X SIP Trunks - Overview and Support

DESCRIPTION

A "trunk" is a connection, either physical (older technologies) or virtual (such as SIP), that typically connects and provides a call path between two switches. When there is more than one trunk that routes numbers the same path, the trunks are configured in a Trunk Group. Therefore a "SIP Trunk" is a trunk that uses Session Initiation Protocol (SIP) for call control. Evolve IP uses SIP Trunks in two ways:

1- to connect our network to a carrier's PSTN gateway switch

2- to connect to a customer's on-premise PBX

• This document focuses on #2, customer PBX interconnection. 

Phone numbers get assigned/routed to a trunk group and are not assigned to any particular trunk within the group. In traditional telco, trunks were assigned trunk numbers; that is not the case with SIP trunks. The same number may consume multiple call paths, up to the max number of trunks within the trunk group.

The below drawing shows the basic difference between an HPBX customer and a SIP Trunk Customer:

WHY SIP TRUNKS

Customer Perspective

Trunks, by their very nature, are not feature rich, nor are they intended to be. Instead features, such as call forwarding, etc, are to be offered by the customer's PBX. Additionally, extension-to-extension dialing remains within the customer's PBX; only calls in/out of the PSTN should cross the trunks; one exception to this rule is if the customer has multiple locations and need to connect a call from one PBX to another, including if the "other" PBX is Evolve IP's Hosted PBX (HPBX); we refer to this as a Hybrid PBX or Hybrid Hosted PBX. So, if trunks are not feature rich, why would a customer want them and why would Evolve IP offer them?

Customers who are not ready to abandon the investment they've made in their on-premise phone system still need to connect to the PSTN. How they accomplish this may be dependent on a variety of factors, including what technologies their phone system supports. Most newer PBX's (in the last 5 years or so) will natively support SIP, however, that may also require specific hardware that they may or may not have purchased. The phone system may still use legacy technologies such as ISDN/PRI, T1 (E&M Wink, Loop Start, Ground Start) trunks, or even individual analog trunks.

Since trunks offer little in the way of features (i.e. "value added" services), the primary purchasing decision for customers is cost. Trunks are usually sold with two cost components:

• • a monthly recurring charge, times the number of trunks (call paths)
  • cost per minute (CPM) usage charges

Evolve IP Perspective

Evolve IP has never offered SIP Trunks as a generally available (GA) product, nor do we plan to, for three primary reasons:

• • We have no interest in being the last cost provider. Evolve IP's products are a collection of high-value services and we want to keep it that way. We consider chasing least-cost businesses as a "race to the bottom" where you're only better if you're cheaper
  • Trunks are generally high volume usage, so it is costly from a network resource perspective
  • Large Service Providers, such as Verizon and Level3, have a legacy network that was built for this kind of business and therefore they can do it better and cheaper

So, if this is not a business we want, why do we do it? The short answer is, opportunity. We may do SIP Trunks for a customer that has an HPBX location (or many), or there are other opportunities to be gained. Additionally, our private label partners may choose to offer SIP Trunks to their end users.

WHAT IF THEIR PHONE SYSTEM DOES NOT NATIVELY SUPPORT SIP?

When a device does not support SIP natively, an integrated access device (IAD), sometimes also referred to as a "gateway router." Evolve IP generally uses Cisco routers for this but there are numerous routers that can support this configuration. In this case, the WAN side of the router uses the SIP protocol to communicate with our network, and uses ISDN PRI to communicate with the customer's equipment. NOTE: while technically feasible to use protocols other than ISDN/PRI for customer premise hand-off, Evolve IP ONLY supports direct ISDN/PRI as a hand-off (or direct SIP, of course). 

Such an IAD connection looks something like this:

TO REGISTER OR NOT TO REGISTER, THAT IS THE QUESTION:

Not all SIP "compliant" devices, including many PBX's, support the SIP REGISTER method. Whenever possible, we want devices to REGISTER to our network, just like our hosted phones do. This gives an added layer of security to the trunk. However, like hosted phones, a trunk is only as secure as the switch (PBX) to which it is connected. Refer to the Fraud section below.

Like an HPBX phone, a SIP Trunk that uses authentication utilizes the same SIP Auth username/password mechanism, however, this is configured in Broadsoft on the Enterprise/Group/Services/TrunkGroup/Profile page.

For devices that do not support the REGISTER method, Voice Engineering must be involved in every turn up because additional configuration is required in Broadsoft as well as in the Session Border Controllers, to allow a level of trust from the customer's IP Address(es). Without this additional configuration, and without being registered, the SBC would deny with a "403 Forbidden" outbound calls from the customer and Broadsoft would not know where to deliver calls that are supposed to terminate to the customer.

PILOT USERS

Particular to Broadsoft is the notion of Pilot Users. These are used for SIP Registration and may or may not have a phone number assigned to them. 

CALL FORWARDING

Similar to devices that do not support the SIP REGISTER method, additional configuration is required in scenarios where an end user (of the customer-premise PBX) call forwards their number to a PSTN number AND they use the original caller's number in the From field. That's because the call would appear to our network as a call FROM an untrusted number TO an untrusted number. In that scenario, the SBC (correctly!) blocks the call with a 403 Forbidden. If this call flow is required, which it often is, Voice Engineering needs to apply additional configuration to Broadsoft and the SBC to allow this call flow, regardless of whether their trunk group is REGISTER'd.

FRAUD

Unlike HPBX, where we have default limits on concurrent calls, a Trunk Group is limited only by the number of call paths that the customer has purchased. Therefore, a PBX that is compromised can generate fraud much faster than a hosted user can. (Some customers have hundreds of trunks inn their trunk group). Hackers can be clever and will exploit whatever vulnerability is left open. Not only can a weak PBX relay calls, it is also possible that a management console that is accessible to the Internet could also be compromised and fraud be generated. Or, a PBX without proper security may allow hackers to register an endpoint directly to the PBX and launch calls as though they are sitting in the office (the calls would still appear to us as though they are coming from the customer's IP. 

ENTERPRISE TRUNK GROUPS

Broadsoft supports a concept they call "Enterprise Trunk Groups."  An Enterprise Trunk Group is essentially a grouping of Trunk Groups. If a customer has multiple trunk groups with us we can then set different routing algorithms, including load balancing, overflow, failover, etc. When using Enterprise trunks, the main Trunk Groups will still utilize Pilot Users for registration purposes, however, the numbers are now assigned to the Enterprise Trunk so that they can route to any of the Trunk Groups within the Enterprise Trunk Group.

EIP STANDARD TRUNK FEATURES

The following services are the standard services that are assigned to Trunk Users:

• Authentication

• Basic Call Logs

• Calling Line ID Delivery Blocking

• Calling Name Retrieval

• Calling Party Category

• Call Waiting

• Charge Number

• Client Call Control

• Customer Originated Trace

• External Calling Line ID Delivery

• Intercept User

• Internal Calling Line ID Delivery

• Malicious Call Trace

• Physical Location

COMMON ISSUES and TROUBLESHOOTING