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Basic Communications Systems Class 8 Today’s Class Topics Internet Protocols IP TCP Others Telecommunications Networks History Voice Call Control Business Services Key Systems, PBX and Centrex Data Communications and Computer Networks Chapter 11 Internet Protocols To support the Internet and all its services, many protocols are necessary. Some of the protocols that we will look at: • Internet Protocol (IP) • Transmission Control Protocol (TCP) • Address Resolution Protocol (ARP) • Domain Name System (DNS) Data Communications and Computer Networks Chapter 11 Internet Protocols Recall that the Internet with all its protocols follows the Internet model. An application, such as e-mail, resides at the highest layer. A transport protocol, such as TCP, resides at the transport layer. The Internet Protocol (IP) resides at the Internet or network layer. A particular media and its framing resides at the interface layer. Data Communications and Computer Networks Chapter 11 The Internet Protocol (IP) IP prepares a packet for transmission across the Internet. The IP header is encapsulated onto a transport data packet. The IP packet is then passed to the next layer where further network information is encapsulated onto it. Data Communications and Computer Networks Chapter 11 Data Communications and Computer Networks Chapter 11 The Internet Protocol (IP) Using IP, a subnet router: Makes routing decision based on the destination address. May have to fragment the datagram into smaller datagrams (very rare) using Fragment Offset. May determine that the current datagram has been hopping around the network too long and delete it (Time to Live). To perform these functions, an IP header is encapsulated onto each transport layer packet. Data Communications and Computer Networks Chapter 11 Data Communications and Computer Networks Chapter 11 The Transmission Control Protocol (TCP) The TCP layer creates a connection between sender and receiver using port numbers. The port number identifies a particular application on a particular device (IP address). TCP can multiplex multiple connections (using port numbers) over a single IP line. Data Communications and Computer Networks Chapter 11 The Transmission Control Protocol (TCP) The TCP layer can ensure that the receiver is not overrun with data (end-to-end flow control) using the Window field. TCP can perform end-to-end error correction (Checksum). TCP allows for the sending of high priority data (Urgent Pointer). Data Communications and Computer Networks Chapter 11 Data Communications and Computer Networks Chapter 11 Internet Control Message Protocol (ICMP) ICMP, which is used by routers and nodes, performs the error reporting for the Internet Protocol. ICMP reports errors such as invalid IP address, invalid port address, and the packet has hopped too many times. Data Communications and Computer Networks Chapter 11 User Datagram Protocol (UDP) A transport layer protocol used in place of TCP. Where TCP supports a connection-oriented application, UDP is used with connectionless applications. UDP also encapsulates a header onto an application packet but the header is much simpler than TCP. Data Communications and Computer Networks Chapter 11 Address Resolution Protocol (ARP) When an IP packet has traversed the Internet and encounters the destination LAN, how does the packet find the destination workstation? Even though the destination workstation may have an IP address, a LAN does not use IP addresses to deliver frames. A LAN uses the MAC layer address. ARP translates an IP address into a MAC layer address so a frame can be delivered to the proper workstation. Address Resolution Protocol (ARP) The ARP Protocol allows any machine to determine the 6-byte LAN address of any other machine on the same subnet if it knows the IP address of that machine Each PC keeps an ARP Table that stores IP address and LAN address of devices on local subnet ARP Table entries are kept up-to-date automatically by sending ARP Broadcast messages when necessary to determine addresses Two Addresses?? In an IP LAN environment, every PC will have two addresses: 4-byte IP address 6-byte LAN (Ethernet) address Two Addresses?? Why?? LAN address can only deliver data to destination on the same LAN (that is, the same IP subnet) IP address is used to deliver data to any destination, regardless of destination network. Two Addresses?? The 6-byte destination address in the Ethernet header is used to deliver the data to the next device: If the packet is not on the final LAN, then Ethernet destination address identifies the address of the next router If the packet is on the final LAN, then Ethernet destination address identifies the address of the destination host (server). IP Routing Example IP Address 140.192.55.10 IP Address 140.86.22.5 Client Server Token Ring LAN IP Network 140.86 Ethernet LAN IP Network 140.192 NFS File Services Software TCP/IP Networking Software NIC Driver SW NIC Card Create OPEN FILE Request in NFS format Add TCP/IP Headers with Server IP Address Add Ethernet