Aggregate Throughput: 3.6 Tbps!

z Note: the 144x144 crossbar can be partitioned into multiple smaller switches Time-Slot Interchange (TSI) Switching

z Write bytes from arriving TDM stream into memory z Read bytes in permuted order into outgoing TDM stream z Max # slots = 125 μsec / (2 x memory cycle time)

1 a Read slots 2 b according to 3 connection dbc … b a zzz permutation a … dc 24 23 2 1 24 23 2 1 Write 22 slots in order of 23 c zIncoming arrival zOutgoing TDM 24 d TDM stream stream

Time-slot interchange Time-Space-Time Hybrid Switch z Use TSI in first & third stage; Use crossbar in middle z Replace n input x k output space switch by TSI switch that takes n-slot input frame and switches it to k-slot output frame

nxk N/n x N/n kxn 1 1 1 nxk N 2 inputs Input TDM Output TDM frame with frame with k nxk 1 3 n slots slots 2 zzz

… n … 2 1 k … 2 1 nxk n N/n Time-slot interchange Flow of time slots between switches

First slot First slot n × k N/n × N/n k × n 1 1 1

n × k k × n 2 2 N/n × N/n 2 … … …

n × k k × n N/n N/n × N/n N/n kth slot k kth slot z Only one space switch active in each time slot Time-Share the

TSI stage Space stage TSI stage

TDM nxk TDM TDM kxn n slots 1 k slots k slots 1

n slots nxk kxn N 2 N/n x N/n 2 N inputs Time-shared outputs n slots nxk space switch kxn 3 3 … … n slots nxk kxn N/n N/n z Interconnection pattern of space switch is reconfigured every time slot z Very compact design: fewer lines because of TDM & less space because of time-shared crossbar Example: A→3, B→4, C→1, D→3 (a) A C B A z3-stage Space Switch C D D B

(b)

B2 A2 B1 A1 B A C A A1 C1 2x3 1 1 1 1 3x2 1 1 zEquivalent TST Switch

D C D C D C D1 B1 B1 D1 2 2 1 1 2x3 1 1 3x2 2 2 Example: T-S-T Switch Design

For N = 960 z Single stage space switch ~ 1 million crosspoints z T-S-T z Let n = 120 N/n = 8 TSIs z k = 2n – 1 = 239 for non-blocking z Pick k = 240 time slots z Need time-multiplexed space switch

For N = 96,000 z T-S-T z Let n = 120 k = 239 z N / n = 800 z Need 800x800 space switch Available TSI Chips circa 2002 z OC-192 SONET Framer Chips z Decompose 192 STS1s and perform (restricted) TSI z Single-chip TST z 64 inputs x 64 outputs z Each line @ STS-12 (622 Mbps) z Equivalent to 768x768 STS-1 switch Pure Optical Switching z Pure Optical switching: light-in, light-out, without optical-to-electronic conversion z Space switching theory can be used to design optical switches z Multistage designs using small optical switches z Typically 2x2 or 4x4 z MEMs and Electro-optic switching devices z Wavelength switches z Very interesting designs when space switching is combined with wavelength conversion devices Chapter 4 Circuit-Switching Networks

The

z User requests connection z Network signaling establishes connection z Speakers converse z User(s) hang up z Network releases connection resources Source Signal Go Signal ahead Message Release

Signal Destination Call Routing

z Local calls routed (a) 4 C D through local network (In U.S. Local Access & 2 3 5 Transport Area)

A B 1 z Long distance calls routed to long distance service provider

(b) Net 1

Net 2

LATA 1 LATA 2 Telephone Local Loop Local Loop: “” z Copper pair from telephone to CO z Pedestal to SAI to Main Distribution Frame (MDF) z 2700 cable pairs in a feeder cable z MDF connects Pedestal z voice signal to telephone switch z DSL signal to routers Serving area Local telephone office interface Distribution cable Switch

Serving Distribution frame area Feeder interface cable

zFor interesting pictures of switches & MDF, see zweb.mit.edu/is/is/delivery/5ess/photos.html www.museumofcommunications.org/coe.html Fiber-to-the-Home or Fiber-to-the-Curve?

