Signaling protocols- MGCP

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Signaling protocols- MGCP
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Signaling architectures

• Distributed Call Control (a.k.a. Intelligent
  endpoint model)
• Centralized Call Control (a.k.a. Call Agent,
  Softswitch model)
• Dumb endpoint
• Decomposed gateway (physically separate signaling
  and media)
• media conversion close to the traffic source or
  sink
• smaller number of controller
• easy to roll out new applications

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Call Agent model

• Intelligence concentrated at Call Agent
• Routing decisions
• Policy decisions
• Call state kept at Call Agent
• Call agent is the master, gateway or endpoint is
  the slave
• Examples
• MGCP (Media Gateway Control Protocol)
• SCCP (Simple Client Control Protocol, Cisco)

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Intelligent endpoint model

• Routing decisions made in conjunction with server
  in cloud
• Policy decisions usually made by a policy server
• Call state kept at endpoint
• Examples SIP (Session Initiation Protocol), ITU
  H.323

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Media Gateway Control Protocol (MGCP)

• IETF RFC 3435
• Text based protocol, easily readable
• Master slave protocol
• Two main entities
• Call Agent(CA), a.k.a. Media Gateway Controller
• Media Gateway (MG)
• One end is circuit switch, the other is RTP port
• Trunking gateway(TGW)
• Residential gateway(RGW)
• Signaling Gateway

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MGC Requirements

• establish media stream and negotiate format
• tones and announcement
• QoS conditioning
• network monitoring
• billing and accounting
• non-stop, incremental equipment / subsystem
  upgrade

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Key MGCP terms

• Endpoints
• Basically physical circuits/resources that reside
  on the gateway
• Single endpoint can have multiple connections
• Connections
• A data path between an endpoint and a peer
  (address, port) on the IP network
• Calls
• Digit maps
• Events/Signals

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Endpoints

• Analog endpoints
• Named aaln/
• follows slot/port hardware hierarchy
• Digital endpoints (ds0 channel)
• Named ds/
• Specifically ds/ltunit-type1gt-ltunit
  gt/ltunit-type2gt-ltunit gt/.../ltchannel gt
• Example ds/ds1/1
• Virtual endpoints
• An audio source like an IVR server, Announcement
  server
• Conference bridge
• Packet relay (e.g. Firewall)
• Wire-tap access point
• Named virtual-endpoint-type/channel
• Example ann/, for announcement resources

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Endpoint Identifier

• GWs Domain Name Local Name
• Local Name
• A hierarchical form X/Y/Z
• trunk4/12/7_at_gateway.somenetwork.net
• To identify DS0 number 7 within DS1 number 12 on
  DS3 number 4 at gateway.somenetwork.net
• Wild-cards
• , any , all
• e.g., trunk1/5/_at_gateway.somenetwork.net
• CA wants to create a connection on an endpoint in
  a gateway and does not really care which endpoint
  is used.
• e.g., trunk1/5/_at_gateway.somenetwork.net
• CA requests statistical information related to
  all endpoints on a gateway.

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Connection

• Between an endpoint, and a peer on the IP network
• Connection ID created by gateway, and is unique
  in its scope
• Connection has a session description, defining
• Remote address, port
• Types of media
• Codec type

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Call

• Notion of Call, and corresponding Call-ID owned
  by the Call Agent(CA)
• Commonly, a Call comprises a set of two
  Connections created by the CA on one or two
  Gateways
• A conference call typically have many connections.

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Digit Maps

• Most calls require the collection of DTMF digits,
  and their lookup in a route database by the CA
• It is inefficient to be sending up each digit
  from the Gateway to the CA, one at a time
• A digit map is a pattern provided by the CA to
  the Gateway to collect digits
• Egrep based
• Digits are collected by the Gateway until the
  pattern is matched.
• At that point, the collected digits are sent up
  to the CA
• Example 408xxxxxxx(408 followed by 7 digits)

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Events/Signals

• There are telephony signals other than DTMF
  digits
• Example line states offhook/onhook etc
• Trunk signaling
• These are communicated using Event/signal
  packages
• Packages are defined by companion RFCs, like RFC
  3660(Basic MGCP packages)
• Generic media
• Line
• Trunk
• Dtmf
• Media
• Announcement

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Basic MGCP call components
CA (Call)
Connection 1
Connection 2
IP Network
GW1
GW2
endpoint
endpoint
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Protocol structure

• Request/Response
• Each request acknowledged by a response
• There is a set of response codes that convey the
  status of the request
• Can be run over UDP

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MGCP Command Format
MGCP 1.0
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MGCP Request

