Network Time Protocol (NTP)

Network Time Protocol (NTP) is a protocol for synchronizing device clocks across TCP-based computer networks. The latest documented version is NTP v3, defined in IETF RFC 1305. NTP uses UDP port 123 for the distribution of Coordinated Universal Time (UTC) in a hierarchical tree structure. Clocks are synchronized based on their Stratum level, which indicates their precision. Stratum 0 clocks refer to devices that keep highly accurate time, such as atomic clocks. Stratum 1 clocks refer to computers that receive time directly from stratum 0 clocks. Stratum 2 clocks refer to computers that receive time from Stratum 1 computers, and so forth. Network administrators typically synchronize the clocks of network infrastructure devices to synchronize timestamps of event logs collected from routers and switches through SYSLOG and SNMP Traps.

Cisco TelePresence endpoints (CTS devices, CTS-MAN, and the CTMS) all support NTP for time synchronization, which is necessary within a TelePresence deployment for scheduling the resources for a meeting. Further, time must also be synchronized with any email and calendaring systems, such as Microsoft Exchange or IBM/Lotus Domino, used to schedule meetings. Finally, it should be noted that Cisco TelePresence deployments also rely on the use of digital certificates for secure communication; such as Secure Shell (SSH) and Hypertext Transer Protocol over Secure Socket Layer (HTTPS) for secure management, and Transport Layer Security (TLS) for secure SIP signaling. These security protocols often rely on the use of digital certificates, which have a range of dates for which the certificate is valid. Therefore, to prevent any issues with these security protocols due to incorrect dates configured within TelePresence equipment, time synchronization through mechanisms such as NTP are recommended.

IEEE 802.3af: Power over Ethernet

The IEEE 802.3af specification defines a standards-based mechanism for providing Power over Ethernet (PoE) to devices. Power is provided inline, using two of the four available pairs (4 wires) of the 8-pin modular jack used for twisted-pair Ethernet connections. The standard introduces two types of devices:

• Power Sourcing Equipment (PSE): Includes devices such as Ethernet switches and power injectors that provide inline power to powered devices
• Powered Devices: Includes devices such as IP Phones, access points, and IP cameras that receive power from PSE

The specification defines a nominal voltage of 48 volts direct current (min 44 Vdc to max 57 Vdc), with a maximum power output of 15.4 watts per PSE (switch) port. To support more intelligent power management, the PSE might optionally classify powered devices. Table 1 shows the currently defined power classifications.

Table 1: IEEE 802.3af Power Classifications 

Class	Max Output Power (Watts)
0		15.4
1		4.0
2		7.0
3		15.4
4	Treat device as Class 0

If the PSE cannot determine the power classification of a powered device, it should default to a Class 0 device with a maximum of 15.4 Watts. Class 4 is reserved for future use. The IEEE 802.3af task force is currently working on an extension to PoE, commonly referred to as PoE+, which might extend the amount of power supplied by a PSE to 24 watts per port.

In Cisco TelePresence deployments, IEEE 802.3af-based PoE is supplied by the primary codec to the attached IP 7975G Phone (Class 3 device) associated with the CTS endpoint. Power can also be provided locally to the IP 7975G Phone if desired. PoE is also supplied to the primary and auxiliary cameras connected to the primary codec. If the CTS endpoint supports multiple codecs, such as with the CTS-3200 or CTS-3000, the secondary codecs supply PoE to their associated primary camera Ethernet connections as well.

IEEE 802.1p/Q Utilization Within Cisco TelePresence Networks

When a TelePresence endpoint is configured to utilize a Voice VLAN (VVLAN) (consistent with current best practice recommendations for IP telephony deployments), the switch port to which the endpoint is connected effectively operates as a VLAN trunk. All traffic (voice, video, signaling, and management) received from the primary codec of the TelePresence endpoint includes an IEEE 802.1Q header with the VLAN tag corresponding to the VVLAN number. Likewise, all traffic sent to the primary codec from the Cisco Catalyst access edge switch also includes a VLAN tag corresponding to the VVLAN number. VLAN tagging is also extended to all traffic to and from the associated IP Phone attached to the primary codec of a CTS endpoint. Figure 1 shows an example of this.

 
Figure 1: Voice VLAN tagging of TelePresence traffic

The implementation of the Voice VLAN that includes TelePresence traffic and traditional VoIP traffic can be used to both isolate access to devices on the Voice VLAN and to provide a separate and consistent quality of service (QoS) for all traffic corresponding to the VVLAN across the network infrastructure. Traffic isolation can be accomplished by access control lists (ACL) defined on network infrastructure devices, which restrict access to the VVLAN only to those devices that require such access. Consistent QoS can be provided by trusting the CoS value of ingress frames from the VVVLAN and mapping CoS values to ingress or egress queues for prioritization. CoS values can also be mapped to DSCP values to provide a consistent QoS and prioritization as the TelePresence traffic flows across Layer 3 uplinks that do not use VLAN trunking.

The implementation of a VVLAN for TelePresence deployments is optional. If a VVLAN is not defined on the Cisco Catalyst access edge switch, all traffic to and from the TelePresence endpoint is sent without any IEEE 802.1Q VLAN tags. In such cases, the switch can be configured to trust the DSCP value within the Layer 3 IP header of the TelePresence traffic and then map the DSCP values into appropriate ingress or egress queues for prioritization across the network infrastructure.