Table 3.5 Data rates of 24-gauge z Fiber connection to the home provides huge Standard Data Rate Distance amount of bandwidth,

T-1 1.544 Mbps 18,000 feet, 5.5 km but cost of optical

DS2 6.312 Mbps 12,000 feet, 3.7 km still high

1/4 STS-1 12.960 4500 feet, 1.4 km z Fiber to the curve Mbps (pedestal) with shorter 1/2 STS-1 25.920 3000 feet, 0.9 km distance from pedestal Mbps to home can provide STS-1 51.840 1000 feet, 300 m Mbps high speeds over copper pairs Two- & Four-wire connections z From telephone to CO, two wires carry signals in both directions z Inside network, 1 wire pair per direction z Conversion from 2-wire to 4-wire occurs at hybrid transformer in the CO z Signal reflections can occur causing speech echo z Echo cancellers used to subtract the echo from the voice signals

Original Transmit pair Received signal signal

zFour Wires Echoed Hybrid signal transformer

zTwo Wires Receive pair Integrated Services Digital Network (ISDN) z First effort to provide end-to-end digital connections z B channel = 64 kbps, D channel = 16 kbps z ISDN defined interface to network z Network consisted of separate networks for voice, data, signaling

Circuit- switched network (BRI): 2B+D Private BRI channel- switched BRI network

Packet- PRI switched networks PRI

Signaling network (PRI): 23B+D Chapter 4 Circuit-Switching Networks

Signaling Setting Up Connections

Manually Automatically z Human Intervention z Management Interface z Telephone z Operator at console sets up connections at z Voice commands & various switches switchboard operators z Automatic signaling z Transport Networks z Request for connection z Order forms & generates signaling dispatching of messages that control craftpersons connection setup in switches Stored-Program Control Switches z SPC switches (1960s) z Crossbar switches with crossbars built from relays that open/close mechanically through electrical control z Computer program controls set up opening/closing of crosspoints to establish connections between switch inputs and outputs z Signaling required to coordinate path set up across network SPC Control Signaling Message Message Signaling z Processors that control switches exchange signaling messages z Protocols defining messages & actions defined z Modems developed to communicate digitally over converted voice trunks

Office A Trunks Office B

Switch Switch

Processor Modem Processor Signaling Signaling Network z Common Channel Signaling (CCS) #7 deployed in 1970s to control call setup z Protocol stack developed to support signaling z Signaling network based on highly reliable network z Processors & databases attached to signaling network enabled many new services: , , call waiting, user mobility Internodal Signaling Signaling System 7 Access Signaling STP STP SCP

STP STP SSP Signaling Network SSP

Transport Network

SSP = service switching point (signal to message) STP = signal transfer point (packet switch) SCP = service control point (processing) Signaling System Protocol Stack

z Lower 3 layers ensure Application layer delivery of messages to signaling nodes Presentation layer TUP TCAP ISUP z SCCP allows messages to be Session layer directed to applications z TCAP defines Transport layer SCCP messages & protocols between applications Network layer MTP level 3 z ISUP performs basic call setup & release layer MTP level 2 z TUP instead of ISUP in Physical layer MTP level 1 some countries

ISUP = ISDN user part MTP = message transfer part SSCP = signaling connection control part TCAP = transaction capabilities part TUP = telephone user part Future Signaling: Calls, Sessions, & Connections

Call/Session Connection z An agreement by two end z Allocation of resources to parties to communicate enable information transfer z Answering a ringing between communicating phone (after looking at parties caller ID) z Path establishment in z TCP three-way telephone call handshake z Does not apply in z Applies in connection-less & connectionless networks connection-oriented z ReSerVation Protocol networks (RSVP) provides for resource z Session Initiation Protocol reservation along paths in (SIP) provides for establishment of sessions in many Internet applications Network Intelligence z Intelligent Peripherals provide additional service capabilities z Voice Recognition & Voice Synthesis systems allow users to access applications via speech commands z “Voice browsers” currently under development (See: www.voicexml.org) z Long-term trend is for IP network to replace signaling system and provide equivalent services z Services can then be provided by telephone companies as well as new types of service companies

External Database

Signaling Intelligent Network Peripheral SSP SSP

Transport Network Chapter 4 Circuit-Switching Networks

Traffic and Overload Control in Telephone Networks Traffic Management & Overload Control z Telephone calls come and go z People activity follow patterns z Mid-morning & mid-afternoon at office z Evening at home z Summer vacation z Outlier Days are extra busy z Mother’s Day, Christmas, … z Disasters & other events cause surges in traffic z Need traffic management & overload control Traffic concentration