Info with Router MAC Address Look up IP Dest. Network in Routing Table Recognize MAC Addr. Remove Ethernet Info Src: 140.192.55.10 Dest: 140.86.22.5 Src: 15 Dest: 17 Translate Server IP Address to Server MAC Address Routing Software NIC Driver SW NIC Driver SW NIC Card NIC Card MAC #17 TCP/IP Ethernet Ethernet NFS Trailer Request Headers Header Execute OPEN FILE Operation IP Router Add Token Ring Info with Server MAC Address Check TCP/IP Info. Remove TCP/IP Headers Recognize MAC Addr. Remove Token Ring Info NFS File Services Software TCP/IP Networking Software NIC Driver SW NIC Card MAC #22 NFS TCP/IP Token Token Request Ring Headers Ring Header Trailer Src: 22 Src: 140.192.55.10 Dest: 23 Dest: 140.86.22.5 Data Communications and Computer Networks Chapter 11 Tunneling Protocols The Internet is not normally a secure system. If a person wants to use the Internet to access a corporate computer system, how can a secure connection be created? One possible technique is by creating a virtual private network (VPN). A VPN creates a secure connection through the Internet by using a tunneling protocol. Data Communications and Computer Networks Chapter 11 BOOTP Every workstation attached to the Internet needs: • Its IP address • Its subnet mask • The IP address of a router • The IP address of a name server Data Communications and Computer Networks Chapter 11 BOOTP Thin client workstations do not have a disk drive, and its ROM does not contain the previous four pieces of information. How do we tell the machine this information? BOOTP. Data Communications and Computer Networks Chapter 11 BOOTP There are two types of BOOTP operations: REQUEST – A workstation asks a server for the information (source IP address = all 0s, destination IP address = all 1s). REPLY – The server returns the information to the workstation. Data Communications and Computer Networks Chapter 11 Dynamic Host Configuration Protocol (DHCP) BOOTP is not dynamic (when a client requests its IP address, it is retrieved from a static table). DHCP is a dynamic extension of BOOTP. When a DHCP client issues an IP request, the DHCP server looks in its static table. If no entry exists, the server selects an IP address from an available pool. Data Communications and Computer Networks Chapter 11 DHCP The address assigned by the DHCP server is temporary. Part of the agreement includes a specific period of time. If no time period specified, the default if one hour. DHCP clients may negotiate for a renewal before the time period expires. Data Communications and Computer Networks Chapter 11 Network Address Translation (NAT) NAT lets a router represent an entire local area network to the Internet as a single IP address. Thus it appears all traffic leaving this LAN appears as originating from a global IP address. All traffic coming into this LAN uses this global IP address. This security feature allows a LAN to hide all the workstation IP addresses from the Internet. Data Communications and Computer Networks Chapter 11 NAT Since the outside world cannot see into the LAN, you do not need to use registered IP addresses on the inside LAN. We can use the following blocks of addresses for private use: •10.0.0.0 – 10.255.255.255 •172.16.0.0 – 172.31.255.255 •192.168.0.0 – 192.168.255.255 Data Communications and Computer Networks Chapter 11 NAT When a user on inside sends a packet to the outside, the NAT interface changes the user’s inside address to the global IP address. This change is stored in a cache. When the response comes back, the NAT looks in the cache and switches the addresses back. No cache entry? The packet is dropped. Unless NAT has a service table of fixed IP address mappings. This service table allows packets to originate from the outside. Data Communications and Computer Networks Chapter 11 Locating a Document on the Internet When a user, running a web browser, enters a URL, how is the URL translated into an IP address? The Domain Name System (DNS) is a large, distributed database of URLs and IP addresses. The first operation performed by DNS is to query a local database for URL/IP address information. If the local server does not recognize the address, the server at the next level will be queried. Domain Name System The Internet utilizes a system of Domain Name System (DNS) servers. These allow a user to refer to a device by DNS Name (i.e. brewster.cs.depaul.edu) rather than by IP address (i.e. 140.192.32.9) brewster.cs.depaul.edu Host subdomain domain top-level domain Domain Name System Top-Level Domains (TLDs): 2-letter top-level domains for each country - .fr for France, .de for Germany, etc. .edu - Educational Institutions .com - Commercial .gov - Government .mil - Military .org - Organizations .net - Network providers .int - Internet organizations ICANN The Internet Corporation for Assigned Names and Numbers (ICANN) has responsibility to: Determine what new Top-Level Domains will be created (.biz? .jobs?) Determine what groups can register domain names Determine some policy on copyright and name ownership issues Who owns what names? Domain Name registrars can