Tdc 464 Converged Multimedia Networks - Lecture 7

Front 1

MGCP is a protocol for the _________ network

Back 1

MGCP is a protocol for the carrier network

Front 2

A protocol is a solution to the problem, so what is the problem that MGCP solves?

Back 2

We have a need to carry both signaling and media bearer traffic over IP

MGCP is a solution to a problem. So, what is the problem?
Up to lecture-06, VoIP is used on the enterprise environment only. There is no direct interworking from IP to the carrier network.
Up to lecture-06, VoIP calls to PSTN must go through ISDN (Q.931). In other words, carriers cannot offer VoIP services directly to the customers.
The need (problem statement) is for carriers to offer VoIP services and to use VoIP within the carrier networks

Front 3

Where and how many MG's and MGC's should there be in the network?

Back 3

One MGC could control many MGs at multiple locations.
MGs should be close to the central offices while MGC could be at any place on the network.
Also, SS7 links are very expensive.

Front 4

Show the new network architecture

What is the key diffference?

Back 4

Separate signaling and media bearer channels

Front 5

What are the MGCP standards?

Back 5

Media Gateway Control Protocol (MGCP)
RFC 2705 => RFC 3435 (January 2003)
Informational (not a standard track)
MEGACO/H.248
MEdia GAteway COntrol
RFC 3015 => RFC 3525 (June 2003)
Standard track
H.GCP => H.248 (ITU-T) – 2002
A logical decomposition of the H.323 gateway function
Technically, this protocol is independent of SIP, H.323, or SS7

Front 6

Media Gateways exist for ....

Back 6

media conversion

PSTN to IP (and vice versa)

Front 7

What type of separations

Back 7

Logical separation between signaling/control traffic and media streams

Front 8

Separation allows

Back 8

Separation allows
centralization of call control functions
rollout of new features more quickly
media conversion as close to source and sink as possible

Front 9

Exam: Why do we need to separate MG from MGCP?

Back 9

Allows for single central control to control many media gateways, and services or features can be implemented with changes to the MGCP

Front 10

Media Gateways should be deployed

Back 10

close to the tandem offices or in the LEC offices the MGC can be located anywhere

Front 11

What are the advantages of the MG MCP distributed architecture?

Back 11

This architecture is very scalable

New service and features can be deployed easily with changes to the MGC only

Front 12

What is the most important protocol in Next Generation networks?

Back 12

MGCP – Media Gateway Control Protocol —
is the most important protocol in nextgeneration
networks because it is
responsible for implementing the
migration from PSTN to IP telephony at
large enterprises, ISPs, and carriers by
converting today’s TDM circuits into
tomorrow’s voice packets.

Front 13

How does MGCP work?

Back 13

Media Gateway Controller Protocol (MGCP)
is a device control protocol developed by
IETF and destined to control devices, like
Media Gateways and Integrated Access
Devices (IADs), by using text format
messages to set up, manage, and
terminate multimedia communication
sessions in a centralized communications
system.

Front 14

What is the difference between the MGCP and other VoIP protocols?

Back 14

The difference between MGCP and
other multimedia control protocol systems
is that MGCP allows the endpoints in the
network to control the communication
session.

Front 15

How does MGCP operate?

Back 15

MGCP is a protocol that operates between
a Media Gateway (MG) and a Media
Gateway Controller (MGC) – (also known
as Call Agents or Soft Switches), allowing
the Media Gateway Controller to control
the Media Gateway. MGCP enforces the
Media Gateway as the fundamental
component of multipoint, next generation,
converged networks. MGCP was developed
as part of the convergence movement,
which brings voice and data together on
the packet-switched Internet.

Front 16

What is the MGCP architecture?

Back 16

MGCP provides a general description of
the Media Gateway/Media Gateway
Controller model. It describes an
architecture in which call control
intelligence is outside the Media Gateways
and handled by Media Gateway
Controllers. These elements synchronize
with one another to send coherent
commands to the Media Gateways under
their control. A control protocol is used to
control VoIP gateways from the external
call agents.

