學號：M9308383 座號：9 姓名：盧相平 指導教授：陳明仕 教授 2005.04.27. Circuit Switches The Telephone Network Signaling Traffic and Overload Control in Telephone Networks 學號：M9308383 座號：9 姓名：盧相平 指導教授：陳明仕 教授 2005.04.27.

通信網路的分類 交換式通信網路：網路中有多個交換點,用以執行交換機能. 每個交換點為交換機, 主要工作為通話路由選擇, 保持通話路暢通沒有阻塞情況. A.電路交換式：電話網路 －只具備實體層的功能, 兩個電話用戶間, 建立一個專用的通信 通路, 此通路在通話完畢之前, 不會給其他的人佔用, 網路中 的資料為透通性傳送, 即中間傳送過程, 資料格式沒有任何的 改變. B.分封交換式：PSN（X.25）、FRN －具備前三層的功能 廣播式通信網路：網路中無中間的交換點, 每一個用戶設備透過共同的有線或無線通信媒體互相通信. A.電腦網路 B.有線電視網路 C.行動通信網路

Chapter 4 Circuit-Switching Networks Circuit Switches

電路交換(Circuit Switch) – 電話網路 －實體線路之連接。 －將主叫用戶之用戶迴路與被叫用戶之用戶迴路連接 在一起，成為一條完整的通信線路。 －這條通話線路在用戶傳送語音/數據之前須先建立， 其它用戶不能再佔用此通話線路。 －用戶語音在傳送過程中不被儲存，通話線路中間的 交換節點也不會更改語音內容，稱為透通性傳送。

Network: Links & switches Circuit consists of dedicated resources in sequence of links & switches across network Circuit switch connects input links to output links Switch Network Control 1 2 3 N Connection of inputs to outputs … User 1 Switch Link User n User n – 1

Circuit Switch Types Space-Division switches Time-Division switches Provide separate physical connection between inputs and outputs Crossbar switches Multistage switches Time-Division switches Time-slot interchange technique Time-space-time switches Hybrids combine Time & Space switching

Crossbar Space Switch N x N array of crosspoints Connect an input to an output by closing a crosspoint Nonblocking: Any input can connect to idle output Complexity: N2 crosspoints N 1 2 N –1 …

Multistage Space Switch Large switch built from multiple stages of small switches The n inputs to a first-stage switch share k paths through intermediate crossbar switches Larger k (more intermediate switches) means more paths to output nk N/n  N/n kn 1 2 N/n N inputs 3 outputs k 2(N/n)nk + k (N/n)2 crosspoints …

Clos Non-Blocking Condition: k=2n-1 Request connection from last input to input switch j to last output in output switch m Worst Case: All other inputs have seized top n-1 middle switches AND all other outputs have seized next n-1 middle switches If k=2n-1, there is another path left to connect desired input to desired output nxk kxn N/n x N/n 1 1 1 … … n-1 busy N/n x N/n Desired input nxk kxn Desired output n-1 j m n-1 busy N/n x N/n … n+1 … # internal links = 2x # external links N/n x N/n 2n-2 nxk kxn N/n N/n x N/n Free path Free path N/n 2n-1

Time-Slot Interchange (TSI) Switching Write bytes from arriving TDM stream into memory Read bytes in permuted order into outgoing TDM stream Max # slots = 125 msec / (2 x memory cycle time) 1 2 3 22    23 24 Write slots in order of arrival Read slots according to connection permutation Time-slot interchange a b c d … Incoming TDM stream Outgoing TDM stream

Time-Space-Time Hybrid Switch Use TSI in first & third stage; Use crossbar in middle 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 kxn N/n x N/n 1 1 1 nxk N inputs 1 2    n Time-slot interchange Input TDM frame with n slots Output TDM frame with k slots n … 2 1 k … 2 1 2 nxk 3 … nxk N/n

Flow of time slots between switches First slot First slot n  k N/n  N/n k  n 1 1 1 k  n n  k 2 2 N/n  N/n 2 … … … k  n n  k N/n N/n N/n  N/n kth slot k kth slot Only one space switch active in each time slot