assign ownership of domain names one level below the TLDs One master database of all domain name owners is kept Example: DePaul U. owns “depaul.edu” Registrars charge a nominal fee ($15-$30 per year) for names Networksolutions.com is the best known registrar Types of TLDs Uncontrolled TLDs Controlled TLDs Anyone can register any name under these TLDs. Examples of uncontrolled TLDs: .com, .net, .org Only certain qualifying groups can register a name under these TLDs – example: .edu. National TLDs Most countries have a national registrar that handles all names for that country’s TLD. Domain Name System Top-Level DNS Servers store address of each Domain Server for a particular top-level domain (i.e. “.edu”) Domain DNS Servers store names and IP addresses for organization (i.e. depaul.edu) Domain Names are registered with the ICANN and associated with IP address prefixes. Example (depaul.edu <=> 140.192.x.x) Domain Name System Example: You type “http://www.ibm.com” Your PC sends DNS request to DePaul DNS server DePaul DNS server sends DNS request packet to “.com” top-level DNS server Top-level DNS server sends back address for “ibm.com” Domain DNS Server at IBM Corp. DePaul DNS server sends DNS request packet to “ibm.com” Domain DNS Server IBM DNS server looks up IP address and sends result back to DePaul DNS Server. DePaul DNS server sends IP address back to your PC. Data Communications and Computer Networks Chapter 11 IP Addresses All devices connected to the Internet have a 32-bit IP address associated with it. Think of the IP address as a logical address (possibly temporary), while the 48-bit address on every NIC is the physical, or permanent address. Computers, networks and routers use the 32-bit binary address, but a more readable form is the dotted decimal notation. Data Communications and Computer Networks Chapter 11 IP Addresses For example, the 32-bit binary address 10000000 10011100 00001110 00000111 translates to 128.156.14.7 There are basically four types of IP addresses: Classes A, B, C and D. A particular class address has a unique network address size and a unique host address size. Network and Host parts So – the first fixed part of the IP address is the Network address or Network Prefix Determines what network address is on Used by routers to determine which direction to send the packet Network and Host parts The rest of the IP address is the Host address or Host Number Determines which particular device on the network is addressed Used by the last router in the path to deliver the packet to the specific machine Data Communications and Computer Networks Chapter 11 Data Communications and Computer Networks Chapter 11 IP Addresses When you examine the first decimal value in the dotted decimal notation: All Class A addresses are in the range 0 - 127 All Class B addresses are in the range 128 - 191 All Class C addresses are in the range 192 - 223 Data Communications and Computer Networks Chapter 11 IP Subnet Masking Sometimes you have a large number of IP address to manage. By using subnet masking, you can break the host ID portion of the address into a subnet ID and host ID. For example, the subnet mask 255.255.255.0 applied to a class B address will break the host ID (normally 16 bits) into an 8-bit subnet ID and an 8-bit host ID. IP Address Subnetting Subnetting splits the bits of the IP address into three address fields: IP Network – identifies network prefix assigned by the ICAAN IP Subnet – identifies internal subnet number determined by local network administrator IP Host – identifies individual machine IP Packet Delivery IP routers deliver data packets as follows: Internet routers only look at IP Network address to determine destination network Once inside destination network, local routers look at IP Subnet address to determine destination LAN The last router attached to the destination LAN looks at IP Host address to determine which particular destination machine. Subnet Masks How do Host machines and routers know which bits are Subnet Bits? A Subnet Mask is entered into every Host machine and every router to tell them: Out of each 32-bit IP address: How many bits are Network bits? How many bits are Subnet bits? How many bits are Host bits? Subnet Masks An IP subnet mask is a 32-bit value defined as follows: Every IP address bit position that should be a Network bit or a Subnet bit position is marked with a 1 bit Every IP address bit position that should be a Host bit is marked with a 0 bit Subnet masks are written in dotted decimal notation, just like IP addresses – but they are not IP addresses. Subnet Mask Example Network Address: Subnet Mask: Host 140 192 30 100 10001100 11000000 00011110 01100100 11111111 11111111 11111111 00000000 Subnet Mask: 255.255.255.0 Data Communications and Computer Networks Chapter 11 The Future of the Internet Various Internet committees are constantly working on new and improved protocols. Examples include: • Internet Printing Protocol • Internet fax • Extensions to FTP • Common Name Resolution Protocol • WWW Distributed Authoring and Versioning Data Communications