• Text format
• ACTION TRANS-ID ENDPOINT VERSION
• Parameter line
• Parameter line
• ..
• Parameter line NAME Value
• Example
• rqnt 4 aaln/1_at_rgw2.whatever.net mgcp 1.0
• r l/hd(n)
• x 234567892

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MGCP Response

• RESP-CODE TRANS-ID RESP-STRING
• Parameter line
• Parameter line
• ..
• Parameter line NAME Value
• Example
• 200 0 ok
• z aaln/1_at_rgw1.whatever.net
• z aaln/2_at_rgw1.whatever.net

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MGCP Requests Actions

• RQNT (Request notification)
• Sent from CA to MGW to request notification on
  change of endpoint status
• NTFY (Notify)
• Sent from MGW to CA in response to RQNT
• CRCX (Create Connection)
• Sent from CA to MGW
• MDCX (Modify Connection)
• Sent from CA to MGW, e.g. provide far end info
• i.e. update info about the connection
• DLCX (Delete Connection)
• Sent from CA to MGW typically. Additionally from
  MGW to CA in error conditions

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MGCP Requests

• AUCX (Audit Connection)
• Sent from CA to MGW to get status of a
  connection.(Useful in CA redundancy/failover)
• AUEP (Audit Endpoint)
• Sent from CA to MGW to get status of an
  endpoint.(Useful in CA redundancy/failover)
• RSIP (Restart in progress)
• Sent from MGW to CA to inform the CA that it is
  rebooting
• EPCF (Endpoint configuration)
• Sent by the CA to MGW to alert configuration of
  the endpoint(things like gain level, codec, etc

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Key parameter types

• X Request id, RQNT/NTFY correlator
• C Callid, String
• N Notified entity (addr of call agent to be
  notified)
• I Connection id
• S Signal request(dialtone, ringback etc)
• M Connection mode (sendonly, recvonly,
  sendrecv)
• T Events to be detected (on/off hook, DTMF)
• O Events detected
• R Events requested
• D Digit map
• B Bearer Information (A-law, mu-law)

during a call?
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MGCP Response codes

• 100-199
• Provisional, in process
• 200-299
• Successful
• 400-499
• Unsuccessful, transient error
• 500-5xx
• Unsuccessful, permanent error
• 800-8xx
• Package specific errors
• 000
• Response acknowledgement

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Reason codes

• The reason for a certain request or response
• As opposed to the Response code which is the
  status of executing that request
• 000 (normal state, used for audit)
• 900-9xx(Package specific)

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transactionID 1111 endpoint1_at_MGA CallID
1234567 Mode reconly
transactionID 1111 connectionID AAAA
RTP format 0 G711mu
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Setup for call flow
Route server/DB
CA1 port 5678
Connection 1
Connection 2
IP Network
Rgw2567 128.96.41.1
Rgw124 128.96.63.25
endpoint
endpoint
Joe
Alice 14083238155
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Call flow sequence
Route server/DB
CA1 port 5678
1(NTFY)
5(RQNT)
0(RQNT)
3(CRCX))
4(MDCX)
2(CRCX)
6(NTFY)
IP Network
7(MDCX)
Rgw2567 128.96.41.1
Rgw124 128.96.63.25
endpoint
endpoint
Joe
Alice 14083238155
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MGCP call flow (0)

• RQNT 1202 aaln/1_at_rgw-2567.whatever.net MGCP 1.0
• N ca_at_ca1.whatever.net5678 Notified Entity CA
  address
• X 0123456789AC Request ID
• R L/hd(A, E(S(L/dl),R(L/oc, L/hu,
  D/0-9T(D)))) Requested Event
• D (0T00Txxxxxxx1xxxxxxxxxx91xxxxxxxxxx9011x
  .T) DigitMap
• S Signal request
• Q process
• T G/ft DetectEvents
• 200 1202 OK

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Call flow (1)

• NTFY 2002 aaln/1_at_rgw-2567.whatever.net MGCP 1.0
• N ca_at_ca1.whatever.net5678
• X 0123456789AC
• O L/hd,D/1,D/4,D/0,D/8,D/3,D/2,D/3,D/8,D/1,D/5,D/
  5 ObservedEvents
• 200 2002 OK

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Call flow(2)

• CRCX 1204 aaln/1_at_rgw-2567.whatever.net MGCP 1.0
• C A3C47F21456789F0 CallID
• L p10, aPCMU LocalConnectionOptions
• M recvonly
• 200 1204 OK
• I FDE234C8
• v0
• o- 25678 753849 IN IP4 128.96.41.1
• s-
• cIN IP4 128.96.41.1 t0 0
• maudio 3456 RTP/AVP 0

