Patent Publication Number: US-7586923-B2

Title: Bandwidth allocation for VoIP traffic in network having interface between frame relay and ATM

Description:
FIELD OF THE DISCLOSURE 
   The present disclosure relates to methods and systems for allocating bandwidth for Voice over Internet Protocol (VoIP) traffic. 
   DESCRIPTION OF THE RELATED ART 
   VoIP imposes stringent Quality of Service (QoS) constraints on network delay, jitter and packet loss. For example, a mouth-to-ear delay is not to exceed 150 ms, an end-to-end jitter is limited to be less than 30 ms, and a packet loss ratio is not to be more than 0.5%. To achieve the QoS, appropriate network resources including bandwidth are allocated to support the VoIP application. 
   In various VoIP applications, up to eight different VoIP coder-decoders (CODECs) may be employed. The different CODECs have different raw bit rates, ranging from 5.6 kbps to 64 kbps. TABLE I shows the raw bit rates of eight commonly used VoIP CODECs. 
   
     
       
         
             
             
             
           
             
                 
               TABLE I 
             
             
                 
                 
             
             
                 
                 
               Bit Rate 
             
             
                 
               CODEC 
               (kbps) 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
          
             
                 
               G.711 
               64 
             
             
                 
               G.723ar53 
               5.3 
             
             
                 
               G.723ar63 
               6.3 
             
             
                 
               G.726r16 
               16 
             
             
                 
               G.726r24 
               24 
             
             
                 
               G.726r32 
               32 
             
             
                 
               G.728 
               16 
             
             
                 
               G.729 
               8 
             
             
                 
                 
             
          
         
       
     
   
   The different CODECs have different packetization intervals, ranging from 10 ms to 100 ms. Consequently, the network bandwidth required for different combinations of CODEC and packetization delay can drastically vary. For example, on layer 3, the bandwidth required by a VoIP call is at least 16 kbps and at most 96 kbps. 
   The bandwidth allocation is more complicated when both asynchronous transfer mode (ATM) and frame relay networks are involved to transport the VoIP traffic end-to-end. This is because ATM and frame relay introduce different overheads for the same VoIP traffic, which in turn can cause bandwidth mismatch at an ATM/frame relay internetworking interface. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is pointed out with particularity in the appended claims. However, other features are described in the following detailed description in conjunction with the accompanying drawings in which: 
       FIG. 1  is a flow chart of an embodiment of a method of allocating bandwidth; 
       FIG. 2  is a block diagram of an embodiment of an IP packet that encapsulates a VoIP sample; and 
       FIG. 3  is a block diagram of an embodiment of a network having a bandwidth allocator that performs the method of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE DRAWINGS 
   Disclosed herein is an ATM bandwidth allocation algorithm that more accurately matches ATM bandwidth to frame relay bandwidth for VoIP traffic. In an exemplary embodiment, the algorithm addresses bandwidth-mismatch-caused VoIP packet drop at an ATM/frame relay internetworking interface. Further, frame relay network bandwidth can be more fully utilized by allocating bandwidth using the algorithm. For instance, with the disclosed method, customers can send VoIP traffic at a rate of about 95% or higher of the frame relay access bandwidth. Since the signaling protocol of VoIP consumes a small portion of the frame relay bandwidth, the disclosed method is capable of reserving enough bandwidth for the signaling traffic. The disclosed method can be used by a telecommunications company to develop a network provisioning template to improve its provisioning process. The disclosed method can also be used to communicate with customers of the telecommunications company, helping them understand the cost and network resources required to support their services. For example, an agreement between the telecommunications company and its VoIP customer can include a condition that the customer cannot unilaterally alter its VoIP CODEC and its packetization interval without asking the telecommunications company to change the customer&#39;s ATM bandwidth. 
   To allocate ATM bandwidth to VoIP traffic, a number of VoIP calls that can be supported by a frame relay access link is determined. Using the herein-disclosed algorithm, the number of VoIP calls is determined based on total frame relay access bandwidth and an amount of frame relay bandwidth required for each call. ATM traffic parameters are determined based on the calculated number of VoIP calls and an estimated bandwidth for VoIP signaling and other protocol traffic. In contrast, other algorithms calculate the number of VoIP calls based on layer-3 bandwidth per call and total layer-3 bandwidth available in the network. 
     FIG. 1  is a flow chart of an embodiment of a method of allocating bandwidth. As indicated by block  10 , the method comprises determining a packetization time interval T S  and sample data size B S  for a particular CODEC. For instance, a G.711 CODEC has four different possible time intervals (10 ms, 20 ms, 30 ms and 40 ms). The sample data size is dependent on the packet time interval. TABLE II shows the sample data size B S  (in bytes) as a function of the packetization time interval T S  (in ms) for a G711 CODEC. For other CODEC standards, the sample data size B S  can be determined based on the packetization time interval T S  by referring to the standards specifications. 
   