and Computer Networks Chapter 11 IPv6 The next version of the Internet Protocol. Main features include: • Simpler header • 128-bit IP addresses • Priority levels and quality of service parameters • No fragmentation Data Communications and Computer Networks Chapter 11 Internet2 A new form of the Internet is being developed by a number of businesses and universities. Internet2 will support very high speed data streams. Applications might include: • Digital library services • Tele-immersion • Virtual laboratories Data Communications and Computer Networks Chapter 11 The Internet In Action: A Company Creates a VPN A fictitious company wants to allow 3500 of its workers to work from home. If all 3500 users used a dial-in service, the telephone costs would be very high. Data Communications and Computer Networks Chapter 11 Data Communications and Computer Networks Chapter 11 The Internet In Action: A Company Creates a VPN Instead, the company will require each user to access the Internet via their local Internet service provider. This local access will help keep telephone costs low. Then, once on the Internet, the company will provide software to support virtual private networks. The virtual private networks will create secure connections from the users’ homes into the corporate computer system. Data Communications and Computer Networks Chapter 11 Data Communications and Computer Networks: A Business User’s Approach Chapter 12 Telecommunication Systems Data Communications and Computer Networks Chapter 12 Introduction Students used to go into either data communications or voice communications. Today, the two fields are merging. Most voice systems are computer controlled and data networks support voice. Anyone studying the field of data communications and networks must learn some basic telecommunications too. Data Communications and Computer Networks Chapter 12 Basic Telephone Systems POTS is the plain old telephone system that connects most homes and small businesses. POTS lines were designed to transmit the human voice, which has a bandwidth less than 4000 Hz. A telephone conversation requires two channels, each occupying 4000 Hz. Data Communications and Computer Networks Chapter 12 Basic Telephone Systems The local loop is the telephone line that runs from the telephone company’s central office to your home or business. The central office is the building that houses the telephone company’s switching equipment and provides a local dial tone on your telephone. If you place a long distance call, the central office passes your telephone call off to a long distance provider. Data Communications and Computer Networks Chapter 12 Data Communications and Computer Networks Chapter 12 Basic Telephone Systems The country is divided into a few hundred local access transport areas (LATAs). If your call goes from one LATA to another, it is a long distance call and is handled by a long distance telephone company. If your call stays within a LATA, it is a local distance call and is handled by a local telephone company. Data Communications and Computer Networks Chapter 12 Basic Telephone Systems A trunk is a special telephone line that runs between central offices and other telephone company switching centers. A trunk is usually digital, high speed, and carries multiple telephone circuits. A trunk is typically a 4-wire circuit, while a telephone line is a 2-wire circuit. Voice Network Structure IXC POP C IX k un Tr Ameritech Central Office em Tand Ameritech T andem Office k Trun Direct IXC Trunk IXC POP Ameritech Central Office Inter-Office T runks s X PB nk T ru PBX Centrex Lines Subscriber Lines POP = Point of Presence PBX = Private Branch Exchange Regulatory History 1876 Hubbard, Gray and Bell all file telephone patents 1877 Bell Telephone Company formed 1878 BTC files patent suit against American Speaking Telephone 1879 National Bell Telephone (NBT) formed via mergers NBT becomes American Bell 1880 Regulatory History 1881 American Bell buys Western Electric from Western Union 1885 AT&T formed from American Bell 1910 Western Union joins AT&T 1913 AT&T divests Western Union in anti-trust suit. Agrees to interconnect independent telcos Regulatory History 1934 FCC formed and universal telephone service is made a national goal (Fed. Comm. Act) 1940 Anti-trust suit against AT&T postponed due to WWII 1949 Anti-trust suit filed against AT&T 1956 AT&T settles by limiting Western Electric manufacturing to Bell companies (Consent Decree) Regulatory History 1966 Computer Inquiry I - Bell System cannot provide data processing 1968 Carterphone decision Compliant non-Bell devices must be allowed to connect. 1974 Anti-trust suit filed against AT&T 1982 AT&T settles by agreeing to divest 22 local operating companies (Consent Decree) Regulatory History 1983 Regional Bell Operating Companies (RBOCs) formed 1983 FCC rules telcos can only provide equipment (CPE) through separate subsidiary. 