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Wired and Wireless Phone Networks
An overview of telephony network technology
Dean Churchill, Ph.D.
Enterprise Architecture, AT&T Wireless, [email protected]
04/10/23 Mobile Phone Networks 2
Outline of Talk1. History of phone networks – analog, and digital
2. Circuit switched vs. packet switched networks
3. Architecture of cellular networks
4. Multiplexing – TDMA and GSM networks
5. GPRS: Internet on a cell phone
6. GPRS Network Architecture
04/10/23 Mobile Phone Networks 3
History•First networks (1800s) were “point-to-point”. Required N*(N-1)/2 phone lines between N phones.
•First telephone offices used patch panels with human operators. Required N phone lines, but required a manual connection for each call placed.
•First mechanical switches developed in late 1800s.
•Digital “land-line” connections began in 1960s.
•Analog wireless phones took off in 70s.
04/10/23 Mobile Phone Networks 4
Circuit Switched Architecture
Calling Telephone Sending Switch Office
Sending call
Receiving Switch Office
Trunk
Receiving Telephone
04/10/23 Mobile Phone Networks 5
Circuit Switched Networks
• Dedicated connections: local loop, switch, and trunk resources are dedicated to the two end-points.
•Must be heavily over-designed to handle peak call volumes
•Expensive, inefficient
•Good for voice quality; bad for Internet connections
•Billing is based on duration of call.
04/10/23 Mobile Phone Networks 6
Packet Switched Land-line Networks
Telephone
Telephone
Telephone
Telephone
Telephone
Telephone
Telephone
Telephone
Sending Switch Office Receiving Switch Office
Trunk
1. Analog "local loop"place calls
2. Multiple calls getdigitized into packets, and
sent over one physicalconnection
3. Receiving side convertspackets back to analog
signals
4. Receiving phones getanalog calls
04/10/23 Mobile Phone Networks 7
Packet Switched Attributes
•Voice, video, images, pure data, get broken into small segmentscalled packets.•Each packet is encapsulated in a frame•Packet streams from multiple users are combined into a single stream•More economical, as multiple users share a common resource.
04/10/23 Mobile Phone Networks 8
Connection-less packet services
•Connection-less packet-switched services send packets that are routed independently of each other. •Can be routed through the network however the network prefers.•Packets may arrive out of order, and higher-level protocol may be needed to reorder packets.•Internet Protocol is an example (with TCP used to reorder)•Very stable networks are produced, that work around failures.•Has high overhead – lots of redundant routing information
04/10/23 Mobile Phone Networks 9
Connection-oriented packet services
•All packets sent from same source access the same pathacross the network, ensuring sequential delivery.•Has low overhead – little redundant routing information•Requires a very reliable infrastructure ( like fiber optics)•Example protocols: X.25, Frame Relay, and ATM
04/10/23 Mobile Phone Networks 10
Voice Wireless Architecture
Radio tower
Base Station
Radio tower
Mobile Switching Station
Regional Switch Office Regional Switch Office
Trunk
5. Destination switch officereceives call, and forwardsto Mobil Switching Station
4. Regional Switch Officeforwards calls
Radio tower Base Station
Radio tower
Mobile Switching Station
1. Cell phone transmits tolocal towers
2. Towers are serviced bya local base station
3. Basestations areservices by MobilSwitching Station
5. Mobile SwitchingStation forwards to Base
Station
6. Base Station forwardsto local towers.
7. Local towers completethe loop
04/10/23 Mobile Phone Networks 11
Provisioning Data Architecture
HomeLocationRegister
VisitorLocationRegister
AuthenticationCenter
EquipmentIdentityRegister
Mobile Switching Center
Databases are used totrack where mobile phonesare located, who is a validcustomer, what equipment
is used, and servicefeatures of the customer.
Billing System
Send call records,
Operation and Maintenance Center
Command and Control
Customer Service Systems
Configure Services
Authorize
Provisioning
Data records aretransmitted from Switching
Centers to the billingsystem
04/10/23 Mobile Phone Networks 12
GSM attributes•Voice and high-speed data (27-115 kbps) - carried over Internet Protocol (IP)
•Packet switched rather than circuit switched
•Short Message Service (over 1 billion messages per month are passed over GSM systems worldwide)
•Unified messaging: integrates e-mail with voice mail. Access both from one mailbox using any phone or Internet-connected computer. Users can listen to their e-mail.