Time-Share the Crossbar Switch nxk N/n x N/n Time-shared space switch kxn 1 2 N/n N inputs 3 outputs TDM n slots k slots TSI stage Space stage … Interconnection pattern of space switch is reconfigured every time slot Very compact design: fewer lines because of TDM & less space because of time-shared crossbar

Example: T-S-T Switch Design For N = 960 Single stage space switch ~ 1 million crosspoints T-S-T Let n = 120 N/n = 8 TSIs k = 2n – 1 = 239 for non-blocking Pick k = 240 time slots Need 8x8 time-multiplexed space switch For N = 96,000 Let n = 120 k = 239 N / n = 800 Need 800x800 space switch

Pure Optical Switching Pure Optical switching: light-in, light-out, without optical-to-electronic conversion Space switching theory can be used to design optical switches Multistage designs using small optical switches Typically 2x2 or 4x4 MEMs and Electro-optic switching devices Wavelength switches Very interesting designs when space switching is combined with wavelength conversion devices

Chapter 4 Circuit-Switching Networks The Telephone Network

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

Call Routing Local calls routed through local network (In U.S. Local Access & Transport Area) (a) 4 C D 2 3 5 A B 1 Long distance calls routed to long distance service provider (b) LATA 1 LATA 2 Net 1 Net 2

Telephone Local Loop Local Loop: “Last Mile” Copper pair from telephone to CO Pedestal to SAI to Main Distribution Frame (MDF) 2700 cable pairs in a feeder cable MDF connects voice signal to telephone switch DSL signal to routers Local telephone office Distribution frame Pedestal Feeder cable Switch Distribution cable Serving area interface For interesting pictures of switches & MDF, see web.mit.edu/is/is/delivery/5ess/photos.html www.museumofcommunications.org/coe.html

Integrated Services Digital Network (ISDN) First effort to provide end-to-end digital connections B channel = 64 kbps, D channel = 16 kbps ISDN defined interface to network Network consisted of separate networks for voice, data, signaling BRI PRI Circuit- switched network Private channel- Signaling Packet- networks Basic rate interface (BRI): 2B+D Primary rate interface (PRI): 23B+D

電話網路組織

電話網路組織階層圖 台北長途局 台中長途局 高雄長途局 彙接局甲 彙接局乙 彙接局丙 市話局甲 市話局乙 市話局丙 用戶甲 用戶乙 用戶丙 PacketStar TM Voice Gateway PacketStar TM Voice Gateway PacketStar TM Voice Gateway 台北長途局 台中長途局 高雄長途局 PacketStar TM Voice Gateway PacketStar TM Voice Gateway PacketStar TM Voice Gateway PacketStar TM Voice Gateway PacketStar TM Voice Gateway 彙接局甲 彙接局乙 彙接局丙 市話局甲 市話局乙 市話局丙 用戶甲 用戶乙 用戶丙

電話網路組成 電話網路之組成－ 1.交換系統 2.傳輸線路【中繼線（Trunk）/用戶迴路（Subscriber Loop）】 3.用戶設備 交換系統與交換系統間連接線路稱中繼線。 交換系統與用戶系統間連接線路稱用戶迴路。 Subscriber Loop TRUNK Subscriber Loop

電信信號 電信信號 指交換系統與交換系統間或交換系統與用戶系統間，用以彼此溝通的一種語言，使交換網路能夠建立接續，釋放接續，監視等工作，以達到各種通信之目的，並完成通信服務有關管理維護等事項之處理。 TRUNK Subscriber Loop 用戶信號 局間信號/中繼信號 用戶信號 CAS : R1 R2 / CCS : SS7 可聞信號 可聞信號 選擇信號 選擇信號 監視訊號

電話呼叫處理(Call Processing)程序 撥號 主叫送回鈴音 通話 用戶操作 被叫振鈴 送撥號音 被叫先掛斷 主叫先掛斷 交換機處理 釋放 強制釋放 撥接號續音 (約200ms) Dial Tone 受碼蓄碼 (約12s) 呼叫處理 (約300ms) 應答監視 (約6s) 通話監視 (約100s) 計時