Front 17

How does a Media Gateway function and provide examples of gateways;

Back 17

The Media Gateway (MG) is a basic device
that terminates PSTN switched circuits
(trunks and local loops) and converts from
pulse code modulated information to
packetized information and vice versa. It
also handles RTP media streams across
the IP network.
Examples of gateways:
• Trunking gateways – interface
between the telephone network and a
VoIP network. Such gateways typically
manage a large number of digital
circuits.
• Voice over ATM gateways – operate
much the same way as VoIP trunking
gateways, except that they interface to
an ATM network.
• Residential gateways – provide a
traditional analog interface to a VoIP
network.
• Access gateways – provide a
traditional analog or digital PBX
interface to a VoIP network.
• Business gateways – provide a
traditional digital PBX interface or an
integrated “soft PBX” interface to a
VoIP network.
• Network Access Servers – can attach
a modem to a telephone circuit and
provide data access to the Internet.
• Circuit switches (packet switches) –
can offer a control interface to an
external call control element.

Front 18

How does the MGC function?

Back 18

The Media Gateway Controller (MGC)
handles registration, management, and
control functionality of resources in the
Media Gateway. It performs protocol conversion between PSTN signaling
protocols and IP telephony. It gathers
information about IP and circuit flows and
provides that information to billing and
management systems.
The MGCP model consists of endpoints
and connections:
• Endpoints are sources or sinks of data
and could be physical or virtual. An
example of a virtual endpoint is an
audio source in an audio-content
server. Creation of physical endpoints
requires hardware installation, while
creation of virtual endpoints can be
done by software
• A connection is an association
between endpoints over which data is
transmitted. Point-to-point and
multipoint connections are possible.
Connections may exists over IP
networks, ATM networks, or internal
connections such as TDM backplanes
or gateway backplanes. For point-topoint
connections, the endpoint of a
connection could be in separate
gateways or in the same gateway

Front 19

Advantages of Separating MG and MGC

Back 19

Flexible distribution of switching functionality
Switching (bearer traffic) is on MG
Interoperability among functional elements
Carrier selection of best-in-class components – open standard and open interface
Rapid introduction of innovative new services

Front 20

What is a softswitch?

Back 20

Combines SS7 protocol and MGC on the carrier network

A media gateway translates traffic from, for instance, an ATM access link, to time-division multiplexer or IP that is used as the backbone of the carrier network. A signaling gateway mediates between the signaling used by access devices and signaling used across the service provider network. With a traditional phone network, this is called Signaling System 7.

The definition of softswitches varies. The International Softswitch Consortium limits the term softswitch to the media gateway controller. Some vendors include the media gateway or signaling gateway, too, as part of the softswitch itself.

Front 21

Requirements for MGC

Back 21

Specified in RFC 2805 (MGCP) Architecture and Requirements
Requirements
The creation, modification and deletion of media streams
The specification of the transformations applied to media streams
Request the MG to report the occurrence of specified events within the media streams, and the corresponding actions

Front 22

MGCP is protocol between

Back 22

MGC and MG

Front 23

What is an alternative term for MGC?

Back 23

Call Agent

Front 24

Call Agents control the operation of

Back 24

Call Agents control the operation of MG's

Front 25

Call Agents have the following two functions

Back 25

Call Agents (CA)
Call-control intelligence
Related call signaling

Front 26

MG's do the following

Back 26

MGs
Do what the CA commands
The establishment and tear-down of connections
A line to an RTP port

Front 27

Can Call Agents communicate with one another?

Back 27

No

Front 28

What are two messages that are required to setup a call between a CA (MGC) and a MG?

Which direction does the two message flow?

Back 28

CRCX and MDCX

From the CA to the MG's

Front 29

What is the objective of the exchange of the CRCX and MD

Back 29

The objective is to open UDP ports. The CA uses the CRCX to find out the media (source) port on one MG and another CRCX message to communicate the soruce port to the second MG and obtain the source port.

The third step is for the MDCX is for the CA to communicate the destination port to the MG originating the call

There is no need for a second MDCX from the CA to the terminating MG

Front 30

There are two types of MG's.

Back 30

The first is an access MG used for POTS line. The second is for trunking on PStN switch

Front 31

What message is used by the MG to alert the CA of an event on the edge?

Back 31

NTFY

Front 32

What is the signaling between two MG's?

Back 32

None. MG's signal to CA (MGC) but not between one another.

Front 33

Can SIP work in this case?

