Cisco Voice Gateways and Cisco Unified Border Element
Because connectivity to the outside world is of utmost importance in Cisco Collaboration solution, this chapter wouldn’t be complete without an overview and a brief discussion of Cisco IOS Voice Gateways and Cisco Unified Border Element (CUBE).
It is important to understand that both traditional voice gateways and CUBE have specific functions (with some degree of overlapping depending on deployment or design). Simply put, a voice gateway terminates time division multiplexing (TDM) signaling and transmits it by way of IP into the network or vice-versa. This allows calls to/from the PSTN network over traditional PSTN trunks, for example, ISDN T1, E1, and BRI trunks. A CUBE on the other hand terminates IP-to-IP calls, with the most common application being a SIP PSTN connection broker for enterprise network with ITSP. CUBE can do protocol interworking, address hiding, and multiple other functions described in the next section.
Cisco Voice Gateways
An access digital trunk gateway connects Cisco Unified Communications Manager to the PSTN or to a PBX via digital trunks such as Primary Rate Interface (PRI), Basic Rate Interface (BRI), or E1 R2 channel associated signaling (CAS). Digital E1 PRI trunks may also be used to connect to certain legacy voice mail systems.
Figure 2-13 gives an overview of an IOS voice gateway connecting the enterprise IP network to traditional PSTN network.
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Figure 2-13 Cisco IOS Voice Gateway Overview
Gateways in a Collaboration network must meet the following core feature requirements:
Dual Tone Multifrequency (DTMF) relay capabilities: DTMF relay capability, specifically out-of-band DTMF, separates DTMF digits from the voice stream and sends them as signaling indications through the gateway protocol (H.323, SCCP, MGCP, or SIP) signaling channel instead of as part of the voice stream or bearer traffic. Out-of-band DTMF is required when a low bit-rate codec is used for voice compression because the potential exists for DTMF signal loss or distortion.
Supplementary services support: Supplementary services are typically basic telephony functions such as hold, transfer, and conferencing.
CUCM redundancy support: CUCM clusters offer CUCM service and application redundancy. The gateways must support the ability to “re-home” to a secondary Cisco Unified Communications Manager in the event that a primary Cisco Unified Communications Manager fails. Redundancy differs from call survivability in the event of a Cisco Unified Communications Manager or network failure.
Fax/modem support: Fax over IP enables interoperability of traditional analog fax machines with IP telephony networks. The fax image is converted from an analog signal and is carried as digital data over the packet network.
From a protocol perspective, CUCM supports the following gateway protocols:
H.323
Session Initiation Protocol (SIP)
Media Gateway Control Protocol (MGCP)
Skinny Client Control Protocol (SCCP)
Cisco Unified Border Element (CUBE)
Cisco Unified Border Element (CUBE) facilitates simple and cost-effective connectivity between enterprise unified communications with the PSTN world by leveraging Session Initiation Protocol (SIP) trunks to the IT Service Provider (ITSP), also known as the SIP Service Provider. A CUBE is primarily an IP-to-IP gateway that helps connect two or more similar or dissimilar networks, while offering a host of features that a regular voice gateway cannot offer. For example, a CUBE router can connect an H.323 network to SIP network or vice-versa, or a SIP network to a SIP provider. The following are some of the features that CUBE offers:
Security demarcation, firewalling, DOS protection, and VPN services
Signaling, protocol, and media interworking (H.323–SIP, SIP–H.323, SIP-SIP)
Transcoding
DTMF relay
Media and signaling control and monitoring
QoS and bandwidth management
Co-existence/co-operation with TDM trunking
Business-to-Business (B2B) audio and video communications
Figure 2-14 gives an overview of CUBE playing a role in B2B communications and connecting Enterprises 1 and 2 to PSTN via ITSP.
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Figure 2-14 CUBE in B2B Communications