新世代網路
Why NGN?
NGN Definition
NGN Fundamental Characteristics
NGN Areas to Study
Related Technologies
NGN Functions
NGN Transport Stratum Functions
NGN Service Stratum Functions
NGN Management Functions
NGN End-User Functions
IP Multimedia Subsystem For NGN
IMS Functional Entities
USE OF SIP AND SDP BY IMS
Enhancements To IMS for NGN Applications
VoIP與PSTN 網路互連
| Why NGN? |
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隨著電子資訊技術之飛躍發展，通訊網路技術在電信自由化之後，更如脫韁野馬般蓬勃發展，
傳統電信服務供應商在主導電信技術與市場數十年之後，竟然在數年之間兵敗如山倒，
自從網路泡沫化之後，AT&T, Lucent, Nortel 等電信大廠紛紛大幅裁員，
股價從數十美元的高股價，幾乎在一夕之間淪為水餃股。
電信自由化引進競爭固然是元兇之一，VoIP技術變成壓倒駱駝的最後一根稻草，
AT&T 所賴以維生的長途及國際電話為行動電話及VoIP 掠奪，導致AT&T 為掌握local access 的
Baby Bell, SBC, 所收購，變成難堪的at&t.
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我們可以預期，未來的通訊網路將會為新一代的技術所取代，
但我們不知道的是:何種技術將會勝出?其時程為何?
何種公司將會成為市場主導者?螞蟻雄兵式經營網路電話的第二類業者，
或是傳統的電信公司會繼續主導?要回答這些問題，牽涉到的不僅是技術問題，
不可忽視的是電信法規以及商業機制，別忘了還有民粹的影響力。
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| NGN Definition |
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ITU-T Study Group 13 defined in Recommendation Y.2001
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A
packet-based network able to provide telecommunication services and
able to make use of multiple broadband, QoS-enabled transport technologies, and in which service-related functions are independent from underlying transport-related technologies. It enables unfettered access for users to networks and to competing service providers and/or services of their choice. It supports generalized mobility which will allow consistent and ubiquitous provision of services to users. |
| NGN Fundamental Characteristics |
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Packet-based transfer
| Separation of control functions among bearer capabilities,
call/session, and application/service
| Decoupling of service provision from transport, and
provision of open interfaces
| Support for a wide range of services, applications, and
mechanisms based on service building blocks (including
real-time/streaming/non-real-time and multimedia services)
| Broadband capabilities with end-to-end quality of
service (QoS)
| Interworking with legacy networks via open interfaces
| Generalized mobility
| Unrestricted access by users to different service
providers
| A variety of identification schemes
| Unified service characteristics for the same service as
perceived by the user
| Converged services between fixed/mobile
| Independence of service-related functions from
underlying transport technologies
| Support of multiple last mile technologies
| Compliance with all regulatory requirements, for
example, concerning emergency communications, security, privacy,
and lawful interception
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| NGN Areas to Study |
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General framework and architectural principles
| Services capabilities and services architecture
| Interoperability of services and network in the NGN
| Telecommunications capabilities for disaster relief
| Architecture models for the NGN
| End-to-end Qos
| Services platforms
| Network management
| Security
| Generalized mobility
| Network control architecture(s) and protocols
| Numbering, naming, and addressig
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| Related Technologies |
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網路電話相關法規 電信網路參考架構 核心網路技術 光纖網路 (DWDM, CWDM, OADM, OXC) Overlay Model Peer Model Router 技術 IP QoS 技術 MPLS/GMPLS 接取網路技術 xDSL, 被動式接取光纖網路 (PON) FTTx + VDSL FTTx + LAN CWDM/DWDM Cable VoIP H.323 SIP, SIP-I, SIP-T MGCP MEGACO SIGTRAN SCTP 穿越 NAT 防火牆之技術 (STUN, etc) 服務開發技術 CPL SIP CGI SIP Servlet JAIN Parlay/OSA Softswitch 相關技術 VoIP 網路監聽技術 ETSI 標準 各式 VoIP 產品 現行第二類網路電話業者經營方式 Internet VoIP (Skype, MSN, Yahoo, etc.) 雙網整合技術 and VoWLAN WiFI/WiMAx/WiBro Push-to-Talk Protocol SCTP |
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| NGN Functions |
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| Transport Stratum Functions |
Service Stratum Functions
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Management Functions
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| End-User Functions
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| NGN Transport Stratum Functions |
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provide connectivity
for all components and physically separated
functions within the NGN.
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provide IP connectivity for both end-user equipment outside
the NGN, and controllers and enablers that
usually reside on servers inside the NGN.
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provid end-to-end QoS
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Transport stratum function is divided into
access networks and the core network, with a
function linking the two portions.
