Circuit-Switches in Telephone Networks

1. Circuit SwitchesThe Telephone NetworkSignalingTraffic 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.

2. 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

3. Chapter 4Circuit-Switching Networks Circuit Switches

4. 電路交換(Circuit Switch) – 電話網路 • Circuit Switch －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。

5. Control 1 1 2 2 3 3 Connection of inputs to outputs … … Link Switch N User n N User n – 1 User 1 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

6. Circuit Switch Types • Space-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

7. 1 2 … N … N –1 2 N 1 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

8. 2(N/n)nk + k (N/n)2 crosspoints kn nk N/n N/n 1 1 1 kn nk 2 N outputs N inputs 2 N/n N/n 2 kn nk 3 3 … … … kn nk N/n N/n N/n N/n k 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

9. 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 kxn nxk N/n x N/n 1 1 1 … … n-1 busy N/n x N/n Desired output Desired input kxn nxk 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 x N/n N/n Free path Free path N/n 2n-1

10. a 1 Read slots according to connection permutation 2 b 3 … … d b c b a a d c    24 2 1 24 2 1 23 23 Write slots in order of arrival 22 23 c 24 d Time-slot interchange 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) • Incoming TDM stream • Outgoing TDM stream

11. Input TDM frame with n slots Output TDM frame with k slots 1 2    n … 2 1 k … 2 1 n Time-slot interchange 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 kxn nxk N/n x N/n 1 1 1 nxk N inputs 2 nxk 3 … nxk N/n

12. Flow of time slots between switches • Only one space switch active in each time slot First slot First slot N/n N/n k n n k 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 kth slot k

13. Space stage TSI stage TSI stage kxn TDM n slots TDM k slots nxk TDM k slots 1 1 kxn nxk n slots N/n x N/n Time-shared space switch N outputs 2 N inputs 2 kxn nxk n slots 3 3 … … n slots kxn nxk N/n N/n Time-Share the Crossbar Switch • 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

14. 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 • T-S-T • Let n = 120 k = 239 • N / n = 800 • Need 800x800 space switch

15. 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

16. Chapter 4Circuit-Switching Networks The Telephone Network

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

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

19. 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 Pedestal Serving area interface Local telephone office Distribution cable Distribution frame Switch Serving area interface Feeder cable Telephone Local Loop • For interesting pictures of switches & MDF, see • web.mit.edu/is/is/delivery/5ess/photos.html www.museumofcommunications.org/coe.html

20. Circuit- switched network Private channel- switched network BRI BRI Packet- switched networks PRI PRI Signaling network 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 Basic rate interface (BRI): 2B+D Primary rate interface (PRI): 23B+D

21. 電話網路組織

22. PacketStar TM Voice Gateway PacketStar TM Voice Gateway PacketStar TM Voice Gateway PacketStar TM Voice Gateway PacketStar TM Voice Gateway PacketStar TM Voice Gateway PacketStar TM Voice Gateway PacketStar TM Voice Gateway 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 市話局乙 市話局丙 市話局甲 用戶甲 用戶乙 用戶丙

23. 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 Subscriber Loop TRUNK

24. Subscriber Loop Subscriber Loop TRUNK 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。 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

25. 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

26. 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

27. 典型電話呼叫接續信號方式-呼叫通話 發信用戶 交換機A 交換機B 交換機C 受信用戶 主叫用戶啟動 Off-Hook 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

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

29. 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

30. Chapter 4Circuit-Switching Networks Signaling

31. 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 Setting Up Connections

32. SPC Signaling Message Control 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

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

34. SCP 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 Access Signaling Dial tone STP STP STP STP SSP SSP Signaling Network Transport Network SSP = service switching point (signal to message) STP = signal transfer point (packet switch) SCP = service control point (processing)

35. 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 Presentation layer TUP TCAP ISUP Session layer SCCP Transport layer Network layer MTP level 3 Data link layer MTP level 2 Physical layer MTP level 1 Signaling System Protocol Stack ISUP = ISDN user part MTP = message transfer part SSCP = signaling connection control part TCAP = transaction capabilities part TUP = telephone user part

36. 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 Future Signaling: Calls, Sessions, & Connections

37. External Database Signaling Network Intelligent Peripheral SSP SSP Transport Network 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

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

39. 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

40. Many lines Fewer trunks Traffic concentration • 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

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

42. (λT)ke–λT k! 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: 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)

43. ac c! Pb = c ∑ ak k! k=0 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 • The utilization is the average # of trunks in use Utilization =λ(1 – Pb) E[X]/c = (1 – Pb) a/c

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

45. Multiplexing Gain • 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

46. Trunk group (a) (b) Tandem switch 2 Tandem switch 1 A D B E F E C D B A C F 90 Erlangs when combined 10 Erlangs between each pair 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 17 trunks for 10 Erlangs 9x17=153 trunks Efficiency = 90/153=53% 106 trunks for 90 Erlangs Efficiency = 85%

47. Tandem switch Alternative route Switch Switch High-usage route Alternative Routing • 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

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

49. Tandem switch 3 Tandem switch 2 Tandem switch 1 Alternative routes Switch B Switch A High-usage route Dynamic Routing • 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

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