Session Border Controller (SBC) Models
SBCs are also heavily used by VoIP SPs for a variety of reasons. This section discusses some of the common-use cases to continue the discussion of VoIP networks and the corresponding components.
In VoIP networks, the most prevalent and common use for SBCs is offloading VoIP traffic to the PSTN. Other usage includes voice transcoding and network hiding. Figure 3-7 describes a network of an SP with SBC deployed at the edge of the SP network. Figure 3-7 also shows various SBC deployment scenarios. It shows SP1 and SP2 connected to each other through SBCs; this connection is through SIP trunks and is also known as an SIP tied trunk. Here, the call flows occur through these SIP trunks from SP1 to SP2. The PSTN connectivity is handled by SP2, representing a PSTN offload network. Another solution for a Call Control Server Farm is also shown in the figure; here, an IMS server is communicating through an SBC to SP1 and at the same time through a set of servers (an SIP Proxy, a Gatekeeper, and MGX/AS5400 trunking gateways) representing the PSTN connectivity network. The other network scenarios reflect the SBC connection from the SBC to an enterprise1 network, a small-business network, and lastly to residential CPE units. All these networks are shown to be carrying different types of traffic along with possibly the VoIP traffic. The key component depicted in the figure is the SBC and how it interconnects with the all the other VoIP networks. The SBC and SIP Router Proxy (SRP) are the tap points for the SIP metrics.
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Figure 3-7 Session Border Controller Deployment Architecture
Key Components Used in SBC Models
Some of the key components used in SBC deployment models are as follows:
- SBC: SBCs are used at the edge of the network; they manage and control the SIP session streams traversing the borders of the networks they sit at. The SBCs provide the functionality for hiding customer networks by NATing. The SIP session streams represent SIP signaling and media communication.
- SIP Router Proxy (SRP): SIP Proxy and SRP perform the same functionality. SRPs basically help route requests to the user's current location, authenticate and authorize users for services, implement provider call-routing policies, and provide features to users.
PSTN Offload
VoIP SPs are offloading the handling of PSTN traffic through PSTN SPs. Typically, these are referred to as the traditional telcos. The telco and the VoIP SP have SBC devices at the edges of their networks, and through SIP trunks between these SBCs, the VoIP traffic is offloaded to the PSTN network. The VoIP SP maintains the quality of the VoIP offload service through SLAs. Telcos being the hosts of the VoIP offload also report the SLAs on this service. The VoIP offload allows the VoIP SP to focus and improve the IP-centric traffic and not worry about managing a PSTN-based network.
The needs or challenges for the VoIP SP that drive this design are numerous and include the following:
- Turning up, maintaining, and growing the VoIP network and interfacing with the PSTN involves a lot of overhead.
- The VoIP SP has to plan in advance the trunk capacity needed to service its subscriber base.
- VoIP SPs not only have to deal with the bearer traffic aspect of the service but also the PSTN/SS7 signaling network. Both of these call bearer and signaling aspects have their own infrastructure, and thus are an overhead that needs to be monitored and maintained. The monitoring through select KPIs allows effective operations.
- The PSTN trunk turnup procedure requires countless hours of interaction with the telco; thus Opex cost is high.
- Maintenance is another challenge, especially if operations work is being performed on network augmentation and Emergency/911 circuits. The downtime for this kind of service has many repercussions, some of which can even lead to lawsuits, if the E911 services are impacted. Effective monitoring of the circuits through KPIs allows quick resolution of the outages. The PSTN offload allows the VoIP SP to basically hand over the liability of managing the 911 trunks to the PSTN provider.
- The VoIP SP has to constantly monitor the capacity and continue to profile the traffic to keep up with the subscriber growth.
In addition to the aforementioned needs or challenges, additional aspects include monitoring and managing the complex network of SIP trunks, which are used to interconnect the VoIP switches or the SBCs. Another key challenge is to keep on top of SIP trunk utilization and call performance metrics. As you will see in Chapters 7 and 8, monitoring and correlating these metrics yield an effective VoIP network management system. The SIP traffic counters constitute the KPIs needed to effectively monitor the SIP network. The main theme here is to provide a background of the various operational overheads. Chapters 7 and 8 look at how the metrics are used to track both the hosted PSTN network and the VoIP offload network, which leads to tracking of key metrics.
Network Hiding
Previously, we mentioned that SBCs are deployed at the edge of the VoIP SP network. This section provides more context for this discussion. The SBC enables VoIP signaling and media to be received from and directed to a device behind a firewall and Network Address Translator (NAT) at the border of an adjacent network. The SBC achieves this by rewriting the IP addresses and ports in the call-signaling headers and the Session Description Protocol (SDP) blocks attached to these messages. SDP is basically used for multimedia session setup. This functionality is offered by all SBCs. This NAT functionality enables the VoIP SP provider to hide the network. Hiding allows the SP to not expose its internal network to the outside work, because NAT translates the internal network to another external-facing network. The VoIP SP's SBC basically gets a tied SIP trunk to the SBC of the PSTN provider and does NAT for the back-end internal network.
Being able to look into the traffic enables the SBC to perform a wide range of functionality, including antispam, QoS, and billing. These features also help the VoIP SP to potentially improve the VoIP QoS, better track the connection billing records, and detect any security violations like spam attacks. To summarize, the SBC-based network is effectively monitored thorough KPIs comprised of SIP traffic counters. The collection, correlation, and reporting of these counters are important items that a VoIP SP should perform.