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| Access Functions |
Access Transport Functions
| responsible for transporting information
across the access network. They also provide
QoS control mechanisms dealing directly with
user traffic, including buffer management, queueing
and scheduling, packet filtering, traffic classification,
marking, policing, and shaping.
| Edge Functions
| for traffic processing when access traffic is
merged into the core network.
| Core Transport Functions
| responsible for ensuring information
transport throughout the core network.
| They provide the means to differentiate the quality of transport in the network, according to interactions with the transport control functions. These functions also provide QoS mechanisms dealing directly with user traffic, including buffer management, queuing and scheduling, packet filtering, traffic classification, marking, policing and shaping, gate control, and firewalls. Network Attachment Control Functions
| provide registration at the
access level and initialization of end-user functions
for accessing NGN services.
| They provide network-level identification/authentication, manage the IP address space of the access network, and authenticate access sessions. They also announce the contact point of the NGN service and application functions to the end user. That is, the network attachment control functions assist end-user equipment in registering and starting use of the NGN. Resource and Admission Control Functions
| The RACFs provide admission control
and gate control functionalities, including
control of network address and port translation
(NAPT) and differentiated services field
code points (DSCPs). Admission control
involves checking authentication based on user
profiles, through the network attachment control
functions. It also involves authorization
based on user profiles, taking into account
operator-specific policy rules and resource
availability.
| The RACFs interact with transport functions to control one or more of the following functionalities in the transport layer: packet filtering, traffic classification, marking and policing, bandwidth reservation and allocation, NAPT, antispoofing of IP addresses, NAPT/FW traversal, and usage metering. Transport User Profile Functions
| represents the compilation of
user and other control data into a single “user
profile” function in the transport stratum.
| This function may be specified and implemented as a set of cooperating databases with functionality residing in any part of the NGN. Gateway Functions
| provide
capabilities to interwork with other networks,
including many existing networks, such as PSTN/
ISDN-based networks and the Internet.
| These functions even support interworking with other NGNs belonging to other administrators. The NNI for other networks applies to both the control and transport levels, including border gateways. Interactions between the control and transport levels may take place directly or through the transport control functionality. Media Handling Functions
| The series of
media handling functions are media resource
processes for providing services, such as generating
tone signals, transcoding, and conference
bridging.
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| NGN Service Stratum Functions |
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These functions provide session-based and nonsession-based services,
including subscribe/notify
for presence information and a message method
for instant message exchange. The service stratum
functions also provide all of the network
functionality associated with existing PSTN/
ISDN services and capabilities and interfaces to
legacy customer equipment.
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| Service and Control Functions | include session control functions,
a registration function, and authentication and
authorization functions at the service level.
They can include functions controlling media resources (i.e., specialized resources). Service User Profile Functions
| These functions
represent the compilation of user data and
other control data into a single user profile function
in the service stratum.
| Application Functions
| NGNs support open
APIs enabling third-party service providers to
apply NGN capabilities to create enhanced services
for NGN users. All application functions
(both trusted and untrusted) and third-party service
providers access NGN service stratum capabilities
and resources through servers or gateways
in the service stratum.
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| NGN Management Functions |
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The management functions enable the NGN operator to
manage the network and provide NGN services
with the expected quality, security, and reliability.
These functions are allocated in a distributed
manner to each functional entity (FE). They
interact with network element (NE) management,
network management, and service management
FEs.
The management functions include charging
and billing functions.
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| NGN End-User Functions |
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The interfaces to the end user are both physical
and functional (control) interfaces.
No assumptions are made about the
diverse customer interfaces and customer networks
that may be connected to the NGN
access network.
All customer equipment categories are supported in the NGN, from singleline legacy telephones to complex corporate networks. End-user equipment may be either mobile or fixed. |
| IP Multimedia Subsystem For NGN |
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An IP multimedia subsystem (IMS) is a set of
core network FEs and interfaces used by a network
service provider to offer SIP-based services
to subscribers.
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For the most part, an IMS is independent of the access
network technology, although there are some
links between the IMS and the underlying
transport functionality, and these may be access
specific.
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An IMS is built on IETF protocols,
with specific profiles and enhancements to provide
a complete, robust multimedia system.
The enhancements and operational profiles
provide support for operator control, charging
and billing, and security.
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In addition to the enhancements and profiles,
an IMS requires a set of vertical interfaces to
provide the following:
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Common interfaces to application servers
for accounting, security, subscription data,
service control, and to service building
blocks such as presence functions
| Coordinated and enforced QoS (session
layer negotiation can be matched with
resources granted at the transport layer, per
operator policy)
| Session-based media gating under operator
control
| Correlated accounting and charging among
the service, session, and transport layers
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Controlled Operational Model
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An IMS is based on a model
where a network operator and service provider
control access to the network and services,
respectively, for which customers are billed.
This is in contrast to the usual Internet model,
where the network is transparent and all services
are provided by endpoints. As a result of
this more controlled environment, users get an
improved experience with managed QoS, single-
sign-on security, and customer support, at
least in theory.
