Category: QoS

VoIP Quick Facts

  1. Voice header is normally 40 bytes, which consists of IP + UDP + RTP (20+8+12). Bandwidth requires to send 40-byte packets at 50 packets per second is 16kbps
  2. RTP hear can be compressed from 40 bytes to 2 (without UDP checksum) or 4 bytes (with UDP checksum), it is called cRTP.
  3. G729a payload is 8kbps, The header (IP+UDP+RTP) takes 16kbps, the total bandwidth for a G729a call is 24kbps
  4. Each voice payload consists of 20ms of voice payload on Cisco IOS devices, which means 50 packets per second.
Popular Voice Codecs and Payload Bandwidth Requirements
Codec Bit Rate for Payload (no header) (in kbps) Size of payload (20-ms Default in Cisco IOS Software)
G.711 Pulse Code Modulation (PCM) 64 160 bytes
G.726 ADPCM 32 80 bytes
G.729 8 20 bytes
G.723.1 ACELP 5.3 20 bytes
Updated Bandwidth Requirements for Various Types of Voice Calls
Bandwidth Consumption, Including L2 Overhead Layer 3 Bandwidth Consumption
802.1Q Ethernet (32 Bytes of L2 Overhead) PPP (9 Bytes of L2 Overhead) MLP (13 Bytes of L2 Overhead) Frame-Relay (8 Bytes of L2 Overhead) ATM (Variable Bytes of L2 Overhead, Depending on Cell-Padding Requirements)
G.711 at 50 pps 80 kbps 93 kbps 84 kbps 86 kbps 84 kbps 106 kbps
G.711 at 33 pps 75 kbps 83 kbps 77 kbps 78 kbps 77 kbps 84 kbps
G.729A at 50 pps 24 kbps 37 kbps 28 kbps 30 kbps 28 kbps 43 kbps
G.729A at 33 pps 19 kbps 27 kbps 21 kbps 22 kbps 21 kbps  
One-Way Delay Budget Guidelines
1-Way Delay (in ms) Description
0–150 ITU G.114’s recommended acceptable range
0–200 Cisco’s recommended acceptable range
150–400 ITU G.114’s recommended range for degraded service
400+ ITU G.114’s range of unacceptable delay in all cases
Voice Delay, Jitter, and Packet Loss Requirement for VoIP
Delay < or = 150 ms
Jitter < or = 30 ms
Packet Loss < 1 percent

Serialization and Propagation Delay

  • Serialization Delay
    • size of packet in bits/bandwidth
    • Serialization Delay is the measure of the time requires to transfer a WHOLE PACKET out of an interface of a device (router or switch or hosts)
    • large packets have larger serialization delay
    • smaller links have larger serialization delay
    • serialization delay is negligible on link over T3 speed. e.g. LAN
  • Propagation Delay
    • Distance in meters/200,000,000 mps (speed of light in fiber optics)
    • Propagation Delay is the measure of how fast a bit travel from one point to another point. It is only affected by distance. Bandwidth of the link DOES NOT improve propagation delay

Three QoS Models in MPLS

  1. Three Models
    1. Pipe Model
      1. Ingress PE defines its MPLS EXP bits (SP Defined)
      2. at egress PE, forwarding is based on EXP
      3. Same QoS for each customer in Egress PE, that is reduces QoS configuration at PE
      4. A change on the outer EXP is not copied to DSCP
      5. When to use: when MPLS core and the cutomer networks have difference QoS policy, and when SP want to use EXP marking to PE to CE link. Typcial for L3VPNv3 Managed CPE customers. PE Config is more complex compare to short pipe since the MPLS EXP info needs to be stored on a temporary field on PE after it was removed on the inbound interface from core. The PE then refers to the tempoery field to set its outbound queueing
    2. Short Pipe Model
      1. Ingress PE defines its MPLS EXP bits (SP Defined)
      2. at egress PE, forwarding is based on DSCP
      3. Different QoS for each customer in Egress PE, mode complex QoS configuration at PE
      4. A change on the outer EXP is not copied to DSCP
      5. When to use: when the MPLS core and the customer networks have difference QoS Policy, and when SP wants to honor customer DSCP marking on egree PE to CE link. Typical for L3VPNv3 unmanaged CPE customers
    3. Uniform Model
      1. EXP bits on all LSP (Top) and Tunnel (Bottom) labels are copied from customer IP Precedence Bits across MPLS network
      2. That means customers define the MPLS EXP bits
      3. A change on the outer EXP is not copied to DSCP
      4. A change of EXP in MPLS will be copied to all labels and DSCP
      5. when to use: when the MPLS network and the Customer network share the same QoS policy, typical for large enterprise that run their own MPLS core
  2. In all three models, customer can set their IP Precedence Bit at ingress, the bits will be preserved at egress.
  3. Pipe and short pipe models only different at the egress PE forwarding.
Tunneling Mode IP –> Label Label –> Label Label –> IP
Uniform Copy IP Prec/DiffServ into MPLS EXP (may be changed by the SP also) MPLS EXP may be changed by SP MPLS EXP copied to IP Prec/DiffServ
Pipe MPLS EXP set by the SP QoS policy Original IP Prec/Diffserv preserved (egress queuing based on MPLS EXP)
Short-Pipe Original IP Prec/Diffserv preserved (egress queuing based in IP Prec/DiffServ)

External Links:

  1. Cisco Config Example

Implicit-Null vs. Explicit-Null label

  • The purpose of null label assignment on Egress PE is to avoid unnecessary MPLS encapsulation on the P LSR connected to the Egress PE, and also to avoid unnecessary MPLS decapsulation on the Egress PE.
  • This however has an effect on QoS on the last hop in the MPLS network. The reason is because EXP bits are in the label header as well.
  • The solution to preserve QoS info on the last hop is to use Explicit-Null label instead of implicit-Null label. When using Explicit-Null label, a label is kept but ignored, just so that the QoS EXP bits on the top label can be used for QoS consideration.
  • Explicit-Null is also used when SP wants to reduce its load on PE by letting the CE to define EXP. The CE will send MPLS null label packets to PE. The setup is considered a Carier’s Carier (CsC) design.


  1. You three options to configure MPLS EXP bits:
    1. Static
    2. copy DSCP from IP packet
    3. copy class of service on L2 frames (i.e. CoS in 802.1Q) if you are transporting L2 protocols in the case of AToM
  2. When you copy DSCP from IP, the ingress PE will copy the DSCP to both labels if the second labels exist. It is so that in the even of PHP, QoS marking is preserved for the last hop to egress PE.