TDM交換系統呼叫處理(Call Processing) 呼叫處理係交換系統為處理用戶之呼叫需求，而建立接續路由，以達成通話之目的。  區分如下： （1）電話呼叫處理 （2）撥號音接續-本地交換機送出撥號音給主叫用戶 （3）呼叫接續（Call Connection） 有下列四種呼叫接續方式： - 自局內呼叫接續:接通收容於同一交換系統之兩部電話用戶 - 出局呼叫接續:本地交換機所收容之電話用戶,撥叫它局交換機所收 容之電話用戶 - 入局呼叫接續:它局交換機電話用戶,撥叫本地交換機所收容之電話 用戶 - 轉接接續:它局交換機之呼叫,經由本地交換機,轉接至另一部交換 機

典型電話呼叫接續信號方式-呼叫通話 通話狀態 發信用戶 交換機A 交換機B 交換機C 受信用戶 Off-Hook 主叫用戶啟動 連接並啟動 Dial Tone 連接並啟動 CAS R1局間中繼信號 撥號 啟動準備完了 選擇信號 KP信號 KP確認 連接並啟動 數碼選擇信號 啟動準備完了 KP信號 KP確認 數碼選擇信號 接通並振鈴鈴流 Ring Back Tone Ring Back Tone Ring Back Tone 被叫用戶應答 應答信號 應答信號 開始計費 通話狀態

典型電話呼叫接續信號方式-釋放通話 通話狀態 發信用戶 交換機A 交換機B 交換機C 受信用戶 On-Hook 被叫用戶掛斷 終話信號 停止計費 主叫用戶掛斷 切斷信號 切斷信號 用戶掛斷 空間狀態 用戶掛斷

用戶信號-可聞音訊號(Audible Tone Signals) 用戶信號：交換系統與用戶設備間的控制信號。 交換系統對用戶系統：可聞音訊號(Audible Tone Signals) -供用戶聽的信號。 1.撥號音（Dial Tone）:僅對本交換系統所屬用戶送出 2.忙音（Busy Tone）:由受話局經中繼電路,發話局送到主叫用戶 3.設備或中繼線全忙音（Reorder Tone） 4.振鈴音（Ring Tone） 5.回鈴音（Ring Back Tone）:由受話局送回,表示交換系統正啟動鈴 流通知被叫用戶 目前已使用當被叫用戶忙線中,直接由錄音服務來截答,以中文告訴主叫用戶“被叫講話中,請等一下再撥”,用以代替Busy Tone 種類 傳送頻率(Hz) 傳送方式 備註 Dial Tone 350+440 連續 頻率與時間誤差小於百分之0.5 Busy Tone 480+620 0.5s續 0.5s斷 Reorder Tone 0.25s續 0.25s斷 Ring Tone 440+480 1s on 2s off Ring Back Tone 1s續 1s斷

Chapter 4 Circuit-Switching Networks Signaling

Setting Up Connections Manually Human Intervention Telephone Voice commands & switchboard operators Transport Networks Order forms & dispatching of craftpersons Automatically Management Interface Operator at console sets up connections at various switches Automatic signaling Request for connection generates signaling messages that control connection setup in switches

Stored-Program Control Switches SPC switches (1960s) Crossbar switches with crossbars built from relays that open/close mechanically through electrical control Computer program controls set up opening/closing of crosspoints to establish connections between switch inputs and outputs Signaling required to coordinate path set up across network SPC Control Signaling Message

Message Signaling Processors that control switches exchange signaling messages Protocols defining messages & actions defined Modems developed to communicate digitally over converted voice trunks Switch Processor Office B Office A Signaling Modem Trunks