Back 33

No.

How does the trunking gateway know the port of acces gateway? Possibly the INVITE message.

How the does access gateway know the port of trunking gateway?

There is no way using SIP.

Front 34

Does this work?

Back 34

SIP is UNI protocol
NNI-->UNI-->NNI does not work

Hint 34

This does not work. There is no way for MG B to send port back to MG A

Front 35

What is the protocol between CA's?

Back 35

SS7

Front 36

What protocols are running on the Media Gateway

Back 36

For both trunking and access gateways MGC will be running. For access gateway Q.931 and/or Station signaling may be present as well.

You will never see SS7 or SIP running on Media Gateway

Front 37

What is the purpose of the IAM message

Back 37

Is to create a CIC from the Media Gateay and PSTN

Front 38

What are the motivations behind using IP for SS7?

Back 38

Recall some of the motivators for VoIP
Opportunity for innovation
Opportunity for cost savings
Convergence around a single technology: IP
Why limit the above benefits to the enterprise?
For VoIP to make an impact in the carrier world, it must:
maintain current SS7 functionality
provide reasons for migrating off of current SS7 infrastructure

Front 39

Is it possible to send SS7 messages over the IP network?

Back 39

Translation (SS7 => SIP => SS7) Does not work

MTP2 provide reliability on a single link. MTP3 provides redundancy

There are many SS7 messages for which there is no one-to-one mapping for SIP

Encapsulation (Put SS7 in the body of SIP) Does not work. The transport protocol for SIP is UDP which is unreliable. TCP would introduce delay

Transportation (use a different transport protocol for SS7 over IP) This works

Front 40

What is SIGTRAN?

Back 40

Signaling Transport (SIGTRAN)

Addressing the issues regarding to the transport of telephony signaling within IP networks
The issues related to signaling performance within IP networks and the interworking with PSTN
SIP/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 (PC) to IP address (Is it?)

Front 41

What is a point code?

Back 41

It's the addressing scheme on MTP3

Front 42

What are the twp parts of SIGTRAN?

Back 42

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.

Front 43

What are the issues with SIGTAN?

Back 43

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 (over MTP)
Can we use TCP or UDP to accomplish it?

Front 44

What is important about SCTP?

Back 44

SCTP: fast delivery of messages (error-free, in sequence  delivery), network-level fault tolerance

Front 45

Describe Transport and Adaptation

Back 45

SCTP: a transport layer protocol (comparable to TCP and UDP) to ensure error-free, in-sequence delivery of messages, enable fast delivery, and avoid head-of-line blocking.
Adaptation layer: to provide an interface between SCTP and upper layer protocol. It provides the primitives to send and receive protocol data. With the adaptation layer, the transport layer (i.e. SCTP) is transparent to the upper layer (ISUP or MTP3).

Front 46

If the upper layer is ISUP then we use

Back 46

M3UA

Front 47

If the upper layer is MTP3 then we use

Back 47

MP2A

Front 48

Why choose SCTP protocol over UDP or TCP?

Back 48

RFC 2960 => RFC 3309 (September 2002)
SCTP is a reliable transport protocol operating on top of a connectionless packet network such as IP.
The design of SCTP includes appropriate congestion avoidance behavior and resistance to flooding and masquerade attacks.
Reliability and Performance

Front 49

Exam: Why not use TCP?

Back 49

TCP provides both reliable data transfer and strict order-of-transmission, but SS7 may not need ordering.
TCP will cause long delay by its packet ordering scheme and the congestion control scheme.
The limited scope of TCP sockets complicates the task of data transmission using multi-homed hosts.
TCP is relatively vulnerable to DoS attack, such as SYN attacks.

Front 50

What is the Layer 1 and Layer 2 protocols with ISUP?

Back 50

MTP

Front 51

What function is resposible for internetworking between SS7 and IP networks

Back 51

NIF (Nodal Interworking Function) is responsible for interworking between the SS7 and IP networks

Front 52

What transport layer does SIGTRAN use in place of TCP and UDP?

Back 52

SCTP: fast delivery of messages (error-free, in sequence  delivery), network-level fault tolerance

Front 53

What is the SIGTRAN protocol between MPT3 and SCTP?

