Voice over Internet Protocol (VoIP), also called as IP phone or network phone, is currently a hot topic in the field of Internet application. VoIP is an advanced communication medium that uses the Internet to transfer digital voice signal on real-time basis for local or long-distance calls.
With the ZXR10 router, developed by ZTE Corporation, the enterprise can use the existing IP network to build its own private VoIP network, thus considerably reducing the expenses for long-distance and fax communication. Meanwhile, the ZXR10 router serves as a voice gateway and can flexibly provide a variety of value-added services. This makes possible the delivery of triple-play services to the enterprise. Therefore, an Intranet can be established capable of delivering effective and integrated services.
1 ZTE´s QoS-Supporting VoIP Equipment
Based on its powerful R&D capability, ZTE rolls out flexible VoIP solutions to meet the requirements for deploying services such as enterprise VoIP private network, carrier-grade IP local call and VoIP supermarket.
The ZXR10 General Access Router (GAR) acts as a voice gateway or voice trunk gateway to provide voice functions and convert data from traditional circuit-switched network to packet-switched network. During the conversion, it is necessary to map the traditional telephone number to the IP address that can be identified by the packet switching. This mapping relation may be completed via the gateway or the GateKeeper (GK).
In addition to delivering basic routing functions, the ZXR10 GAR can also be used to carry voice services via the IP network among all remote branch offices of an enterprise. For a typical networking, the headquarters and all branch offices of the enterprise are equipped with ZXR10 GARs, and these ZXR10 GARs can be interconnected over IP through a dedicated line or the Internet. Moreover, serving as a VoIP gateway, the ZXR10 GAR performs signaling conversion from Public Switched Telephone Network (PSTN) to H.323, voice coding/decoding and voice transmission.
The ZXR10 GAR has distinct advantages over various VoIP products available in the market.
Firstly, the ZXR10 GAR supports multiple voice interfaces for interconnection with ordinary telephone, such as: Foreign Exchange Station (FXS) with traditional PSTN switch，Foreign Exchange Office (FXO) and E1. It supports various coding/decoding standards including G.711, G.723, G.728 and G.729. Besides, it complies with the standard H.323 protocols to support two modes of calling: the fast and the non-fast. It offers all functions of the router such as routing, forwarding, secure encryption, load balancing, Virtual Private Network (VPN), backup and configuration management. It also supports Voice Activity Detection (VAD), echo cancellation and comfortable noise generation to provide users with better voice quality. In addition, it has the GK Client function to support interoperation with external standard GKs, so that the GK can be used to implement the VoIP solution.
Secondly, the ZXR10 GAR provides multiple QoS methods to ensure voice quality. On one hand, it uses the hardware to implement coding/decoding and encapsulation of voice packets, thus maximally reducing the packet processing delay. On the other hand, it uses traffic classification, queuing (PQ/CQ) and other methods to mark high priority for first forwarding, thus greatly reducing the transmission delay.
Thirdly, the ZXR10 GAR provides the jitter buffer function. The jitter refers to the variation of time delay, which results in reduced voice quality. The ZXR10 GAR can reduce the jitter in three ways: reducing the jitter via the queue buffer, providing QoS to ensure the bandwidth of voice messages to be sent so as to avoid the variation of delay, and implementing Compressed Real-time Transmission Protocol (CRTP) Header to reduce bandwidth occupancy and to make sure the jitter is reduced even for a low-speed link. With these three ways, the jitter is well controlled.
Fourthly, as a gateway, the ZXR10 GAR has the following advantages.
(1) It supports high-density voice interfaces, with the capability to simultaneously connect a maximum of 64 phones and 64 remote calls originated from local. If the trunk line is also taken into consideration, a maximum of 8 lines are available for connection with E1 of the traditional PSTN switch, i.e., total of 240 calls can be supported.
(2) It adopts time slot switching rather than IP to switch calls on the router, which improves the voice quality and reduces the CPU load as well.
(3) It supports Multiple Protocol Label Switching (MPLS) VPN, so the VPN technology can be adopted to enhance privacy and security of VoIP.
Finally, the ZXR10 GAR supports Network Address Translation (NAT) traversing. With the wide applications of VoIP, its failure in supporting the NAT equipment of the H.323 protocol, on which the VoIP application is based, has received growing attentions. The ZXR10 GAR is a good solution to this problem.
In addition to rich VoIP functions, the ZXR10 GAR supports Time Division Multiplexing over Internet Protocol (TDMoIP) that is a kind of new voice technology developed based on Time Division Multiplexing (TDM) and IP. It is capable of transparent transmission of E1 circuit signals over IP network.
TDMoIP is transparent transmission to E1. Therefore, it has excellent compatibility with traditional telecom networks. All conventional services such as protocols, signalings, data, voice and image can make full use of this new technology without any alternation to the related devices.
