Applying QoS for Voice Traffic

QoS is very important for Voice traffic which is delay sensitive. I won’t go into details of QoS over here and will just explain the configuration we normally use on a Voice gateway for QoS. class-map match-any Voice-RTP match ip precedence 5 match ip dscp ef match ip rtp 16384 16383 class-map match-any Voice-Cntl match ip precedence 3 match ip dscp af31 match access-group name voice-signal ! ip access-list extended voice-signal permit tcp any any range 2000 2002 permit udp any any eq 2427 permit tcp any any eq 2428 permit tcp any any eq 1720 permit tcp any any … Continue reading Applying QoS for Voice Traffic

Scenario#27 – RightFax Server not working – MGCP

I had an interesting scenario where customer logged a case with us regarding their RightFax server not working. Those of you who don’t know what is a RightFax server can read about it here. When you dial the fax number from outside, it reaches the gateway and you hear one ringback and then a fast busy. The call flow was something like this: 0XXXX23422 >>>> Birmingham GW  FXO >>> on CCM we have FXO port with number 3422 >> on CCM there was a RP 3422 with a RL pointing towards an Intercluster trunk to The was the … Continue reading Scenario#27 – RightFax Server not working – MGCP

DSP, PVDM & Media Resources

The Cisco High-Density Packet Voice Digital Signal Processor (DSP) Module (PVDM2) enables Cisco Integrated Services Routers to provide high-density voice connectivity, conferencing, and transcoding capabilities in Cisco IP Communications solutions. PVDM stands for packet voice DSP module; it is the Cisco product name for the module that provides digital signal processing resources to a system. DSP stands for Digital Signal Processor; it is a generic Industry terminology. A PVDM module could be staffed with one or multiple DSPs. PVDM2-8 = 8ch (G), 4ch (H), 4ch (M) PVDM2-16 = 16ch (G), 6ch (H), 8ch (M) PVDM2-32 = 32ch (G), 12ch (H), … Continue reading DSP, PVDM & Media Resources

Scenario#10 – Phones showing Incorrect time

I don’t remember how many times I came across customers complaining that their phones are either x minutes ahead or x minutes behind the normal time. This is what I do whenever I come across time issues. This is for applicance based Call manager (5.x, 6.x, 7.x, 8.x) Go to OS Admin > Settings > NTP server Is there any NTP Server? If there is an IP address mentioned then try to Ping that IP from OS Admin > Services > Ping Most of the time customers add the CUCM IP address as NTP address. This is not correct. Under … Continue reading Scenario#10 – Phones showing Incorrect time

MGCP Call Preservation

MGCP gateway is a Survivable endpoint unlike H323 which is not survivable. What does it mean? It means that an active call through an MGCP gateway will not tear-down and the call will be preserved. As discussed already in another section that an MGCP gateway terminates Layer 2 signalling at the gateway while Backhauling Layer 3 Q931 Signalling to CUCM through TCP port 2428. Terminating Layer 2 Signal (Q921) at gateway means the D-channel will remain up even if one of the call manager goes down. There are three steps involved in MGCP call preservation which are transparent to user: … Continue reading MGCP Call Preservation

MGCP Gateway Setup

MGCP Configuration Using Trombone Feature: To register an MGCP gateway to call manager, follow these steps: Do a show inventory OR show diag to check the position of E1/T1 Enable the E1/T1 by using command – card type T1 0 0 OR card type T1 0 1 (whatever the case may be) Do show ver to check what gateway is it isdn switch-type primary-ni network-clock-particpate wic 0 network-clock-select 1 E1 0/0/0 Go to CCM and add MGCP gateway as per type Make sure the name and domain are exactly the same as they are on gateway If domain is … Continue reading MGCP Gateway Setup


In a typical ISDN setup for an Inbound call from PSTN to an IP phone (-SCAC-) CCM  –  GW  – PSTN Setup <——- Call proceeding ——-> Alerting ——-> Connect ——> Gateway and PSTN switch exchange Q921 Layer 2 information. Gateway registers with ISDN switch and terminates Q921 at gateway. A Q931 Setup message arrives. The gateway puts it in a packet and send it to CCM via TCP port 2428 (Backhaul) CCM then sends three Q.931 messages – Call proceeding, Alerting and connect After the call is brought up, CCM sends the gateway an MGCP MDCX to tell it parameters … Continue reading MGCP Call Flow – ISDN PRI