Interworking Internet Telephony and Wireless Telecommunications Networks

1
Interworking Internet Telephony and Wireless
Telecommunications Networks
Jonathan Lennox
Kazutaka Murakami
Mehmet Karaul
Thomas F. La Porta
lennox kmurakami
karaul
tlp _at_bell-labs.com
_at_bell-labs.com _at_bell-labs.com
_at_bell-labs.com
Bell Labs, Lucent Technologies

• Presented By Matt Vidal

July 1,2003
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Introduction

• Wireless Mobile Telephony and Internet Telephony
  are both growing at a rapid pace
• Mobile Global System for Mobile communication
  (GSM)
• Internet Telephony Session Initiation Protocol
  (SIP)
• What is the best way for calls to be delivered
  and routed between the two networks?
• Both SIP and GSM are designed to interface with
  Public Switched Telephone Network (PSTN)
• PSTN Interface is a bad idea as it can be very
  inefficient in the mobile realm

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Introduction (2)

• Triangular Routing in mobile networks due to PSTN
• Causes highly inefficient routing of both
  signaling and media packets

Triangular Routing in Mobile Networks
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Background GSM Mobility and Call Delivery

• MSC Mobile Switching Center
• Serving MSC routes all mobile calls within its
  serving area
• Gateway MSC directs calls from PSTN into mobile
  access network
• VLR Visitor Location Register
• Stores information about devices in its serving
  area
• HLR Home Location Register
• Maintains subscriber information and location
• GSM device has International Mobile Subscriber
  Identity (IMSI)
• HLR gets mobiles location using GSM Mobile
  Application Part (MAP) Protocol

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Background GSM Mobility and Call Delivery (2)

• GSM call placed from Mobile Station ISDN Number
  (MSISDN) Sent to gateway MSC (2 Phases)
• 1. Gateway MSC obtains temporary Mobile Station
  Routing Number (MSRN) by contacting subscribers
  HLR
• 2. The call is routed to the serving MSC using
  ISDN User Part Protocol (ISUP)
• Intra-MSC Handover (Between base stations within
  one MSCs control) media traffic is simply
  redirected
• Inter-MSC Handover (To a different serving MSC)
  old serving MSC is required as an anchor,
  extending the calls media circuit across the PSTN

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Background GSM Mobility and Call Delivery (3)
Elements of a GSM Network
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Background SIP Mobility and Call Delivery

• Simpler than GSM
• All devices communicate by IP
• All signaling occurs with SIP
• GSM / SIP Mapping
• GSM SIP
• HLR Registrar
• Gateway MSC Home Proxy Server
• Serving MSC End System (for REGISTER)
• MSISDN User Address (in INVITE)
• IMSI User Address (in REGISTER)
• MSRN Device Address

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Background SIP Mobility and Call Delivery (2)

• End Systems Contact Registrars Directly (Not 2
  Phases)
• New SIP device sends SIP REGISTER to its
  Registrar
• SIP Address is in the form of User_at_Domain
• New call sends SIP INVITE to SIP Address
• Call is established and media flows directly
  between the endpoints
• Handles handovers by reconfiguring the IP Address
  it is sending to without having to relay from
  home address as in mobile IP.

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Background SIP Mobility and Call Delivery (3)
Elements of a SIP Network
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Architecture SIP/GSM Interworking

• Three primary issues to consider when
  interworking Internet Telephony and Wireless
  Systems
• 1. How Calls May Be Placed From SIP to GSM
• 2. How Calls May Be Placed From GSM to SIP
• 3. How In-Call Mobility (Handovers) Is Handled
• Issues two and three are easily dealt with
• The first issue is the most complicated and will
  be investigated thoroughly

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SIP/GSM Interworking Calls From GSM To SIP

• Calls from GSM to SIP are functionally similar to
  calls originating from PSTN to SIP
• Difference is that they are addressed in
  telephone form rather than User_at_Domain form
• Solution is to perform a lookup (Enum) based on
  the telephone number
• Maybe use custom dialing plan prefixes
• Translate number into User_at_Domain form
• Proceed as usual PSTN-to-SIP

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SIP/GSM Interworking In-Call Handover

• Two Categories
• 1. Intra-MSC
• 2. Inter-MSC
• Intra-MSC needs no special treatment for SIP/GSM
  Interworking
• Could optimize by using different IP Addresses to
  correspond to different base stations under
  serving MSCs control
• Inter-MSC affects SIP/GSM Interworking
• Remains for future study

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SIP/GSM Interworking Mobile-Terminated Calls

• Most Complex Aspect of SIP/GSM Interworking
• Need direct media stream between caller and
  serving MSC
• To do so, SIP signaling needs to reach Serving
  MSC for
• IP Address
• Port Assignment Conventions
• Media Characteristics
• Three proposals for how SIP devices can determine
  the MSC at which the GSM device is registered
• Each has various degrees of complexity, signaling
  traffic and call setup delay

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Proposal 1 Modified Registration