Many Fewer lines trunks

z Traffic fluctuates as calls initiated & terminated z Driven by human activity z Providing resources so z Call requests always met is too expensive z Call requests met most of the time cost-effective z Switches concentrate traffic onto shared trunks z Blocking of requests will occur from time to time z Traffic engineering provisions resources to meet blocking performance targets Fluctuation in Trunk Occupancy

zNumber of busy trunks

N(t) All trunks busy, new call requests blocked

t

1 zactive 2 zactive 3 zactive zactive 4 zactive 5 zactive Trunk number 6 zactive zactive 7 zactive zactive Modeling Traffic Processes z Find the statistics of N(t) the number of calls in the system

Model z Call request arrival rate: λ requests per second z In a very small time interval Δ, z Prob[ new request ] = λΔ z Prob[no new request] = 1 - λΔ z The resulting random process is a Poisson arrival process: (λT)ke–λT Prob(k arrivals in time T) = k! z Holding time: Time a user maintains a connection z X a random variable with mean E(X) z Offered load: rate at which work is offered by users: z a = λ calls/sec * E(X) seconds/call (Erlangs) Blocking Probability & Utilization z c = Number of Trunks z Blocking occurs if all trunks are busy, i.e. N(t)=c z If call requests are Poisson, then blocking probability

Pb is given by Erlang B Formula ac c! P = b c ∑ ak k=0 k! z The utilization is the average # of trunks in use

Utilization = λ(1 – Pb) E[X]/c = (1 – Pb) a/c Blocking Performance

za To achieve 1% blocking probability: a = 5 Erlangs requires 11 trunks a = 10 Erlangs requires 18 trunks Multiplexing Gain

Load Trunks@1% Utilization z At a given Pb, the 1 5 0.20 system becomes more 2 7 0.29 3 8 0.38 efficient in utilizing 4 10 0.40 trunks with increasing 5 11 0.45 system size 6 13 0.46 7 14 0.50 z Aggregating traffic 8 15 0.53 flows to share centrally 9 17 0.53 10 18 0.56 allocated resources is 30 42 0.71 more efficient 50 64 0.78 z This effect is called 60 75 0.80 90 106 0.85 Multiplexing Gain 100 117 0.85 Routing Control z Routing control: selection of connection paths z Large traffic flows should follow direct route because they are efficient in use of resources z Useful to combine smaller flows to share resources z Example: 3 close CO’s & 3 other close COs z 10 Erlangs between each pair of COs

Trunk (a) (b) Tandem group Tandem A D switch 1 switch 2

B E

C F A B C D E F 10 Erlangs between each pair 90 Erlangs when combined

17 trunks for 10 Erlangs 9x17=153 trunks 106 trunks for 90 Erlangs Efficiency = 90/153=53% Efficiency = 85% Alternative Routing

Tandem switch

Alternative route

Switch Switch High-usage route z Deploy trunks between switches with significant traffic volume z Allocate trunks with high blocking, say 10%, so utilization is high z Meet 1% end-to-end blocking requirement by overflowing to longer paths over tandem switch z Tandem switch handles overflow traffic from other switches so it can operate efficiently z Typical scenario shown in next slide Typical Routing Scenario

Tandem Tandem switch 1 switch 2

Alternative routes for B-E, C-F

Switch D Switch A Switch B Switch E High-usage route B-E Switch C Switch F High-usage route C-F Dynamic Routing

Tandem Tandem Tandem switch 1 switch 2 switch 3

Alternative routes

Switch A Switch B High-usage route z Traffic varies according to time of day, day of week z East coast of North America busy while West coast idle z Network can use idle resources by adapting route selection dynamically z Route some intra-East-coast calls through West-coast switches z Try high-usage route and overflow to alternative routes Overload Control Carried load Network capacity Offered load Strategies z Overload Situations z z z z z z Mother’s Day,Xmas Call request pacing Code blocking Outbound first Direct routes first Network Faults Catastrophes Chapter 4 Circuit-Switching Networks

Cellular Telephone Networks Radio Communications z 1900s: Radio demonstrated z 1920s: Commercial radio broadcast service z 1930s: Spectrum regulation introduced to deal with interference z 1940s: z Police & ambulance radio service z Single covers transmission to mobile users in city z Less powerful car antennas transmit to network of antennas around a city z Very limited number of users can be supported Cellular Communications

