B2BUA – A New Type of SIP Server

Name: Stephen Cipolli

Title: System Architect

Date: Feb. 12, 2004

PSTN IP

VoIP NetworksVoIP Networks

SIPH.323

MEGACO MGCP

IP-PBXIP-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-PBXTraditional PBX + Gateway

IP-PBXTraditional PBX + Gateway

PBX

MG

SIP

SIPGateway

RTP

IP-PBXPBX supporting IP-based Phones

IP-PBXPBX supporting IP-based Phones

PBX

MG

SIP

PSTN

RTP

IP PBXIP-based Phones/Decoupled Media

IP PBXIP-based Phones/Decoupled Media

IP PBX

MG

SIP RTP

SIP

Analog

IP-PBXDecoupled Media + Proxy/Gateway

IP-PBXDecoupled Media + Proxy/Gateway

IP-PBX/Proxy

MG

SIPRTP

SIP

PSTN

Gateway

B-Channels

D-Channels

Peer-to-Peer Call ControlPeer-to-Peer Call Control

User Agent 2

IP

User Agent 1

ProxyInviteInvite

LocationDB

B2BUAB2BUA

?

Types of IP-PBXsTypes 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 SignalingStimulus-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 PBXStimulus-based Signaling

IP PBXStimulus-based Signaling

IP PBX

MG

MGCP

Off-Hook

Digits

Play Dialtone

Create Media

RTP

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 ServersSIP 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 ProxySIP Proxy

User Agent 2

IP

User Agent 1

ProxyInviteInvite

LocationDB

SIP RedirectSIP Redirect

User Agent 2

IP

User Agent 1

Redirect

Server

Invite

Invite

“Contact this address”

LocationDB

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 B2BUASIP 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 WorkBack-2-Back UA Transparent

SIP Servers at WorkBack-2-Back UA Transparent

User Agent 2

Response

IP

User Agent 1

SIP

Requ

est

Request

Resp

onse

B2BUA

UAS Dialog created

UAC Dialog created

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

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

User Agent 1

200 no media

IP

User Agent 2

INV no mediaB2BUA

B2BUA acts as 3PCC to create

call between UA1 & UA2

INV no SDP

200 SDP1INV SDP1

200 SDP2ACK SDP2

ACK

Call between

UA1 & UA2 connected

RTP

ACK

SIP B2BUA UsesSIP 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)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

Session-based Signaling:Peer-to-Peer Model

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:Centralized Control Model (B2BUA)

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

SummarySummary

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

www.radvision.com

scipolli@radvision.com