Cellular Networks Presentation

Wired and Wireless Phone Networks

An overview of telephony network technology

Dean Churchill, Ph.D.

Enterprise Architecture, AT&T Wireless, [email protected]

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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

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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.

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Circuit Switched Architecture

Calling Telephone Sending Switch Office

Sending call

Receiving Switch Office

Trunk

Receiving Telephone

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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.

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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

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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.

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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

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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

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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

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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

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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

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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

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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.

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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

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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.

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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.

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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

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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

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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.

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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

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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.