1984 Consent decree is implemented as modified by Judge Greene (Modified Final Judgement). Modified Final Judgement 1984 7 Regional Holding Companies (RHCs) or Regional Bell Operating Companies (RBOCs) formed Each RHC composed of multiple Local Exchange Companies (LECs) (ex. Ameritech) Interexchange Carriers (IXCs) provide long-distance service. (ex. AT&T) Data Communications and Computer Networks Chapter 12 Modified Final Judgement (1984) 231 Local Access and Transport Areas (LATAs) defined in US. LECs can provide only intra-LATA service. IXCs provide only inter-LATA service. LECs must give customers equal access to all IXCs. Problems with MFJ of 1984: No single company (LEC or IXC) can provide all communications services for its customers. This is inefficient. LATA boundaries limit LEC services. Many other companies (Cable TV, Competitive Access Carriers, etc.) are not constrained by MFJ. The Solution? Deregulation! Telecommunications Act of 1996 Allows IXCs to offer local service Allows LECs to offer long-distance service (when they can show competition) Allows Competitive LECs (CLECs) to share LEC facilities (for fair competition). Allows combination of voice, data, video service packages. Telecommunications Act (1996) LECs must give competitors: All services at wholesale prices. Access to telephone numbers, operator services, directory listings. Access to poles, ducts, and right-of-ways. Physical co-location of equipment within LEC buildings. Telecommunications Act (1996) A LEC can offer inter-LATA services when substantial competition exists on its network. The courts are still deciding exactly what this means... Merger-Mania!! - LECs SBC Communications buys: Qwest Communications buys: U.S. West – 6/14/96 Bell Atlantic buys: Pacific Telesis – 4/1/96 Ameritech – 5/11/98 NYNEX Corporation – 4/22/96 GTE Corporation – 7/28/98 BellSouth is only original LEC still unmerged Merger-Mania!! - IXCs WorldCom Inc. buys: MFS Communications – 8/26/96 MCI Communications – 10/1/97 Sprint Corporation – 10/5/99 (failed) AT&T buys: McGaw Cellular – 8/17/83 TCI, Inc. (cable) – 6/24/98 MediaOne Group Inc. (cable) – 4/22/99 Telecommunications Act (1996) December 21, 1999: Bell Atlantic is the FIRST regulated Local Exchange Carrier to be approved for long distance services by the FCC. Decision was immediately challenged by AT&T – more court dates pending… Problems - Telecom Act 1996: Competition is slow to arrive IXCs are moving slowly on providing local service due to wiring costs. CLECs/LECs are still in court deciding: Exactly how they must share their facilities Exactly when they can offer long-distance service. Cable TV carriers find it expensive to offer voice services over cable. Problems - Telecom Act 1996: Competition reduces quality Local telephone service is getting worse Time to get a new phone installed Time to fix problems Number of outages Some Reasons: Too many telephony carriers are competing for too few technical personnel Emphasis is on business customers ($$$) Data Communications and Computer Networks Chapter 12 Basic Telephone Systems A telephone number consists of an area code, an exchange, and a subscriber extension. The area code and exchange must start with the digits 2-9 to separate them from long distance and operator services. Data Communications and Computer Networks Chapter 12 Address Signaling Rotary Pulse Dial 697 Hz 770 Hz 852 Hz ABC DEF 1 2 3 GHI JKL MNO 4 5 6 PRS TUV WXY 7 8 9 Dual Tone Multifrequency (DTMF) or Touch Tone Oper 941 Hz * 0 # 1209 Hz 1336 Hz 1447 Hz After pulling dial for digit ‘N’, telephone opens and closes subscriber loop ‘N’ times as dial returns to its original position. One frequency assigned to each row and each column. Pressing any key sends a 2frequency signal tone. North American Numbering Plan Administration (NANPA) Determines format for dialed numbers. Chooses new area codes, exchange codes Format before 1995: Prefix + 0/1 + N(0/1)X - NNX - XXXX (N digits are 2-9, X digits are 0-9) Format after January, 1995: Prefix + 0/1 + NXX - NXX - XXXX (N digits are 2-9, X digits are 0-9) www.nanpa.com North American Numbering Plan (NANP) Prefix + 0/1 + NNX - NXX - XXXX (N digits are 2-9, X digits are 0-9) NNX - Numbering Plan Area / Area Code NXX - Central Office Prefix / Exchange Code Determines geographic area. Determines Central Office handling this number. XXXX - Station Number Identifies particular end station on C.O. North American Numbering Plan (NANP) Prefix + 0/1 + NNX - NXX - XXXX (N digits are 2-9, X digits are 0-9) Prefix - Can be used to select IXC carrier 10-XXX 101-XXXX (as of 1998) Selects carrier associated with Carrier Identification Code (CIC) XXX. Selects carrier associated with Carrier Identification Code (CIC) XXXX. 0/1 - Used to request “operator assistance”: 0 = Collect, calling card, etc. Central Office Switch Technologies Central Office Wire Center Components Central Office / Wire Center C.O. Switch Service Circuits Common Control Line Cards M ain Distribution Frame (M DF) Cable Side (Protected) Cable Vault Main Distribution Frame (MDF) Trunk Cards Switch Side Outside Plant Switches Data Store Switch Matrix Central Office Cable Vault Wire Center Components Central Office / Wire Center C.O. Switch Service Circuits Common