Troubleshooting guide

CHAPTER
4-1
Cisco Broadband Local Integrated Services Solution Troubleshooting Guide
OL-5169-01
4
Troubleshooting with Call Flows
This chapter suggests ways to use call flows to help diagnose network problems. It includes the
following sections:
• Understanding MGCP, page 4-2
• Understanding SS7, page 4-10
• Call Flow Analysis, page 4-13
• Voice Quality Problems, page 4-22
Making a phone call on a VoIP over cable network is a complex process that involves many software and
hardware components. The network elements directly involved in call setup include MTAs, CMTSs,
trunk gateways, the Call Agent, and announcement servers, which are all described in Chapter 1,
“Solution Overview”. These network elements communicate call setup information to each other using
Signaling System 7 (SS7) and the Media Gateway Control Protocol (MGCP). The sequence of messages
that must be exchanged to setup and teardown the call is referred to as a call flow.
Call flow analysis can be used to identify which element in the network is not behaving to specification,
a procedure known as trouble isolation. It allows you to initially treat each network element as a “black
box,” with a known set of inputs and a known set of expected outputs or behavior. Identifying the
non-conforming network element can point you in the right direction to further investigate a problem,
although it does not always yield the root cause of the problem. Finding the root cause of a problem
within a network element usually requires further investigation with element-specific debug tools, logs,
or trouble reports.
This chapter includes an overview of the MGCP and SS7 protocols and the key parts that are useful for
call flow analysis. The following sections review strategies for collecting MGCP protocol data, and for
identifying which MGCP messages are related to a single call attempt. It also includes an overview of
SS7 messages for off-net call setup and teardown and guidelines for troubleshooting certain specific call
flow-related problems.