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B2BUA – A New Type of SIP Server. Name: Stephen Cipolli Title: System Architect Date: Feb. 12, 2004. PSTN. IP. VoIP Networks. H.323. SIP. MEGACO. MGCP. IP-PBX. What is an IP-PBX? Multiple definitions Traditional PBX + VoIP Gateway PBX supporting IP-based Phones

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B2BUA – A New Type of SIP Server

Name: Stephen Cipolli

Title: System Architect

Date: Feb. 12, 2004

voip networks

PSTN

IP

VoIP Networks

H.323

SIP

MEGACO

MGCP

ip pbx
IP-PBX
  • What is an IP-PBX?
    • Multiple definitions
      • Traditional PBX + VoIP Gateway
      • PBX supporting IP-based Phones
      • Decoupled Signaling and Media
      • Server components for Peer-to-Peer Call Control
      • Back-2-Back User Agent (B2BUA)
ip pbx decoupled media proxy gateway

Analog

IP-PBXDecoupled Media + Proxy/Gateway

IP-PBX/Proxy

SIP

D-Channels

Gateway

B-Channels

PSTN

SIP

RTP

MG

peer to peer call control

Location

DB

Peer-to-Peer Call Control

IP

Proxy

Invite

Invite

User Agent 1

User Agent 2

types of ip pbxs
Types of IP-PBXs
  • Stimulus-based Signaling
    • Client/Server model
    • Intelligence is in the PBX
    • Phones are “dumb” devices
    • Centralized Call Control
    • Maps easily to Traditional PBX model
  • Session-based Signaling
    • True Peer-to-Peer Model (e.g. “Pure” SIP)
    • Centralized Control Model (e.g. SIP B2BUA)
stimulus based signaling
Stimulus-based Signaling
  • Protocols
    • MGCP & MEGACO/H.248
  • Phones
    • Send events to PBX (e.g. Hook-state, button state, etc.)
    • Play tones on command from PBX (e.g. ring, busy, etc.)
    • Create/modify/destroy Media connections on command from PBX
  • PBXs
    • Manages Call state based on Phone events
    • Commands Phones to ring, play dial tone, etc.
    • Commands Phones to create/modify/destroy media connections
ip pbx stimulus based signaling
IP PBXStimulus-based Signaling

IP PBX

Create Media

Digits

Play Dialtone

Off-Hook

MGCP

RTP

MG

session based signaling peer to peer
Session-based Signaling: Peer-to-Peer
  • Protocols
    • SIP & H.323
  • Phones
    • Implement call control
    • Implement services (e.g. Transfer, Forward, …)
    • Implement Presence
  • “PBXs” are SIP Server components such as:
    • Proxy Server
    • Redirect Server
    • Registrar
    • Event Server/Presence Server
sip servers
SIP Servers
  • The SIP specification defines a SIP server as “a network element that receives requests in order to service them and sends back responses to those requests”
  • Examples of SIP Servers:
    • Registrar Server– accepts user location messages (registrations) and updates a location DB
    • Proxy Server– forwards requests and responses
      • Similar to a router, but at the SIP level
    • Redirect Server– returns “contact this address” response
    • SIP Events Server – Handles subscription requests (i.e. Presence) from watchers and notifies them of status change.
sip proxy

Location

DB

SIP Proxy

IP

Proxy

Invite

Invite

User Agent 1

User Agent 2

sip redirect

Location

DB

SIP Redirect

“Contact this address”

IP

Invite

Redirect

Server

Invite

User Agent 1

User Agent 2

session based signaling centralized control b2bua
Session-based Signaling:Centralized Control (B2BUA)
  • Protocols
    • SIP (H.323 has a similar capability)
  • Centralized Call Control
    • Similar to traditional PBX call control
    • Enables traditional (& non traditional) PBX services
      • Transfer
      • Conference
      • Forwarding
      • Etc…
  • Allows Third Party Call Control (3PCC)
  • Enables PBX to manage and track calls from beginning to end
  • Allows interworking with other networks (e.g. H.323)
sip b2bua
SIP B2BUA
  • Back-to-back user agent (B2BUA)
    • Takes what is traditionally a SIP end-to-end call and mediates it through a central SIP server for the entire length of the call.
sip servers at work back 2 back ua transparent

Request

SIP Request

Response

Response

SIP Servers at WorkBack-2-Back UA Transparent

UAS Dialog created

UAC Dialog created

B2BUA

IP

User Agent 1

User Agent 2

sip back 2 back ua at work third party call control

RTP

ACK

ACK

INV SDP1

INV no media

INV no SDP

ACK SDP2

200 SDP1

200 SDP2

200 no media

Call between UA1 & UA2 connected

SIP Back-2-Back UA at Work – Third Party Call Control

B2BUA acts as 3PCC to create call between UA1 & UA2

IP

B2BUA

User Agent 2

User Agent 1

sip b2bua uses
SIP B2BUA Uses
  • Call Center
    • Connecting inbound/outbound calls to free agent allowing monitoring, agent training and recording.
  • Prepaid
    • Allows third party to play mid-call announcements.
    • Allows third party to disconnect call
  • Create value added services
    • Call recording
    • Send a song/recorded message
    • Dating – IM/MMS -> call creation
    • Etc…
  • Traversal of NAT
  • Policy Servers (QoS)
stimulus based signaling mgcp megaco h 248
Advantages

Simpler to

Implement Phones

Add feature

Model closer to traditional PBX model

Smaller footprint

Lower cost

Third-party Call Control

Disadvantages

Can’t be used end-to-end to phones not on PBX (need Peer-to-Peer protocols anyway)

Can’t take advantage of standardized advanced VoIP concepts

Presence

Instant Messaging

Stimulus-based Signaling(MGCP & MEGACO/H.248)
session based signaling peer to peer model
Disadvantages

Complex to

Implement Phones

Add Features

Phones typically have larger footprints

No Third Party Call Control (3PCC) Support

Advantages

Advanced standardized end-to-end features

Presence

Instant Messaging

Integration with Internet applications

Proxy/redirect behavior

Forking

Etc.

Phones operate independently of centralized call control.

Simplifies “PBX”

Session-based Signaling:Peer-to-Peer Model
session based signaling centralized control model b2bua
Disadvantages

Complex to

Implement Phones

Implement PBX

Phones typically have larger footprints

Breaks end-to-end services of SIP

End-to-end security (S/MIME) will not work

Single point of failure

Introduces more load on the network

B2BUA MUST stay in call signaling path

Call statefulness requires more memory

Advantages

May take advantage of standardized features

Presence

Instant Messaging

Integration with Internet applications

Proxy/redirect behavior

Forking

Third Party Call Control

Centralized Control of Services

Session-based Signaling:Centralized Control Model (B2BUA)
summary
Summary
  • There are many Roads to IP-PBX
  • Choices range from the traditional to the leading-edge
  • Different solutions provide different services and challenges
  • Standards-based solutions are well-defined across the range of choices
thank you

Thank You

www.radvision.com

scipolli@radvision.com

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