Control Data Store Central Office Switch Components Switch Matrix Line Cards Trunk Cards Switch Side M ain Distribution Frame (M DF) Outside Plant Cable Side (Protected) Cable Vault Common Control is central processor Data Store contains all customer feature and billing information Service Circuits are shared circuits for ringing, tones, digit collection, etc. Wire Center Components Central Office / Wire Center C.O. Switch Service Circuits Common Control Data Store Central Office Switch Components Switch Matrix Line Cards Trunk Cards Switch Side M ain Distribution Frame (M DF) Outside Plant Cable Side (Protected) Cable Vault Switch Matrix can interconnect Line/Trunk cards and Service Circuits Line Cards perform per-line processing (BORSCHT functions). Trunk Cards perform per-trunk processing. Wire Center Components Central Office / Wire Center Distribution Frame C.O. Switch Service Circuits Common Control Data Store Switch Matrix Line Cards Trunk Cards Switch Side M ain Distribution Frame (M DF) Outside Plant Cable Vault Outside Plant Cable Side (Protected) Cable Vault Patch cables connect switch interfaces (Switch Side) to outside cable (Cable Side) Fuses protect against foreign potential Subscriber loops Trunks How is a Voice Call Made? A Voice Call Central Office Service Common Circuits Control Data Store Originating phone goes off-hook. Switch Matrix Line Cards Trunk Cards MDF Line Card sees current, notifies Common Control Common Control checks customer records Common Control sets up connections to Service Circuits: Digit Decoder circuit Dial Tone Generator circuit A Voice Call Customer dials number. Central Office Service Common Circuits Control Data Store Switch Matrix Dial Tone is disconnected after 1st digit. Digit Decoder de-allocated after all digits dialed. Common Control: Line Cards Trunk Cards MDF Consults Routing Table. Determines features of terminating line. Connects originating line to Ringback Generator Connects terminating line to Ringing Circuit. A Voice Call Central Office Service Common Circuits Control Data Store Call is answered. Switch Matrix Line Cards Trunk Cards MDF Ringing Line Card detects offhook and informs Common Control Common Control disconnects ringing circuit Common Control connects originating line card to terminating line card through Switch Matrix Talking path has been established. A Voice Call Central Office Service Common Circuits Control Data Store Phones go on-hook. Switch Matrix Line Cards Trunk Cards MDF Tear down connection (after time-out period if one phone still off-hook). Store call record. Inter-Office Calls Central Office Central Office Service Common Circuits Control Service Common Circuits Control Data Store Switch Matrix Switch Matrix Line Cards Data Store Trunk Cards Trunk Cards Line Cards MDF MDF Inter-Office Trunk Business Service Pricing Fixed Price Services Same monthly cost regardless of how much service is used Example: Leased Line Measured Services Monthly cost based on minutes of usage Example: Direct Distance Dialing (DDD or long distance calling) Private Line Services Private Line Service provides a dedicated circuit between two locations Fixed connection is made on the distribution frame Central Office provides no dial tone service Customer pays flat rate each month Also called a “dedicated circuit” or “leased line” service Private Line Services Central Office Central Office Service Circuits Service Circuits Com m on Data Contr ol Store Switch Matrix Switch Matrix Line Cards Com m on Data Contr ol Store Trunk Cards Trunk Cards Line Cards MDF MDF Inter-Office Trunk Private Line / Leased Line / Dedicated Line Direct circuit between 2 locations No switching service from Central Office (only transmission service). Flat Rate charge based on distance and bandwidth. Private Line Services Foreign Exchange (FX) Line Allows a telephone to get dial tone from a C.O. switch other than the closest one Off Premises Extension (OPX) Allows a telephone to connect to a PBX at a different customer location Private Line Services Central Office Central Office Service Circuits Service Circuits Com m on Data Contr ol Store Switch Matrix Switch Matrix Line Cards Com m on Data Contr ol Store Trunk Cards Trunk Cards Line Cards MDF MDF Inter-Office Trunk Foreign Exchange (FX) Line Provides local switched telephone service from a Central Office outside of the subscriber’s exchange area. Private Line Services Central Office Central Office Service Circuits Service Circuits Com m on Data Contr ol Store Switch Matrix Switch Matrix Line Cards Com m on Data Contr ol Store Trunk Cards Trunk Cards Line Cards MDF MDF Inter-Office Trunk PBX Off Premises Extension (OPX) Private line service that allows a remote business telephone to access local PBX services. FX Line Example GregCo Corporation in Chicago has many customers in New York DDD Chgo-NY costs 15 cents per minute FX line Chgo-NY costs $800 per month GregCo makes about 25 calls to NY each day, each call lasting about 10 minutes SHOULD GREGCO GET AN FX LINE? Private Line Services Tie Line Used to directly connect PBX switches