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| IMS Functional Entities |
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| Application Server (AS) | Breakout Gateway Control Function (BGCF) | Call Session Control Functions (CSCF)
| Media Gateway (MGW)
| Media Resource Function Controller (MRFC)
| Media Resource Function Processor (MRFP)
| Subscription Locator Function (SLF)
| User Equipment (UE)
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Application Server (AS)
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provides service control
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Connections:
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to an S-CSCF
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via an Open Services Architecture
(OSA) gateway for third-party based
applications over an SIP-based ISC reference point.
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interact with the home subscriber server (HSS)
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to support various telephony-type services,
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such as call forwarding and number translation,
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to support other services
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such as presence, conference control, and online charging.
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Breakout Gateway Control Function (BGCF)
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receives session requests forwarded by an SCSCF
or another BGCF, and it selects the network
in which PSTN breakout is to occur.
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It selects a local MGCF or peer BGCF in another network.
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This ability to select a BGCF in another
network provides the capability to optimize
routing from the visited network to the PSTN,
where desired and supported by the operators
involved.
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Call Session Control Functions (CSCF)
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provide session control for the IMS.
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They coordinate
with other network elements to control
session features, routing, and resource allocation.
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| Serving CSCF (S-CSCF) | the main home
network session control point for the user
equipment (UE) for originating or terminating
sessions
| Interrogating CSCF (I-CSCF)
| the contact
point into the UE’s home network
from other networks
| Proxy CSCF (P-CSCF)
| the contact point
into the IMS from the UE
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An S-CSCF acts as a registrar, as defined in
IETF RFC 3261 [1]. In this role it accepts SIP
REGISTER requests and creates a binding
between the public user ID and the terminal
location. The S-CSCF retrieves the subscriber
profile from the HSS, including filter criteria
that indicate the ASs providing service control
for this user. To support service control, the SCSCF
interacts with these ASs during SIP signaling.
During session establishment or
modification, the S-CSCF monitors the Session
Description Protocol (SDP) to ensure that the
session is within the boundaries of the subscriber’s
profile.
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The S-CSCF uses the filter criteria to involve
ASs as needed in order to provide the services
and features to which the user subscribes. It forwards
SIP messages to each AS in the order
indicted by the filter criteria. After the last AS is
contacted, the SIP message is then sent toward
the intended destination. The filter criteria can
be set on various service trigger points, including
any known SIP method (e.g., REGISTER,
INVITE), the presence or absence of any header,
the content of any header, the direction of
the request with respect to the served user, and
SDP.
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The S-CSCF also performs routing of SIP
messages on behalf of the originating UE. It
obtains the address of an I-CSCF (or other IP
endpoint) for the network operator serving the
destination subscriber from a domain name server
(DNS) by using the destination name of the
terminating subscriber; it then forwards the SIP
request toward the destination. If the destination
name of the terminating subscriber is determined
to be a PSTN address, the S-CSCF forwards
the request to a BGCF for routing toward
the PSTN. On behalf of the destination endpoint,
the S-CSCF forwards the SIP request to a
P-CSCF according to the subscriber’s registered
location, or, for an unregistered subscriber, it
may send or redirect the SIP request to an alternate
endpoint according to call forwarding or a
similar service.
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The I-CSCF serves as the initial point of contact
to the IMS home network from other networks.
It performs a stateless SIP proxy function.
It routes received SIP requests to the S-CSCF
assigned to the user or selects an S-CSCF if one
is not currently assigned. The I-CSCF assigns SCSCFs
upon initial UE registration and when
terminating services for unregistered users.
The P-CSCF serves as the initial point of
contact for a user terminal to the IMS. It performs
a stateful SIP proxy function, sending SIP
REGISTER requests from the UE to an I-CSCF
in the home network, which is determined using
the home domain name provided by the UE.
The P-SCCF sends all subsequent SIP messages
received from the UE to the S-CSCF whose
name it has received as a result of the registration
procedure.
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The P-CSCF also ensures that a valid public
user identity for the IMS user is inserted into
UE-initiated SIP requests. It performs SIP message
compression to reduce the amount of data
sent to or from the UE. It may also support
resource and admission control capabilities by
interacting with the transport layer for networks
where this approach is employed.
Home Subscriber Server — The HSS contains
a subscription database for the IMS, including
subscription-related information to support the
network entities that actually handle calls or sessions.
It supports IMS-level authentication and
authorization and holds the IMS subscriber profiles.
The HSS also stores the currently assigned
S-CSCF.
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A home network may contain one or several
HSSs. The number of HSSs depends on the
number of subscribers, the capacity of the equipment,
and the organization of the network.
Media Gateway Control Function — The
MGCF supports interworking between the IMS
and the PSTN. It supports SIP-to-ISUP protocol
conversion and controls the media gateway for
bearer-level conversion.