What is the SIGTRAN protocol between TCAP/ISUP and SCTP?

Back 53

What is the SIGTRAN protocol between MPT3 and SCTP? MP2A

What is the SIGTRAN protocol between TCAP/ISUP and SCTP?MP3A

Front 54

What is the purpose of Stream Control Transmission Protocol

Back 54

M2PA: MTP2 Peer Adaptation Layer
M3UA: MTP3 User Adaptation Layer

Purpose: carrying SS7 messages over an IP network

Front 55

What problem are we trying to address with SCTP?

Back 55

We want the benefits of SS7 without the cost. Foe example a single 64K circuits can cost $1500. Obvioulsy IP will be much cheaper.

Front 56

What are the features we need in the transport layer protocol

Back 56

Reliability
Performance
Sequence
Fault Tolerance

Front 57

What is SCTP?

Back 57

SCTP: a transport layer protocol (comparable to TCP and UDP) to ensure error-free, in-sequence delivery of messages, enable fast delivery, and avoid head-of-line blocking.

Front 58

What is head of line blocking?

How does SCTP avoid HOLO

Back 58

Head-of-line blocking (HOL blocking) is a performance-limiting phenomenon that occurs in buffered telecommunication network switches.


with multiple streams

Front 59

What are the interfaces provided by SCTP

Back 59

Adaptation layer: to provide an interface between SCTP and upper layer protocol. It provides the primitives to send and receive protocol data. With the adaptation layer, the transport layer (i.e. SCTP) is transparent to the upper layer (ISUP or MTP3).

Front 60

If the uppler layer is ISUP what interface is used

Back 60

M3UA

Front 61

If the MGC (SSP) has MPT3 at the top of the stack what is the interface

Back 61

MP2A

Front 62

Show the protocol stack for interworking between the new and old worlds

Back 62

Front 63

How we prevent denial of service attack

Back 63

4-way handshake.

End-point B will authenicate end-point A using a cookie. End-point will send the ACK back with a COOKIE. Ed-pont A will respond with a COOKIE ECHO

Front 64

SCTP

Back 64

n computer networking, the Stream Control Transmission Protocol (SCTP) is a Transport Layer protocol, serving in a similar role to the popular protocols Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). It provides some of the same service features of both: it is message-oriented like UDP and ensures reliable, in-sequence transport of messages with congestion control like TCP.

The protocol was defined by the IETF Signaling Transport (SIGTRAN) working group in 2000,[1] and is maintained by the IETF Transport Area (TSVWG) working group. RFC 4960 defines the protocol. RFC 3286 provides an introduction.


Features of SCTP include:

Multihoming support in which one or both endpoints of a connection can consist of more than one IP address, enabling transparent fail-over between redundant network paths.
Delivery of chunks within independent streams eliminate unnecessary head-of-line blocking, as opposed to TCP byte-stream delivery.
Path selection and monitoring select a primary data transmission path and test the connectivity of the transmission path.
Validation and acknowledgment mechanisms protect against flooding attacks and provide notification of duplicated or missing data chunks.
Improved error detection suitable for Ethernet jumbo frames.
The designers of SCTP originally intended it for the transport of telephony (Signaling System 7) over Internet Protocol, with the goal of duplicating some of the reliability attributes of the SS7 signaling network in IP. This IETF effort is known as SIGTRAN. In the meantime, other uses have been proposed, for example, the Diameter protocol[citation needed] and Reliable server pooling (RSerPool).[4]

Front 65

Issues with TCP use with SS7

Back 65

TCP provides both reliable data transfer and strict order-of-transmission delivery of data. Some applications need reliable transfer without sequence maintenance, while others would be satisfied with partial ordering of the data. In both of these cases, the head-of-line blocking offered by TCP causes unnecessary delay.
The stream-oriented nature of TCP is often an inconvenience. Applications must add their own record marking to delineate their messages, and must make explicit use of the push facility to ensure that a complete message is transferred in a reasonable time.
The limited scope of TCP sockets complicates the task of providing highly-available data transfer capability using multi-homed hosts.
TCP is relatively vulnerable to denial-of-service attacks, such as SYN attacks.

Front 66

SCTP Security

Back 66

Although Encryption was not part of the original SCTP design, SCTP was designed with features for improved security, such as 4-way handshake (compared to TCP 3-way handshake) to protect against SYN flooding attacks, and large "cookies" for association verification and authenticity.