at 2 customer sites Dedicated Access Line to IXC POP Directly connects PBX switch to IXC point of presence. Eliminates LEC access charges for calls made on this line (replaces them with flat monthly fee for private line) Private Line Services Central Office Central Office Service Circuits Service Circuits Com m on Data Contr ol Store Switch Matrix Switch Matrix Line Cards Trunk Cards Trunk Cards Line Cards MDF MDF Inter-Office Trunk PBX Com m on Data Contr ol Store PBX Tie Line A dedicated circuit between two PBXes. Tie lines may be used due to cost savings (as compared with switched service) or to allow proprietary signaling between PBXes. Private Line Services IXC Switch LEC Switch Service Circuits Service Circuits Data Store Common Control Switch Matrix Switch Matrix Line Cards Trunk Cards Trunk Cards Line Cards MDF MDF PBX Data Store Common Control IXC Trunk Dedicated access to IXC POP A dedicated circuit between customer site and IXC POP Eliminates LEC access charges for long-distance calls over this line. Switched Line Services Direct Distance Dialing (DDD) Wide Area Telecommunications Service (WATS) Plain long-distance telephone service Reduced rates to certain geographic areas IN-WATS (800, 888 services) Business pays for each call received on this line. Switched Line Services 900 Services International WATS Allows customer to charge whatever they want for calls to this number Reduced rates to certain countries Remote Call Forwarding; Ex: Business in Chicago can advertise local phone number in New York. May be cheaper than FX line. RCF Example GregCo Corporation in Chicago has many customers in New York. GregCo wants to offer a local New York number. DDD Chgo-NY costs 15 cents per minute RCF service costs $50 per month FX line Chgo-NY costs $800 per month Customers in NY make about 10 calls to GregCo each day, each call lasting about 10 minutes SHOULD GREGCO GET AN FX LINE? Virtual Private Networks Virtual Private Networks (VPNs) are an alternative service, where The carrier does not dedicate an end-to-end circuit to the customer circuit. Carrier charges fixed rate per month for calls between 2 locations Carrier guarantees a particular service level (i.e., 99.99% of calls get through on first try). Virtual Private Networks So, Virtual Private Networks look and perform a lot like Private Lines, but they cost less. Virtual Private Networks Example: Private line from Chicago - New York Cost: $800 per month Reliability: 99.999% uptime VPN Service from Chicago - New York Cost: $600 per month Reliability: 99.99% calls get through on first try Which would you choose? Private Line Features Fixed cost per month regardless of usage Typically best value when lots of calls go between two fixed locations Can be imitated by Virtual Private Network carrier service Switched Service Features Variable cost per month depending on usage May provide special service (like 800 number) Lower cost than leased lines when there is not much call traffic Price discounts offered for large call volumes predictable call patterns (for example, WATS services to particular geographic regions) Key Telephone Systems Customer Premises Central Office KSU Multi-Button Key Sets Business Lines Key System Unit (KSU) ties many multi-line sets into a set of C.O. lines. User selects outside line by pressing corresponding line button on key set. Typical Key System Features Call Transfer Call Hold Music on Hold Conference Calling Speed dialing Intercom calling between key sets Paging Call Pickup and Call Barge-In Speakerphones / Hands-free calling Automatic Call Information Recording A Call using a Key System Frank calls Sam in New York Customer Premises Central Office KSU Business Lines Frank picks up phone, hits LINE #2 button Key system connects set to C.O. line #2 Frank hears dial tone from the C.O. Frank dials 1-212-894-6622. A Call using a Key System Frank calls Sam in New York Customer Premises Central Office KSU Business Lines C.O. routes call to New York, rings Sam’s phone. Sam answers phone. Frank and Sam talk for a while A Call using a Key System Frank calls Sam in New York Customer Premises Central Office KSU Business Lines Frank decides to consult with Jane Frank hits HOLD button - Key System puts current call to Sam on hold. Frank hits INTERCOM button and dials Jane’s set number A Call using a Key System Frank calls Sam in New York Customer Premises Central Office KSU Business Lines Jane hears a special intercom ring Jane and Frank talk A Call using a Key System Frank calls Sam in New York Customer Premises Central Office KSU Business Lines Frank hits LINE #2 button again and now talks with Sam again. A Call into a Key System Joe calls Frank Customer Premises Central Office KSU Multi-Button Key Sets Business Lines Joe picks up phone, dials number associated with LINE #2 of Frank’s Key System A Call into a Key System Joe calls Frank Customer Premises Central Office KSU Multi-Button Key Sets Business Lines C.O. rings LINE #2 connected to KSU KSU sends ringing signal out to all phones programmed for access to LINE #2 A Call into a Key System