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Call flow(3)

• CRCX 1205 aaln/1_at_rgw-124.whatever.net MGCP 1.0
• C A3C47F21456789F0
• L p10, aPCMU
• M sendrecv
• X 0123456789AD
• R L/hd
• S L/rg
• v0
• o- 25678 753849 IN IP4 128.96.41.1
• s-
• cIN IP4 128.96.41.1
• t0 0
• maudio 3456 RTP/AVP 0

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Call flow(3)

• 200 1205 OK
• K
• I DFE233D1
• v0
• o- 4723891 7428910 IN IP4 128.96.63.25
• s-
• cIN IP4 128.96.63.25
• t0 0
• maudio 3456 RTP/AVP 0

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Call flow(4)

• MDCX 1210 aaln/1_at_rgw-2567.whatever.net MGCP 1.0
• C A3C47F21456789F0
• I FDE234C8
• M recvonly
• X 0123456789AE
• v0
• o- 4723891 7428910 IN IP4 128.96.63.25
• s-
• cIN IP4 128.96.63.25
• t0 0
• maudio 3456 RTP/AVP 0
• 200 1206 OK

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Call flow(5)

• RQNT 1201 aaln/1_at_rgw-124.whatever.net MGCP 1.0
• N ca_at_ca1.whatever.net5678
• X 0123456789AC
• R l/hd(N)
• S l/rg
• 200 1201 OK

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Call flow(6)

• NTFY 2002 aaln/1_at_rgw-124.whatever.net MGCP 1.0
• N ca_at_ca1.whatever.net5678
• X 0123456789AC
• O L/hd
• 200 2002 OK

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Call flow(7)

• MDCX 1210 aaln/1_at_rgw-2567.whatever.net MGCP 1.0
• C A3C47F21456789F0
• I FDE234C8
• M sendrecv
• X 0123456789AE
• v0
• o- 4723891 7428910 IN IP4 128.96.63.25
• s-
• cIN IP4 128.96.63.25
• t0 0
• maudio 3456 RTP/AVP 0
• 200 1206 OK

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Teardown(on hook, alice)

• NTFY 2002 aaln/1_at_rgw-124.whatever.net MGCP 1.0
• N ca_at_ca1.whatever.net5678
• X 0123456789AC
• O L/hd
• 200 2002 OK

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Teardown(Joe)

• DLCX 1210 aaln/1_at_rgw-2567.whatever.net MGCP 1.0
• C A3C47F21456789F0
• I FDE234C8
• 200 1210 OK
• P PS1245, OS62345, PR780, OR45123, PL10,
  JI27, LA48

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Teardown(Alice)

• DLCX 1210 aaln/1_at_rgw-124.whatever.net MGCP 1.0
• C A3C47F21456789F0
• I FDE234C8
• 250 1210 OK
• P PS1245, OS62345, PR780, OR45123, PL10,
  JI27, LA48

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Provisional responses/ acknowledgements

• 1xx class responses to requests are allowed
• Eventually must be followed by a final response
• Final response 2/4/5xx
• In this case, requestor ACKs the final response
  that is what the 000 code is for

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Thoughts on MGCP

• MGCP is a protocol that provides low level
  primitives for a master(CA) to control a
  slave(MGW)
• Usually more than one way to implement a certain
  call flow/feature
• Feature implementation is in the CA and CAs are
  usually very complex

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Variations

• Callee busy
• Wrong number
• Etc.
• Two/Multiple Call Agents
• Call Agent to Call Agent signaling is usually an
  intelligent endpoint model protocol, such as
  SIP.

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A typical PSTN usage scenario
Signaling Controller (Signaling plane)
TDM Switch
TDM Switch
Data plane(TDM)
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Typical MGCP usage scenario
MGCP CA (Signaling plane)
SCP STP
SS7 GW
IP NetworkData Plane
Switch
Switch
MGW
MGW
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Reading exercise

• Read RFC 3435, Sections 1 to 2.1.7, and the
  example in G.2.1, G.3.1

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SCCP(Cisco)

• Another example of the call agent model
• Skinny Client Control Protocol
• Binary, not text
• Even simpler than MGCP(no digit maps, for
  example)
• Applicable to IP-PBX, low RTT between phones and
  CA
• Features implemented on the CA, not on the phone

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Wrapping up the CA model

• Sometimes the CA is also termed the MGC(Media
  gateway controller)
• H.248(also known as MEGACO) is an ITU alternative
  to MGCP, and operates on the same principles