     
       
         
             
             
             
           
             
                 
               TABLE II 
             
             
                 
                 
             
             
                 
               Packetization 
               Sample Size 
             
             
                 
               Time (ms) 
               (bytes) 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
          
             
                 
               10 
               80 
             
             
                 
               20 
               160 
             
             
                 
               30 
               240 
             
             
                 
               40 
               320 
             
             
                 
                 
             
          
         
       
     
   
   As indicated by block  12 , the method comprises determining a layer-3 data rate for a VoIP call based on the sample data size B S  and the packetization time interval T S . The layer-3 data rate is higher than the CODEC raw data rate due to at least one header in each IP packet.  FIG. 2  is a block diagram of an embodiment of an IP packet that encapsulates a VoIP sample. The IP packet comprises voice sample data  30 , a real-time protocol (RTP) header  32 , a user datagram protocol (UDP) header  34 , and an Internet protocol (IP) header  36 . The combined size of the RTP header  32 , the UDP header  34  and the IP header  36  is 40-bytes in this embodiment. Therefore, in this embodiment, the layer-3 data rate R IP  can be determined using the following equation.
 
 R   IP =8*(40 +B   S )/ T   S   (1)
 
   If an alternatively-sized header(s) are used, the “40” in equation (1) is replaced by an alternative combined size B IP  of at least one header in the IP packet, which results in R IP =8*(B IP +B S )/T S . 
   As indicated by block  14 , the method comprises determining a frame relay data rate for the VoIP call based on the sample data size B S  and the packetization time interval T S . Since frame relay adds its own header and trailer to a VoIP IP packet, the frame relay data rate is also based on the size of the frame relay header and/or trailer, herein denoted by B HT  in units of bytes. The frame relay data rate R FR  can be determined using the following equation:
 
 R   FR =8*( B   HT   +B   IP   +B   S )/ T   S   (2)
 
   where B IP =40 in one embodiment. 
   As indicated by block  16 , the method comprises determining an ATM data rate R ATM  for the VoIP call based on the sample data size B S  (in bytes) and the packetization time interval T S  (in ms). The particular equation used to determine the ATM data rate R ATM  depends on the type of encapsulation used. If sub-network access protocol (SNAP) encapsulation is used, the ATM data rate R ATM  is based on a number of cells NUM CELL  determined using the following equation:
 
 NUM   CELL =ceiling[(16+40 +B   S )/48]  (3)
 
   where ceiling[n] represents the closest integer that is not smaller than n. If virtual-circuit-based multiplexing (Vcmux) encapsulation is used, the ATM data rate R ATM  is based on a number of ATM cells NUM CELL  determined using the following equation:
 
 NUM   CELL =ceiling[(8+40 +B   S )/48]  (4)
 
   The ATM data rate R ATM  (in units of kbps if T S  is in ms) is determined using the following equation.
 
 R   ATM   =NUM   CELL *53*8 /TS   (5)
 
   As indicated by block  18 , the method comprises determining a number of VoIP calls, NUM VOIP , that can be supported by a frame relay access link. NUM VOIP  is determined based on a bandwidth of the frame relay access link that is allocated to VoIP traffic, and the frame relay data rate R FR . 
   The bandwidth of the frame relay access link is denoted by B FR . Since some of the bandwidth is to be reserved for VoIP signaling protocol and other routing protocols, only a fraction R P  of the frame relay bandwidth B FR  can be allocated to VoIP data traffic. R P  may be predetermined as 95% in one instance, but other percentage values are also within the scope of this disclosure. 
   In one embodiment, NUM VOIP  is determined using the following equation:
 
 NUM   VOIP =floor[ R   P   *B   FR   /R   FR ],  (6)
 
   wherein floor[ ] rounds a number down to its nearest integer. 
   As indicated by block  20 , the method comprises determining one or more ATM bandwidth parameters based on NUM VOIP , the frame relay data rate R FR  and the ATM data rate R ATM . The parameters may include a peak cell rate (PCR), a sustained cell rate (SCR), and a maximum burst size (MBS). 
   The SCR can be determined based on NUM VOIP  using the following equation.
 
 SCR=NUM   VOIP   *R   ATM +( B   FR   −NUM   VOIP   *R   FR )*53/48  (7)
 
   It is noted that in other embodiments, the constant 53/48 in the above equation may be replaced with another constant. 
   The MBS can be determined based on a product of NUM VOIP  and NUM CELL  using the following equation.
 
 MBS=NUM   VOIP   *NUM   CELL   (8)
 
   The PCR can be set to any number allowed by the network. For instance, the PCR can be set to be equal to SCR. 
   As indicated by block  22 , the method comprises allocating bandwidth in an ATM network for VoIP traffic based on the one or more ATM network parameters. This act may comprise outputting values of the one or more ATM network parameters in a machine-readable form to another device responsible for allocating the bandwidth in the ATM network for VoIP traffic. Alternatively, this act may comprise outputting the values of the one or more ATM network parameters in a human-readable form (e.g. on a hard copy or by an electronic display such as a computer display), wherein an operator reads the values and allocates the bandwidth in the ATM network. 
   As indicated by block  24 , the method optionally comprises determining one or more layer-3 policing parameters based on the number of VoIP calls that can be supported by the frame relay access link, NUM VOIP . The layer-3 policing parameters may include a committed information rate (CIR) and a committed burst size (Bc). The CIR can be determined using the following equation.
 