• Enhance MSCs Registration Behavior
• Serving MSC registers with Subscribers HLR and
  with Home SIP Registrar
• MSC Needs to find SIP Registrar
• This can be done with an Enum lookup using GSM
  devices MSISDN number from registration
• Initial Registration (Dual Registration)
• 8-10 GSM MAP messages, 6 DNS messages, 4 SIP
  messages
• Call Setup (Standard SIP)
• 1 SIP message, 4 DNS messages

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Proposal 1 Modified Registration (2)
Modified Registration Procedure
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Proposal 1 Modified Registration (3)

• Advantages
• Minimal Infrastructure Modifications
• Requires updates to the serving MSC
• Requires the addition of an Enum type of Database
• Call Setup is SIP, one phase lookup
• Disadvantages
• With two databases, inconsistency could be a
  problem
• GSM HLR is persistent
• SIP Registrations require refreshing
• Dual registration requires extra signaling
  overhead

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Proposal 2 Modified Call Setup

• Adapt GSM Call Setup
• GSM Registration untouched
• GSM call setup behaves like SIP
• SIP Proxy Server determines MSISDN based on SIP
  Address
• Queries HLR for MSRN
• Enum Lookup for serving MSC SIP Address
• SIP INVITE message sent to serving MSC
• Initial Registration (Standard GSM)
• 8-10 GSM MAP messages
• Call Setup
• 4 GSM MAP messages, 6 DNS messages, 1 SIP message

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Proposal 2 Modified Call Setup (2)
Modified Call Setup Procedure
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Proposal 2 Modified Call Setup (3)

• Advantages
• GSM database remains the same
• No possibility of out-of-sync database like in
  Modified Call Setup
• Low registration overhead
• Same as GSM
• Disadvantages
• Three Phase Call Setup!
• GSM MAP query for the MSRN
• Enum lookup for SIP device address
• Call initiation
• SIP and HLR need to be in communication

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Proposal 3 Modified HLR

• GSM HLR in Charge
• Standard GSM registration
• HLR determines the SIP device address at the
  serving MSC
• HLR performs a Enum lookup to gather the serving
  MSCs SIP domain (taking advantage of SIP address
  format)
• HLR returns SIP Address (mapped from GSM
  registration, MSISDN_at_hostname.of.serving.MSC) to
  the SIP Proxy Server
• Initial Registration
• 8-10 GSM MAP messages, 2 DNS messages
• Call Setup
• 4 DNS messages, 1 SIP message

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Proposal 3 Modified HLR (2)
Modified HLR, Registration Procedure
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Proposal 3 Modified HLR (3)
Modified HLR, Call Setup Procedure
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Proposal 3 Modified HLR (4)

• Advantages
• Very low signaling overhead
• Nearly matches best registration overhead
• Has the best call setup overhead
• Low Call Setup Latency
• Disadvantages
• Significant modification of GSM HLR
• SIP Proxy Server and HLR must be co-located, or
  have an interface protocol designed

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Analysis

• Two important measures for SIP and GSM Proposals
• Signaling Load
• Setup Delay (to be investigated at a later time)
• To compare total signaling load, signaling
  messages were weighted and compared
• Modified HLR has lowest signaling load (20-30
  less)
• The other two depend on traffic parameters
• Modified Call Setup best with low incoming call
  rate and low call/mobility ratio (higher setup
  time)
• Modified Registration best with high incoming
  call rate and high call/mobility ratio (higher
  registration time)

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Analysis (2)
Weighted Signaling Load of all Three Proposals
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Implementation

• Implemented Modified Call Setup
• Enhanced Mobile Call Processing (EMCP) of Bell
  Labs Router for Integrated Mobile Access (RIMA)
• RIMA composed of Base Station Controller (BSC),
  PSTN Media Gateway (PSTN-GW), RTP Media Gateway
  (RTP-GW) and EMCP network
• BSC Circuit Voice Formatted to/from Packet Voice
• PSTN-GW RTP/IP Packet Voice Translated to/from
  Circuit Voice (RIMA ltgt PSTN)
• RTP-GW RIMA Media Connections to Internet
• EMCP Implements RIMAs MSC and VLR Functionality
  (IP Telephony Signaling Gateway)

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Implementation (2)
RIMA-based Network Implementation
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Implementation (3)
EMCP Processing Engine
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Implementation (4)
EMCP Network Architecture
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Conclusions

• Proposed Three Schemes to Directly Interconnect
  SIP Internet Telephony and GSM Mobile Telephony
  Systems
• Modified Registration
• Modified Call Setup
• Modified HLR
• Eliminates Triangular Routing Created by PSTN
• More Efficient Media Packet Routing
• Eliminates Unnecessary Transcodings Due to PSTN
• Better Voice Quality

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Conclusions (2)

• Modified Registration
• Very Efficient When High Incoming Call Rate And
  High Call/Mobility Ratio (Higher Registration
  Overhead)
• Modified Call Setup
• Very Efficient When Low Incoming Call Rate And
  Low Call/Mobility Ratio (Higher Call Setup
  Overhead)
• Modified HLR
• Very Low Signaling Burden
• (20-30 Less Than Modified Registration /
  Modified Call Setup)
• Significant GSM Equipment Modification
• Development is in progress for a Modified HLR
  scheme

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Questions?