VoIP Fundamentals



VoIP Fundamentals:
 

Loop Start and Ground Start Signaling:

Analog phone circuits are composed of a pair of wires. One wire is "grounded" aka the positive side of the connection (the tip). The other wire is called the "battery" aka the negative side (the ring). They are ofter referred to as "tip and ring.

Loop start signaling:
  • Used in home environments
  • Loop start is susceptible to "glare"
* Glare: Occurs when the phone is picked up to make an outgoing call and an incoming comes in at the exact same time before the phone rings.

 Supervisory Signaling:

  • On-hook signaling: No electrical signals pass because the tip and ring wires are disconnected
  • Off-hook signaling: Electric signals flow through the phone, the tip and ring connection is initiated
  • Ringing: An alternating current (AC) is sent down one of the wires this generates the ring signal

Informational Signaling:

  • Dial tone: The phone company is ready to receive digits
  • Busy: The remote phone is already in use
  • Ring back: The remote phone is ringing
  • Congestion: Indicates the long distance telephone network is not able to complete the call
  • Recorder: Indicates the local telephone company is not able to complete the call
  • Receiver off hook: The local receiver has been off the hook for a long time
  • No such number: The dialed number is invalid
  • Confirmation: The telephone company is attempting to complete the call
Address Signaling: 

There are 2 types of address signaling in the world


  • Dual-tone multifrequency (DTMF): The buttons on the telephone keypad use a pair of high and low electrical frequencies.
  • Pulse: The rotary dial wheel of a phone connects and disconnects the local loop circuit as it rotates around the signal specific digits.


Benefits of VoIP in Today's Networks

 Reduced cost of communicating
  • Cable reduction
  • Ability to move from location to location by simply disconnecting and reconnecting your phone
  • IP SoftPhones
  • Unified e-mail, voice mail and fax
  • Open compatible standards

The Cisco VoIP Structure:- Think of the VoIP structure the same way you think about the OSI layer

The endpoints:

  • IP Phone
  • Wireless cell phone
  • Video phone
  • IM Clients
Applications:

  • Voice Mail
  • Conference Call Apps
  • Call center apps
  • 911 services
Call Processing:

  • Unified Communications Manager
  • Unified Communications Manager Express
  • UC500
Infrastructure:

  • ASA Firewall
  • Voice Router gateway Voice switch
Power over Ethernet (POE):

  • Cisco Inline Power is pre-standard, as the IEEE 802.3 standard does not include any specifications for power over Ethernet. Cisco Inline Power is a proprietary standard.
  • IEEE 802.3af  technology describes a system to safely pass electrical power, along with data, on Ethernet cabling.
802.3 Power Classes:
  • Class 0: 15.4W
  • Class 1: 4.0W
  • Class 2: 7.0W
  • Class 3: 15.4W

 Cisco IP Phone boot process:

  1. The phone is connected to a POE capable switch. If the switch is POE capable it will provide inline power to the phone.
  2. The phone powers on and receives VLAN information from the switch via CDP.
  3. The phone then sends a DHCP request asking for the correct VLAN information..
  4. The DHCP server responds with an IP address offer. If the phone accepts the offer it will get DHCP options as coded by option 150.  Option 150 directs the phone to a known TFTP server.
  5. Once the phone has the IP address of the TFTP server it will contact it and download it's config files.
  6. The phone will attempt to contact the first CME router in it's config file list.

Now that we are familiar with the IP phone boot process I will provide the necessary commands to configure a Cisco router as a DHCP server.







 Here is a brief walk through of the commands needed to successfully configure a DHCP server

  1. Define a group of addresses to be excluded from the DHCP pool.
  2. Create a name for the DHCP pool
  3. Specify the network range
  4. Specify the default router: This is usually the IP address of the default gateway
  5. Specify the DNS server to be used
  6. Specify option 150 which will point the IP phone to a TFTP server.
Note: Other options can also be specified for the DHCP pool, lease time is an example.


Data DHCP pool configuration:

DHCP_Server(config)#ip dhcp excluded-address 10.0.0.1 10.0.0.15
DHCP_Server(config)#ip dhcp pool DATA_POOL
DHCP_Server(dhcp-config)#network 10.0.0.0 255.224.0.0
DHCP_Server(dhcp-config)#default-router 10.0.0.1
DHCP_Server(dhcp-config)#dns-server 8.8.8.8
DHCP_Server(dhcp-config)#exit

Voice DHCP pool configuration:

DHCP_Server(config)#ip dhcp excluded-address 10.32.0.1 10.32.0.15
DHCP_Server(config)#ip dhcp pool DATA_VOICE
DHCP_Server(dhcp-config)#network 10.32.0.0 255.224.0.0
DHCP_Server(dhcp-config)#default-router 10.32.0.1
DHCP_Server(dhcp-config)#dns-server 8.8.8.8
DHCP_Server(dhcp-config)#OPTIon 150 IP 10.32.0.1
DHCP_Server(dhcp-config)#



Extract cme to flash of the router:

Note: The following config is done under global config mode

CME# archive tar /xtract tftp://10.32.0.1/cme-4.0.1.tar flash:

Configure Cisco router as a tftp server allowing to serve files when requested by the phone:

tftp-server flash:/phone/7940-7960/P00308000500.bin alias  P0308000500.bin
tftp-server flash:/phone/7940-7960/P00308000500.loads alias P00308000500.loads
tftp-server flash:/phone/7940-7960/P00308000500.sb2 alias P00308000500.sb2
tftp-server flash:/phone/7940-7960/P00308000500.sbn alias P00308000500.sbn


Configure Cisco router as a tftp server allowing to serve ring tones when requested by the phone:

 tftp-server flash:/ringtones/RingList.xml
tftp-server flash:/ringtones/RingList.xml alias RingList.xml
tftp-server flash:/ringtones/Jamaica.raw alias Jamaica.raw
tftp-server flash:/ringtones/disticnt.xml alias disticnt.xml
tftp-server flash:/ringtones/Analog1.raw alias Analog1.raw
tftp-server flash:/ringtones/Ring5.raw alias Ring5.raw
tftp-server flash:/ringtones/Analog2.raw alias Analog2.raw
tftp-server flash:/ringtones/Vibe.raw alias Vibe.raw
tftp-server flash:/ringtones/Analog2.raw alias Analog2.raw
tftp-server flash:/ringtones/Vibe.raw alias Vibe.raw




One of the most important things that must be configured in a CME voice network is the time. All IP phones in the network must carry the same time. This feature is very important to users. It also allows voice mail to have the correct time stamps. Cisco routers use NTP to synchronize time on the network where the IP phones are concerned.

Note: The Network Time Protocol (NTP) is a protocol used for synchronizing clocks throughout the packet switched network.








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