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Media Gateway (MGW)
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operates under
the control of the MGCF to support interworking
between the IMS and the PSTN.
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It terminates
bearer channels from circuit-switched
networks and media streams from packet switched
networks, and performs media conversion
functions such as transcoding. In addition, it
supports dual tone multifrequency (DTMF)
detection and generation.
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Media Resource Function Controller (MRFC)
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controls the media stream resources of
the MRFP.
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It interprets information from an AS
or SIP endpoint and controls the MRFP accordingly
to support media services such as transcoding
and conferencing. The MRFC may be
collocated with an AS to provide specialized AS
services.
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Media Resource Function Processor (MRFP)
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provides resources under the control of
the MRFC for media processing.
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It supports
media stream mixing, tone and announcement
generation, transcoding, media analysis, and
other functions.
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Subscription Locator Function (SLF)
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serves as a front-end for distributed HSS systems.
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It may be queried by an I-CSCF during
registration and session setup to get the name of
the HSS containing the required subscriber-specific
data. The SLF may also be queried by the
S-CSCF during registration, or by the AS in conjunction
with the Sh interface.
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The SLF is not required in a single-HSS environment,
or in certain other HSS environments
such as a server-farm architecture. It is also not
required when an AS is configured and managed
to use a predefined HSS.
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User Equipment (UE)
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represents the functionality of a variety of user terminal devices.
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It supports the specific capabilities of the access
network within which it is used.
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In addition, it
supports the user agent capabilities of an IMS
client.
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supports the SIP methods, as defined by the IMS, for REGISTRATION,
INVITE, and so forth.
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| USE OF SIP AND SDP BY IMS |
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The IMS uses a specific SIP profile.
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It defines
extensions to headers and parameters to address
the specific needs of telecommunications operators.
Some specific parameter examples include
the following:
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| A new auth-param parameter defined for the Web-authenticate header | to pass
the integrity key and cipher key during the registration
process for setting up the integrity-protected
relationship between a UE and a P-CSCF
| A new tokenized-by parameter
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to carry encrypt/decrypt strings within SIP
headers to implement the I-CSCF’s topologyhiding
interworking gateway function
| A new icn-charging-info parameter defined
for the P-Charging-Vector header
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to include IP connectivity network charging
information
| A new parameter defined for P-Access-Network-
Info
| provides information on the
access network serving the UE
The IMS architecture has introduced several
private headers (P-Headers) into IETF in order
to meet telephony needs. P-Headers are optional
extensions to SIP:
| P-Asserted-Identity
| Enables the network
(e.g., a P-CSCF) to assert a public user
identity for a calling user.
| P-Called-Party-ID
| Enables the terminating
UE to learn the dialed public user identity
that triggered the call
| P-Access-Network-Info
| Enables the UE
to provide information related to the access
network that it is using (e.g., a cellular ID)
| P-Visited-Network-ID
| Enables the home
network to discover, via registration, the
identities of other networks utilized by the
user
| P-Associated-URI
| Enables the home
network (e.g., an S-CSCF) to return a set of
uniform resource identifiers (URIs) associated
with the public user identity under registration
| P-Charging-Function-Addresses
| Enables
distribution of the addresses of charging
functional entities
| P-Charging-Vector
| Enables sharing of
charging correlation information
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Restrictions w.r.t. the use of SDP
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It bans encryption of SDP payloads, so the operator
network is able to read them and can enforce
QoS and policies when processing INVITE
requests. For an initial INVITE, the IMS
requires the SDP payload to include terminal
capabilities, with codecs listed in priority order.
For video and audio media types, the proposed
bandwidth for each media stream will be included
in the SDP payload.
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| Enhancements To IMS for NGN Applications |
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IMS specifications were developed for use
with cellular access networks
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Needs for Enhancements
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To support xDSL-based access networks,
the IMS may also need to interface with the network
attachment functions of the IP-CAN for
the purpose of accessing location information.
No equivalent interface exists in the base IMS
specifications.
| Support for IPv4 has to be taken into
account,
leading to a requirement to support
NAPT functionalities. There are at least two rea-
| Support for NAPT functionality
| Relaxing the constraints on bandwidth
scarcity may lead to consideration of optional
support for some features that are currently considered
mandatory (e.g., SIP compression).
| Differences in location management will
impact various protocols that convey this information,
in terms of both signaling interfaces and
charging interfaces.
| Differences in resource reservation procedures
in the access network will require changes
to the IMS resource authorization and reservation
procedures, as the resource reservation procedures
for xDSL access networks will have to
be initiated by a network entity (i.e., a P-CSCF
in the case of SIP-based services), on behalf of
end-user terminals.
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| VoIP與PSTN 網路互連 |
|---|
VoIP 與 PSTN 網路互連