Reliability was also a key aspect of the security design of SCTP. Multi-homing enables an association to stay open even when some routes and interfaces are down. This is of particular importance for SIGTRAN as it carries SS7 over an IP network using SCTP, and requires strong resilience during link outages to maintain telecommunication service even when enduring network anomalies.

Front 67

What is the difference between TCP 3-way handshake and SCTP 4 way handshake as it relates to resource allocation

Back 67

The resource is allocated during the SYN-ACK from the receiver with TCP. SCTP does not allocate the resource until the COOKIE ECHO is returned and the sender is authenicated.

Front 68

SCTP Robustness

Back 68

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
An association include one or more IP addresses (a primary address + several secondary addresses).
Multi-homes hosts
SCTP ensures that endpoint is aware of the reachability of another endpoint through the following mechanisms.
Active data => association is OK.
HEARTBEAT if an association is idle

Front 69

Exam: What service does M3UA provide?

How

Back 69

Provides similar interface as MTP3.

Using AS and ASP which provide the redundancy architecture for uppler layer interface

Front 70

M3UA Operation

Back 70

M3UA over SCTP
Application Server (AS)
A logical entity handling signaling for a scope; for example, handling ISUP signaling for a SS7 DPC/OPC/CIC-range
An AS contains a set of Application Server Processes (ASPs)
Application Server Process (ASP)
A process instance of an AS
Can be spread across multiple IP addresses (multi-homed)
Robustness: active ASPs and standby ASPs

Front 71

MP2A Operation

Back 71

IP-based SS7 links
No FISUs sent; only LSSUs and MSUs
Note: an IP is a shared resource, while SS7 links are dedicated signaling functions.
Establish SCTP associations between M2PA peers
Two streams: one for MSU, and the other for LSSU

MP2A is simulate MTP2.

Creates the signaling as if there is MTP2

Front 72

EXam: Which MTP2 message is not needed

Back 72

MP2A is for MTP2. There are three MTP messages FISUs sent; only LSSUs and MSUs

FISU's are similar to heartbeat and are sent when there is no data and is not needed since this is handeld by SCTP.

Front 73

Summary

What is SIGTRAN?

Back 73

Mechanism Allows us to send SS7 message over IP network.

SIGTRAN is about the communication between MGC which is SS7 message over IP

Three approaches

translation
transportation
interworking

Front 74

What is ISUP encapsulation
What are the issues

Back 74

No one-to-one mapping of message and reliability

Does not work for all call flows

When the softswitch network provides a transit function, interworking often leads to a lowest common denominator result
Retry-after header
ISUP -> SIP -> ISUP
Are the ISUP messages the same?
The reliable delivery of provisional response
SIP for Telephones (SIP-T) – RFC 3372
Best Current Practice (not a standard)


NNI-UNI-NNI will never work.
There are many more SS7 messages than SIP messages.
No one-to-one mapping – a requirement for translation and encapsulation.
Reliability of the transport layer (SIP over UDP)
Performance of the transport layer (SIP over TCP)

Encapsulating some ISUP in SIP and translating some ISUP to SIP

Front 75

AS and ASP defined

Back 75

Application Server (AS) - A logical entity serving a specific Routing
Key. An example of an Application Server is a virtual switch element
handling all call processing for a signalling relation, identified by
an SS7 DPC/OPC. Another example is a virtual database element,
handling all HLR transactions for a particular SS7 SIO/DPC/OPC
combination. The AS contains a set of one or more unique Application
Server Processes, of which one or more is normally actively
processing traffic. Note that there is a 1:1 relationship between an
AS and a Routing Key.

Application Server Process (ASP) - A process instance of an
Application Server. An Application Server Process serves as an
active or backup process of an Application Server (e.g., part of a
distributed virtual switch or database). Examples of ASPs are
processes (or process instances) of MGCs, IP SCPs, or IP HLRs. An
ASP contains an SCTP endpoint and may be configured to process
signalling traffic within more than one Application Server.

Association - An association refers to an SCTP association. The
association provides the transport for the delivery of MTP3-User
protocol data units and M3UA adaptation layer peer messages.