Two basic concepts: z Frequency Reuse z A region is partitioned into cells z Each cell is covered by z Power transmission levels controlled to minimize inter-cell interference z Spectrum can be reused in other cells z Handoff z Procedures to ensure continuity of call as user moves from cell to another z Involves setting up call in new cell and tearing down old one Frequency Reuse

z Adjacent cells may not 2 use same band of frequencies 7 3 z Frequency Reuse 1 Pattern specifies how frequencies are reused 6 4 z Figure shows 7-cell reuse: frequencies 5 2 divided into 7 groups & 2 7 3 reused as shown z Also 4-cell & 12-cell 7 3 1 reuse possible 1 6 4 z Note: CDMA allows adjacent cells to use 6 4 5 same frequencies 5 (Chapter 6)

Base station z Transmits to users on forward channels z Receives from users on reverse channels

BSS BSS Mobile Switching MSC Center SS7 HLR STP z Controls connection VLR Wireline EIR terminal setup within cells & AC PSTN to telephone network

AC = authentication center MSC = mobile switching center BSS = base station subsystem PSTN = public switched telephone network EIR = equipment identity register STP = signal transfer point HLR = home location register VLR = visitor location & Connection Control z Setup channels set aside for call setup & handoff z Mobile unit selects setup channel with strongest signal & monitors this channel z Incoming call to mobile unit z MSC sends call request to all BSSs z BSSs broadcast request on all setup channels z Mobile unit replies on reverse setup channel z BSS forwards reply to MSC z BSS assigns forward & reverse voice channels z BSS informs mobile to use these z rings Mobile Originated Call z Mobile sends request in reverse setup channel z Message from mobile includes serial # and possibly authentication information z BSS forwards message to MSC z MSC consults Home Location Register for information about the subscriber z MSC may consult Authentication center z MSC establishes call to PSTN z BSS assigns forward & reverse channel Handoff z Base station monitors signal levels from its mobiles z If signal level drops below threshold, MSC notified & mobile instructed to transmit on setup channel z Base stations in vicinity of mobile instructed to monitor signal from mobile on setup channel z Results forward to MSC, which selects new cell z Current BSS & mobile instructed to prepare for handoff z MSC releases connection to first BSS and sets up connection to new BSS z Mobile changes to new channels in new cell z Brief interruption in connection (except for CDMA) z Users subscribe to roaming service to use service outside their home region z Signaling network used for message exchange between home & visited network z Roamer uses setup channels to register in new area z MSC in visited areas requests authorization from users Home Location Register z Visitor Location Register informed of new user z User can now receive & place calls GSM Signaling Standard z Base station z Base Transceiver Station (BTS) z Antenna + Transceiver to mobile z Monitoring signal strength z Base Station Controller z Manages radio resources or 1 or more BTSs z Set up of channels & handoff z Interposed between BTS & MSC z Mobile & MSC Applications z Call Management (CM) z (MM) z Radio Resources Management (RRM) concerns mobile, BTS, BSC, and MSC Cellular Network Protocol Stack

CM Um Abis A CM

MM MM

RRM RRM RRM RRM

SCCP SCCP

MTP MTP Level 3 Level 3

MTP MTP LAPD LAPDm LAPD LAPD m Level 2 Level 2

Radio Radio 64 64 64 64 kbps kbps kbps kbps Base Base Mobile station MSC transceiver station station controller Cellular Network Protocol Stack

CM Um Radio Air Interface (Um)

MM z LAPDm is data link control adapted to mobile RRM RRM z RRM deals with setting up of radio channels & handover

LAPDm LAPDm LAPD

Radio Radio 64 kbps Base Mobile station transceiver station Cellular Network Protocol Stack

Abis Abis Interface z 64 kbps link physical layer

RRM RRM z LAPDm z BSC RRM can handle SCCP handover for cells within its MTP control Level 3 MTP LAPDm LAPD LAPD Level 2

Radio 64 64 64 kbps kbps kbps Base Base transceiver station station controller Cellular Network Protocol Stack

CM A CM Signaling Network (A) Interface MM MM z RRM deals handover RRM RRM RRM involving cells with different BSCs SCCP SCCP z MM deals with MTP MTP mobile user location, Level 3 Level 3 authentication MTP LAPD LAPD MTP m Level 2 Level 2 z CM deals with call

Radio 64 64 64 setup & release kbps kbps kbps using modified ISUP Base MSC Mobile station station controller What’s Next for Cellular Networks? z Mobility makes cellular phone compelling z Cell phone use increasing at expense of telephone z Short Message Service (SMS) transfers text using signaling infrastructure z Growing very rapidly z Multimedia cell phones z Digital camera to stimulate more usage z Higher speed data capabilities z GPRS & EDGE for data transfer from laptops & PDAs z WiFi (802.11 LAN) a major competitor