Top Cisco Voice Voip 10 Interview Question and Answers | Part 1

 1. Explain the Cisco IP Phone boot up process?

  1. The Cisco IP Phone connects to an Ethernet switchport. If the IP phone and switch support PoE, the IP phone receives power through either Cisco-proprietary PoE or 802.3af PoE.
  2. As the Cisco IP Phone powers on, the Cisco switch delivers voice VLAN information to the IP phone using CDP as a delivery mechanism. The Cisco IP Phone now knows what VLAN it should use.
  3. The Cisco IP Phone sends a DHCP request asking for an IP address on its voice VLAN.
  4. The DHCP server responds with an IP address offer. When the Cisco IP Phone accepts the offer, it receives all the DHCP options that go along with the DHCP request. DHCP options include items such as default gateway, DNS server information, domain name information, and so on. In the case of Cisco IP Phones, a unique DHCP option is included, known as Option 150. This option directs the IP phone to a TFTP server.
  5. After the Cisco IP Phone has the IP address of the TFTP server, it contacts the TFTP server and downloads its configuration file. Included in the configuration file is a list of valid call processing agents (such as Cisco Unified Communications Manager or Cisco Unified Communications Manager Express CME agents).
  6. The Cisco IP Phone attempts to contact the first call processing server (the primary server) listed in its configuration file to register. If this fails, the IP phone moves to the next server in the configuration file. This process continues until the IP phone registers successfully or the list of call processing agents is exhausted.


2. Which transport layer protocols are used for VoIP audio and video payload?

  • The Real-time Transport Protocol is a network protocol used to deliver streaming audio and video media over the internet, thereby enabling the Voice Over Internet Protocol (VoIP). RTP is generally used with a signaling protocol, such as SIP, which sets up connections across the network.
  • The Media Gateway Control Protocol is a signaling and call control communications protocol used in voice over IP telecommunication systems. It implements the media gateway control protocol architecture for controlling media gateways connected to the public switched telephone network. 


3. What is RTP in case of VoIP? Which layer of OSI model does it work on?

  • Applications such as voice over Internet Protocol (VoIP) generally run RTP on top of the User Datagram Protocol (UDP), which provides multiplexing and checksum services. In the context of the OSI Reference Model, RTP falls into both the Session Layer (Layer 5) and the Presentation Layer (Layer 6).


4. What is RTCP and what is its use?

  • Real-Time Transport Control Protocol (RTCP) is a protocol that works with Real-Time Protocol (RTP) to monitor data delivery on large multicast networks. The purpose of monitoring delivery is to determine whether RTP is providing the necessary Quality of Service (QoS) and to compensate for delays, if needed.
  • RTP Control Protocol (RTCP) – the optional companion protocol to Real-time Transport Protocol (RTP) in a Voice Over Internet Protocol (VoIP) network. It is used to monitor the transmission and quality-of-service (QoS) of streaming media, however, it is not needed for RTP to work.


5. What are codec and DSP?

  • A codec is a device or computer program which encodes or decodes a digital data stream or signal. Codec is a portmanteau of coder-decoder. In electronic communications, an endec is a device which acts as both an encoder and a decoder on a signal or data stream, and hence is a type of codec.
  • One common application for CODECs is with Digital Signal Processors (DSPs) in audio electronics devices. CODECs convert sound (using the ADC) to digital signals for a processor to compress and store and for playback, the stored digital data is decompressed and converted to analog signals via the DAC.
  • The DSP is the one that takes care of taking analog data into digital and vice versa. TDM is 8 samples of 8 bits per second, but ANALOG. so you need something to take that into DIGITAL. that's the reason you need DSPs because VoIP is DIGITAL and traditional trunks are ANALOG

6. What are the different versions of PVDM?

  • A Packet Voice Digital Signal Processor Module (PVDM) is a hardware module that provides digital-signal-processor (DSP) resources to the system. A PVDM module can be populated with one or more DSPs. The DSP resources on a PVDM provide collaboration services including voice sessions, transcoding sessions, conference sessions, and video.
  • Four PVDM versions are currently supported: the PVDM2, PVDM3, and PVDM4 modules and the SM-XPVDM service modules. The Cisco 2800 and 3800 Series Integrated Services Routers (ISRs) support only the PVDM2 modules (please note the PVDM2 end-of-sale and end-of-life announcement). The Cisco 2900 and 3900 Series ISRs support both the PVDM2 and PVDM3 modules. The PVDM3 modules provide higher density (up to four times higher) than the PVDM2s. The Cisco 4300 and 4400 Series ISRs support the PVDM4, which has the same density as the PVDM2. PVDM4s are available on T1/E1 network interface modules (NIMs) as well as on the motherboard. On the 4451, 4431, and 4351 routers, which have service- module slots, high-density DSP-Farm PVDM service modules are also available, which can support up to 3080 G.711 transcoding channels


7. What is Cisco FLP?

  • Cisco prestandard PoE connects to switch physically. The switch sends a Fast Link Pulse (FLP) tone signal to unpowered device. ... The unpowered device boots and communicates its actual power requirement to the switch using CDP (Cisco discovery protocol) which Cisco devices use to detect directly attached neighbors.


8. What are the two POE methods?

  • EndspanThe PoE endspan method uses a switch with integrated PoE. You connect your end device to the switch, and it will detect whether the end device is PoE compatible and, if so, enables power automatically.
  • MidspanThe PoE midspan method uses PoE injectors to add PoE to regular non-PoE links. You can use these to add PoE to your network without adding or replacing your non-PoE switches. These are intelligent devices, and like a PoE switch, they detect whether the PD requires power and, if so, enables power. PoE injectors are useful when you only have a few devices you want to power.


9. What is 802.3af and 802.3at?

  • The big difference between 802.3af (PoE) and 802.3at (PoE+) is the amount of power delivered over each standard. 802.3af (PoE) can deliver 15.4 watts over Cat5 cables. 802.3at (PoE+) can deliver up to 30 watts over Cat 5 cables with 25.5 watts available to devices.


10. Why is it recommended to use a separate Vlan for IP Phones?

  • VLAN tags are used to provide high priority to voice packets. Using a VLAN for VoIP also prevents VoIP devices from competing with other traffic on the network, helping to avoid delays in delivering voice packets. A VoIP VLAN also makes it easier to troubleshoot VoIP issues since the VoIP traffic is isolated.

Top Cisco Voice Voip 10 Interview Question and Answers  Part 1

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