2 ZTE´s Enterprise VoIP Solutions
ZTE has a variety of networking solutions available for enterprise VoIP. These solutions differ due to many factors including the enterprise scale, the number of branch offices, the investments in existing equipment, and whether to allow the extension phone to dial a PSTN call. Three typical networking solutions for enterprise VoIP are discussed here.
2.1 Simple VoIP Networking Solution
The word "simple" here refers to small- or middle-sized enterprises without traditional PSTN switch. Additionally, each branch office is less demanding on the total number of extension phones and has no requirement for dialing a PSTN call through the extension phone on the router. The networking solution is shown in Figure 1.
This solution enables a smooth network upgrade. In the beginning, the voice boards are configured according to the number of actual users. When the headquarters or the branch offices need to increase the number of users, it is only necessary to add voice interface boards. Each ZXR10 GAR supports a maximum of 64 calls, which is generally sufficient for ordinary small- or middle-sized companies. If more calls are required, a total of n ×64 calls can be supported by adding related ZXR10 GARs (n represents the number of ZXR10 GARs).
Since such an enterprise has few branch offices, the phone number analysis can be made directly by the gateway other than the GK, which saves the customer´s investment. Reasonable phone number configuration brings convenience and flexibility in using the intranet calls.
The ZXR10 GAR uses the message classification, priority re-marking, and queue scheduling technologies to ensure first forwarding of voice messages. However, the QoS of transmission over the IP network should be guaranteed. Therefore, the IP transmission service provider is required to provide bandwidth or QoS guarantee. To do this, it is necessary to sign a Service Level Agreement (SLA) with the service provider. The QoS guarantee for the transmission and message processing at both ends ensures high voice quality.
2.2 VoIP Solution to Interconnection with PSTN
In the solution mentioned above, each extension phone is only used for Intranet calls and cannot be used to dial an external PSTN call. To meet the requirement for dialing a PSTN call, it is only necessary to lease a telephone line from the telecom office and connect it to the GAR, as shown in Figure 2.
This solution supports external PSTN calling over IP. Therefore, the user can conveniently dial a PSTN call over IP network, and only need to pay the local call fee. As the user shall adopt a leased E1 or an ordinary telephone line to access the PSTN, the ZXR10 GAR offers related E1 and FXO interface respectively. One FXO interface allows one external call, while one E1 line can simultaneously support a maximum of 30 external calls.
2.3 VoIP Solution for Middle- and Large-Sized Enterprises
Middle- and large-sized enterprises generally have their own PSTN switches, numbers of branch offices and demands for other services (such as fax). Therefore, the VoIP networking solution for them should consider these features. ZTE´s networking solution is illustrated in Figure 3.
A GK is added in this solution. With regard to a middle- or large-sized enterprise with many branch offices, all the phone numbers of the branches, if without any GKs, need to be configured locally. Therefore, it would be complicated to configure and manage these phone numbers.
However, with an additional GK, it is only necessary to add the access numbers of a new branch office to the GK. There is no need to configure these numbers at the other branches and make alteration to the configuration of their GARs. This will greatly facilitate the maintenance work.
In this solution, the Private Branch Exchange (PBX) is connected to the ZXR10 GAR via E1, FXO or FXS. The phones of all the branch offices are connected either to the PBX or to the ZXR10 GAR as required, and an Intranet call can be dialed in many ways. Furthermore, as it is common for the middle- or large-sized enterprises to use fax, this solution allows the fax service to be transferred over IP, thus saving toll fees. The fax is also connected either to the PBX or the ZXR10 GAR. Both the PBX and the ZXR10 GAR support connection to the phone, so the maximum number of phones (faxes) supported by each branch office is equal to the total number supported by both the PBXes and ZXR10 GARs.
2.4 A Successful Application Case
In the case illustrated in Figure 4, ZTE used ZXR10 T64G, ZXR10 GER, ZXR10 GAR(7208/3608) and ZXR10 2826E/2626 to construct the police metroplitan network of Changsha, southwest China´s Hunan Province.
The ZXR10 GAR provides the customer with access to phone or the PBX via the analog telephone interface, subscriber loop trunk interface and E1 voice trunk interface to implement VoIP, while protecting the customer´s existing investments in PSTN. It also supports H.323 protocols and functions to offer the user multi-service solutions.
Moreover, the ZXR10 GAR supports TDMoIP to enable transparent transfer over IP via one or more E1s, making possible a seamless connection between the TDM network and the IP network. Therefore, it helps data operators to offer traditional leased line and dedicated line services.
Manuscript received: 2005-06-18
The Voice over Internet Protocol (VoIP) network greatly saves the communication expanses for enterprises. This paper introduces ZXR10 General Access Router (GAR), the VoIP equipment developed by ZTE Corporation, and its technical advantages. Moreover, it discusses three ZXR10-based VoIP solutions for different scale enterprises.