•mobile commerce applications
•E911 location service
04/10/23 Mobile Phone Networks 13
General Packet Radio Service (GPRS)
•Wireless extension of the Internet all the way to mobile device
•Optimized for “bursty” packet-data traffic (e.g. web site downloads).
•Billing may be based on the amount of data transferred, rather than duration of call.
•Allows a customer to view web pages “on line” without incurring additional cost.
•Has higher transfer rates, shorter access times, improved utilization of radio spectrum
04/10/23 Mobile Phone Networks 14
GPRS Description continued•Provides packet-data transport at rates from 40 to 271 kilobits per seconds (as one to eight channels are used)
•Voice and data operations can occur concurrently.
04/10/23 Mobile Phone Networks 15
GPRS Spectrum Utilization
Total radio spectrum used: 890 - 960 MHz
Downlink: 890 -915 MHZ
Uplink : 935-960 MHz
124 TDMA Channels,each is 200 kHZ in width
8 TMDA Frames PerTDMA channel (992
total)
Each Frame is 576.9microseconds long
04/10/23 Mobile Phone Networks 16
GPRS Characteristics
•A single mobile device can transmit on one to eight channels of the same frame. Channels are allocated only when data packets are sent or received, and they are released after the transmission. Users experience being “on-line”, though the data channels of the radio connection may have been reallocated.Channels can be mapped dynamically into packet switched (GPRS) service or circuit switched (conventional) service on demand.
04/10/23 Mobile Phone Networks 17
GPRS Characteristics (continued)
Multiple users can share one channelUpload and download can occur simultaneouslyHigh speed downloads available on demand Command and control channels allow mobile device to acquire data channels dynamically.
04/10/23 Mobile Phone Networks 18
GPRS Network
ContentProviders
OtherGGSNs
Mobile Device
Radio towerBase Station Controller
Local Switch Center
Regional Data Center
SGSN
National Data Center
GGSN
Base Transceiver Station
InternetService
Providers
Firewall
GSM Switch
PublicSwitched
TelephoneNetwork
Voice and Data RoutingArchitecture
Inter-ExchangeCarriers
VOICE
DATA
SIG
HLR VLR
GGSN - Gateway GPRS Support Node
SS7 Internet Gateway
SGSN - Serving GPRSSupport Node
04/10/23 Mobile Phone Networks 19
Gateway GRPS Support Node (GGSN)
GGSN is the interface between GPRS Network and external packet data networks (e.g. Internet).
•Converts GPRS packets from SGSN into IP packets
•Receives IP packets and converts them into GPRS packets
•Sends packets to the SGSN
•Performs authentication and charging functions
04/10/23 Mobile Phone Networks 20
Authentication Center (AuC)•Stores security-related data, such as encryption keys.
•Indexed by the IMSI
•Stores a secret key (Ki)
•Key, Kc, is used for data encryption of the radio channel using the value of Ki
•Kc is requested by the VLR during the setup of a connection.
•The Ki and Kc pair are used for authentication and identification.
04/10/23 Mobile Phone Networks 21
Authentication and Encryption
GPRS Network
Secret key, Ki, is stored permanently onphone and in SSGN at inception of service,
along with IMSI
Random number sent - 128 bits long
Cipher Key, Kc, computed usingrandom number, value of Ki, and
IMSI
Send Kc
If value of Kc received from phonematches the value computed bynetwork, the phone is authentic
Secret key Ki is nevertransmitted
"Start ciphering"
User and signalling data areencrypted with key Kc
Encrypted digital stream transmits
Signal and user data aredecrypted
Tim
e
04/10/23 Mobile Phone Networks 22
Summary•Phone networks are evolving from analog-only, land-line circuit switched networks to include wireless digital packet-switched networks•Next phase of the industry is to provide high-speed Internet service on top of voice phone service. •Heavy reliance on TCP/IP networking for the digital, packet-based components.•Global roaming on GSM phones is a likely result in the long run.