 CIR=NUM   VOIP   *R   IP   (9)
 
   The Bc can be determined using the following equation.
 
 Bc=NUM   VOIP *( B   S +40)  (10)
 
   As indicated by block  26 , the method optionally comprises monitoring and/or policing a portion of the network based on the one or more layer-3 parameters. 
   For purposes of illustration and example, the aforementioned equations are used to determine ATM bandwidth and layer-3 policing parameters for a CODEC G.711 with 20 ms packetization interval. For the packetization interval of T S =20 ms, B S =160 bytes in this CODEC standard. The IP data rate is:
 
 R   IP =(40+160)*8/(20 ms)=80 kbps.
 
   On a T1 frame relay access link, 7 bytes of frame relay header/trailers are added to each VoIP IP packet. Therefore, the frame relay data rate is:
 
 R   FR =(7+200)*8/(20 ms)=82.8 kbps.
 
   The total number of ATM cells is:
 
 NUM   CELL =ceiling[(16+200)/48]=5 cells.
 
   The ATM data rate for the VoIP call is:
 
 R   ATM =5*53*8/(20 ms)=106 kbps.
 
   The ATM traffic parameters are:
 
 SCR= 17*106 kbps+(1536−17*82.8 kbps)*53/48=1943.7 kbps, and
 
 MBS= 17*(5 cells)=85 cells.
 
   The layer-3 policing parameters are:
 
 CIR= 17*80 kbps=1360 kbps, and
 
 Bc= 17*(160 bytes+40 bytes)=3400 bytes.
 
     FIG. 3  is a block diagram of an embodiment of a network having a bandwidth allocator that performs the method of  FIG. 1 . From a customer edge (CE) router  40 , one or more customers use frame relay to access the network. A frame relay access link  42 , such as a permanent virtual circuit (PVC), communicates frame relay traffic between the CE router  40  and an FR/ATM internetworking interface  44 . The FR/ATM internetworking interface  44  converts between frame relay traffic and ATM traffic. The FR/ATM internetworking interface  44  may be located at an ATM end switch. 
   The ATM traffic is communicated via an ATM network  46  between the FR/ATM internetworking interface  44  and an ATM switch  50 . The ATM network  46  is the backbone of the network. The ATM switch  50  may comprise an ATM edge switch. An ATM access link  52 , such as an ATM PVC, communicates ATM traffic between the ATM switch  50  and a provider edge (PE) router  54 . 
   Thus, in one embodiment, the frame relay connection starts from a user&#39;s CE router  40  and ends at the ATM end switch (having the FR/ATM internetworking interface  44 ), and the ATM connection starts from the ATM end switch and ends at the PE router  54 . 
   User VoIP traffic is encapsulated in a frame relay packet and sent to the ATM end switch (having the FR/ATM internetworking interface  44 ). The VoIP frame relay packet is de-capsulated by the FR/ATM internetworking interface  44 , and converted to ATM cells that are sent to the ATM network  46 . The PE router  54  receives the ATM cells, reconstructs the VoIP packet based on the ATM cells, and delivers the VoIP packets to a user on its end. 
   To mitigate bandwidth mismatch at the FR/ATM internetworking interface, a bandwidth allocator  56  allocates bandwidth in the ATM network  46  for VoIP traffic. If the ATM bandwidth were set too low, bandwidth in the frame relay access link  42  would be wasted. If the ATM bandwidth were set too high, VoIP traffic would be dropped at the FR/ATM internetworking interface  44  and cause service quality degradation. To avoid these conditions, the bandwidth allocator  56  performs the method described with reference to  FIG. 1  to determine the one or more ATM network parameters, and to allocate ATM bandwidth to the VoIP traffic based on the parameter(s). The herein-disclosed calculation of ATM bandwidth more accurately estimates the actual ATM bandwidth used for VoIP traffic, and allows up to 95% or more of the frame relay access bandwidth to be used for VoIP traffic. 
   The bandwidth allocator  56  can be implemented by a computer system having at least one computer processor. Acts performed by the at least one computer processor are directed by a computer-readable medium having computer-readable program code stored therein. 
   It will be apparent to those skilled in the art that the disclosed embodiments may be modified in numerous ways and may assume many embodiments other than the forms specifically set out and described herein. For example, the acts described with reference to  FIG. 1  can be performed in a different order than that depicted in  FIG. 1  without affecting their results. 
   The above disclosed subject matter is to be considered illustrative, and not restrictive, and the appended claims are intended to cover all such modifications, enhancements, and other embodiments which fall within the true spirit and scope of the present invention. Thus, to the maximum extent allowed by law, the scope of the present invention is to be determined by the broadest permissible interpretation of the following claims and their equivalents, and shall not be restricted or limited by the foregoing detailed description.