VoIP and SS7
Introduction
Channel Associated Signaling (CAS)
Still widely deployed today
Considered as old technology
Common Channel Signaling (CCS)
Separation of signaling and call paths
Signaling System 7 (SS7)
To enable a wide range of services to be provided to the end-user
SS7 Network Architecture
A typical SS7 network arrangement
This configuration serves several purposes.
A fully meshed signaling network is not required.
The quad arrangement ensures great robustness.
SS7 Protocol Suite
TCAP ISUP SCCP
MAP
MTP Level 3 MTP Level 2
OSI Layers Application Presentation
Session Transport
Network Data Link
INAP
MTP Levels 1 & 2
Message Transfer Part
Responsible for getting a particular message from the source to the destination
Level 1
Handling the issues related to the signals on the physical links between one signaling node and another
Level 2
Dealing with the transfer of messages on a given link from one node to another
Providing error detection/correction and sequenced delivery of the SS7 messages
MTP Level 3
Signaling message handling
Providing message routing between signaling points in the SS7 network
Signaling network management
Rerouting traffic to other SS7 signaling links in the case of link failure, congestion or node failure
Load-sharing
ISDN User Part (ISUP)
Used as the protocol for setting up and tearing down phone calls between switches
Initial Address Message (IAM)
To initiate a call between two switches
Address Complete Message (ACM) - Optinal
To cause a one-way audio path opened from the destination switch to the originating switch (the caller can hear a ring- back tone)
Call Progress Message (CPG) – Optional
To provide additional information to the calling switch regarding the handling of the call
Answer Message (ANM)
To indicate that a call has been accepted by the called party
Release Message (REL)
To initiate call disconnection
ISUP Call Establishment and Release
SCCP
Signaling Connection Control Part
Used as the transport layer for TCAP-based services
freephone (800/888), calling card, wireless roaming
Both connection-oriented and connectionless
Mostly connectionless signaling
Global title translation (GTT) capabilities
The destination signaling point and subsystem number is determined from the global title
TCAP, MAP and INAP
TCAP (Transaction Capabilities Applications Part)
Supporting the exchange of non-circuit related information between signaling points
Queries and responses sent between SSPs and SCPs are carried in TCAP messages
INAP (IN Application Part)
MAP (Mobile Application Part)
Performance Requirements for SS7
VoIP networks should support the performance requirements
specified for SS7.
A given route set should not be out of service for more than 10
minutes per year.
No more than 1x10-7 messages should be lost.
No more than 1x10-10 messages should be delivered out of
sequence.
In ISUP, numerous timing requirements must be met.
How to make sure that VoIP
networks can emulate the signaling performance of SS7.
SIGTRAN (Signaling Transport) group of IETF
Softswitch Architecture
Signaling (SS7) Gateway
Trunking Gateway
Call Agent SCP
Residential Gateway
Internet
SS7 Network
STP
Trunking GatewayTrunking
Gateway CO
Switch
Residential GatewayResidential
Gateway MGCP/
MEGACO
MGCP/
MEGACO
RTP
SIGTRAN
Signaling Transport (SIGTRAN)
Addressing the issues regarding the transport of signaling within IP networks
The issues related to signaling performance within IP networks and the interworking with PSTN
SIP/MGCP(MEGACO)/ISUP Interworking
Translating the MTP-based SS7 message (e.g., IAM) to IP-based message (e.g., IP IAM)
Just a simple translation from point code to IP address ???
SIGTRAN
Issues discussed in SIGTRAN
Address translation
How can we deploy an SS7 application (e.g., ISUP) that expects certain services from lower layers
such as MTP when lower layers do not exist in the IP network?
For transport layer, the ISUP message must be
carried in the IP network with the same speed and reliability as in the SS7.
UDP x
TCP x
RFC 2719, “Framework Architecture for Signaling
SIGTRAN Architecture
Signaling over standard IP uses a common transport protocol that ensures reliable signaling delivery.
Error-free and in-sequence
Stream Control Transmission Protocol (SCTP)
An adaptation layer is used to support specific primitives as required by a particular signaling application.
The standard SS7 applications (e.g., ISUP) do not realize that the underlying transport is IP.
ISUP Transport to MGC
NIF (Nodal Interworking Function) is responsible for inetworking between the SS7 and IP networks
SCTP
To offer the fast transmission and reliability required for signaling carrying.
SCTP provides a number of functions that are critical for telephony signaling transport.
It can potentially benefit other applications needing transport with additional performance and reliability.
SCTP must meet the Functional Requirements
of SIGTRAN.
Why not use TCP?
TCP provides both reliable data transfer and strict order-of-transmission, but SS7 may not need ordering.
TCP will cause delay for supporting order-of- transmission.
Head-of-line Blocking
The limited scope of TCP sockets complicates the task of data transmission using multi-
homed hosts.
TCP is relatively vulnerable to DoS attack,
What Supported By Using SCTP?
To ensure reliable, error-free, in-sequence delivery of user messages (optional).
To support fast delivery of messages and avoid head-of-line blocking.
To support network-level fault tolerance that is critical for carrier-grade network
performance by using multi-home hosts.
To provide protection against DoS attack by
using 4-way handshake and cookies.
SCTP Endpoint & Association
Endpoint
The logical sender/receiver of SCTP packets.
Transport address = IP address + SCTP port number
An endpoint may have multiple transport addresses (for multi-homed host).
Association
A protocol relationship between SCTP endpoints.
Before applications at two endpoints can communicate, an association must be established.
Two SCTP endpoints MUST NOT have more than one SCTP association.
Multi-Homed Host
Host A
SCTP User
Host B
One IP address
One SCTP association with multi-homed redundant
SCTP
SCTP User SCTP
One IP address One IP address
SCTP Streams
A stream is a one-way logical channel between SCTP endpoints.
The number of streams supported in an
association is specified during the establishment of the association.
To avoid head-of-line blocking and to ensure in-sequence delivery
In-sequence delivery is ensured within a single stream.
SCTP Functional View [1/5]
Association Startup
and Takedown
Sequenced Delivery User Data Segmentation
Acknowledgment and Congestion Avoidance
Chunk Multiplex Message Validation
---SCTP User Application
Path Management
Functional View [2/5]
Association Startup and Teardown
An association is initiated by a request from the SCTP user.
SCTP provides for graceful close of an active association.
On request from the SCTP user
SCTP allows ungraceful close.
On request from the user (ABORT primitive) or as a result of an error condition detected within the SCTP layer
Functional View [3/5]
Sequenced Delivery within Streams
Stream is used to refer to a sequence of user messages that are to be delivered to the ULP.
SCTP ensures that messages are delivered to the SCTP user in sequence within a given stream.
SCTP provides a mechanism for bypassing the sequenced delivery service.
Functional View [4/5]
User Data Fragmentation
Fragmenting user messages to conform the lower layer MTU
Acknowledgement and Congestion Avoidance
Responsible for packet retransmission when timely acknowledgement has not been received
Packet Validation
Verification Tag
Checksum
Functional View [5/5]
Chunk Bundling
The SCTP user has the option to request bundling of more than one user message into a single SCTP packet.
Path Management
To monitor reachability of the far-end endpoint through heartbeats
SCTP Robustness
Robustness is a key characteristic of any carrier- grade network.
To handle a certain amount of failure in the network without a significant reduction in quality
The network should provide a graceful rather than a drastic degradation in the event of failures or overload.
Congestion and Flow Control
Multi-home hosts
SCTP ensures that endpoint is aware of the reachability of another endpoint through the following mechanisms.