Signaling Network Common Channel Signaling (CCS) #7 deployed in 1970s to control call setup Protocol stack developed to support signaling Signaling network based on highly reliable packet switching network Processors & databases attached to signaling network enabled many new services: caller id, call forwarding, call waiting, user mobility Internodal Signaling Signaling System 7 SCP Access Signaling Dial tone STP STP 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 Lower 3 layers ensure delivery of messages to signaling nodes SCCP allows messages to be directed to applications TCAP defines messages & protocols between applications ISUP performs basic call setup & release TUP instead of ISUP in some countries Application layer Transport layer Network layer Data link layer Physical layer Presentation layer Session layer SCCP MTP level 3 MTP level 2 MTP level 1 ISUP TCAP TUP 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 An agreement by two end parties to communicate Answering a ringing phone (after looking at caller ID) TCP three-way handshake Applies in connection-less & connection-oriented networks Session Initiation Protocol (SIP) provides for establishment of sessions in many Internet applications Connection Allocation of resources to enable information transfer between communicating parties Path establishment in telephone call Does not apply in connectionless networks ReSerVation Protocol (RSVP) provides for resource reservation along paths in Internet

Network Intelligence Intelligent Peripherals provide additional service capabilities Voice Recognition & Voice Synthesis systems allow users to access applications via speech commands “Voice browsers” currently under development (See: www.voicexml.org) Long-term trend is for IP network to replace signaling system and provide equivalent services Services can then be provided by telephone companies as well as new types of service companies SSP Transport Network Signaling Network Intelligent Peripheral External Database

Chapter 4 Circuit-Switching Networks Traffic and Overload Control in Telephone Networks

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

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

Fluctuation in Trunk Occupancy Number of busy trunks N(t) t All trunks busy, new call requests blocked 1 2 3 4 5 6 7 Trunk number active active active active active active active active active active

Modeling Traffic Processes Find the statistics of N(t) the number of calls in the system Model Call request arrival rate: l requests per second In a very small time interval D, Prob[ new request ] = lD Prob[no new request] = 1 - lD The resulting random process is a Poisson arrival process: (λT)ke–λT k! Prob(k arrivals in time T) = Holding time: Time a user maintains a connection X a random variable with mean E(X) Offered load: rate at which work is offered by users: a = l calls/sec * E(X) seconds/call (Erlangs)

Blocking Probability & Utilization c = Number of Trunks Blocking occurs if all trunks are busy, i.e. N(t)=c If call requests are Poisson, then blocking probability Pb is given by Erlang B Formula Pb = ac c! k! ∑ ak k=0 c The utilization is the average # of trunks in use Utilization = λ(1 – Pb) E[X]/c = (1 – Pb) a/c

Blocking Performance a = 5 Erlangs requires 11 trunks To achieve 1% blocking probability: a = 5 Erlangs requires 11 trunks a = 10 Erlangs requires 18 trunks

Multiplexing Gain Load Trunks@1% Utilization 1 5 0.20 2 7 0.29 3 8 0.38 4 10 0.40 11 0.45 6 13 0.46 14 0.50 15 0.53 9 17 18 0.56 30 42 0.71 50 64 0.78 60 75 0.80 90 106 0.85 100 117 At a given Pb, the system becomes more efficient in utilizing trunks with increasing system size Aggregating traffic flows to share centrally allocated resources is more efficient This effect is called Multiplexing Gain

Routing Control Routing control: selection of connection paths Large traffic flows should follow direct route because they are efficient in use of resources Useful to combine smaller flows to share resources Example: 3 close CO’s & 3 other close COs 10 Erlangs between each pair of COs Tandem switch 2 Tandem switch 1 B C A (b) Trunk group E F D 90 Erlangs when combined E F D B C A (a) 10 Erlangs between each pair 17 trunks for 10 Erlangs 9x17=153 trunks Efficiency = 90/153=53% 106 trunks for 90 Erlangs Efficiency = 85%

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

Typical Routing Scenario High-usage route B-E Tandem switch 1 Alternative routes for B-E, C-F High-usage route C-F Switch B Switch C Switch E Switch D Switch F Tandem switch 2 Switch A

Dynamic Routing Traffic varies according to time of day, day of week High-usage route Alternative routes Switch A Switch B Tandem switch 3 Tandem switch 1 Tandem switch 2 Traffic varies according to time of day, day of week East coast of North America busy while West coast idle Network can use idle resources by adapting route selection dynamically Route some intra-East-coast calls through West-coast switches Try high-usage route and overflow to alternative routes

Overload Control Overload Situations Mother’s Day, Xmas Catastrophes Network Faults Strategies Direct routes first Outbound first Code blocking Call request pacing Carried load Offered load Network capacity