Joe calls Frank Customer Premises Central Office KSU Multi-Button Key Sets Business Lines Frank answers phone and talks to Joe Private Branch Exchange (PBX) Customer Premises Central Office PBX PBX Station Sets Business Trunks PBX is a true intelligent switching system. Provides local switching between station sets or access to C.O. Users dial access code (like ‘9’) to get C.O. service. Provides advanced intelligent features to users. PBX Features (not found on Key Systems) Private Dialing Plans 4-digit, special prefixes for WATS, FX, etc. Automatic Route Selection PBX collects dialed digits and intelligently decides how to route this call for lowest cost Over FX line Over WATS trunk Over DOD trunk PBX Features (not found on Key Systems) Voice Mail Automatic Call Distribution (ACD) Routes incoming calls (usually INWATS) to the best station set and location Voice Response Unit (VRU) Provides recorded messages and responds to touch-tone requests from callers PBX Features (not found on Key Systems) Class-of-Service Authentication Codes Permission to use each PBX feature can be given or taken away from each user. PBX users can identify themselves with “passwords” when dialing calls to get special access permissions Data Services ISDN, Dial-Up digital services A Call using a PBX Frank calls Sam in New York Customer Premises Central Office PBX Business Trunks Frank picks up the phone and gets dial tone from the PBX. Frank dials 9-1-212-894-6622. PBX consults routing tables, determines that this call should go over the FX trunk to New York A Call using a PBX Frank calls Sam in New York Customer Premises Central Office FX Trunk to New York PBX Business Trunks PBX sends dialed number over FX trunk, dropping area code (sends “894-6622”). Call to New York is completed. Frank and Sam talk. A Call into a PBX Joe calls Frank Customer Premises Central Office PBX PBX Station Sets Business Trunks Joe picks up phone. Dials Frank’s public number 1-312-362-6587 C.O. chooses trunk to PBX (could choose ANY trunk). A Call into a PBX Joe calls Frank Customer Premises Central Office PBX PBX Station Sets Business Trunks C.O. sends OFF-HOOK signal to PBX over the trunk PBX sends WINK-BACK signal to C.O. C.O. sends last 4 digits - “6587” (this is a DID trunk). A Call into a PBX Joe calls Frank Customer Premises Central Office PBX PBX Station Sets Business Trunks PBX looks into its station address table and determines that “6587” corresponds to Frank’s station set. PBX rings Frank’s station set. A Call into a PBX Joe calls Frank Customer Premises Central Office PBX Business Trunks Frank picks up telephone handset. PBX sends ANSWER signal back to C.O. indicating that the call has been answered Frank and Joe talk PBX Trunk Signaling Customer Premises Ameritech Central Office PBX PBX Station Sets Business Lines or T runks 1-way trunks: A call can only be initiated from one end (incoming or outgoing). 2-way trunks: A call can be initiated from either end of the trunk (PBX or C.O.) PBX Trunk Signaling Customer Premises Ameritech Central Office PBX PBX Station Sets Business Lines or T runks Trunk signaling for PBX trunks: Loop start : Like subscriber loop. Ground start : Either side can ground a wire to initiate a call. Better control than loop start. E&M : Separate wire pair used for signaling. PBX Trunk Types Customer Premises Ameritech Central Office PBX PBX Station Sets Business Lines or T runks CO Trunk (1-way outgoing) WATS (1-way outgoing) Standard switched service. Switched service to limited geographic area. INWATS (1-way incoming) Trunk for incoming 800 number calls. PBX Trunk Types Customer Premises Ameritech Central Office PBX PBX Station Sets Business Lines or T runks DID (Direct Inward Dial) Trunk 1-way incoming trunk CO sends dialed number after call connect so PBX can automatically ring station. DOD (Direct Outward Dial) Trunk 1-way outgoing trunk PBX repeats dialed number to C.O. Centrex Customer Premises Central Office Centrex Electronic Key Sets Centrex Lines Centrex is a service that imitates all PBX features at the C.O. Private dial plans, special features, etc. are all available at a cost from your local telco. Centrex Advantages (compared with PBX system) Low up-front cost - no need to buy expensive PBX or Key System equipment System Maintenance and backups are all done by the LEC carrier rather than customer. Easy to combine multiple geographic locations into one logical Centrex group Advanced features are available that may not be included on all PBX or Key systems Centrex Disadvantages (compared with PBX system): Customer has less direct control With PBX, customer can modify switch data at any time needed (i.e. add new telephone set, change phone number, etc.) With Centrex, customer must ask LEC to make changes using service order. Centrex may be more expensive if most calls are internal (between phones on PBX) Fixed costs: Centrex is per-line, PBX is per-trunk. Fewer trunks needed if calls mainly internal. Usage costs: Lines see much more usage than trunks if calls are mainly internal. Line-to-line calls are “free” with PBX.