Category Archives: Voice and Video

Voice and Video Networking

QoS on Catalyst 3750

This is probably the best QoS for Cisco Catalyst 3750 out there:

Understanding the QoS architecture of a switch could be a daunting task even for a seasoned network engineer, how many of us remember how many queues and how many thresholds are on which switch model and the features and limitations on each of them? Probably not many. While remembering the details is not required to be a good network designer, understanding various QoS concepts like queueing, scheduling, buffer, CoS and DSCP are essential skills to be a good network designer.

skinny inspection on Cisco ASA breaks skinny

The users on one of my remote sites reported they experienced five seconds of delay when transfer calls. It is a Cisco Communication Manager system, on Cisco phones and skinny protocol. The CM is on the data center, the WAN connection is an IPSEC VPN between an ISR on the remote site and a Cisco ASA in the data center. When the delay happened, Cisco Phone would display “UCM down, features disabled” message.

It turns out that the lost of communication between Cisco phone and the CM was due to the ISR CBAC skinny inspection. As soon as I turned off the skinny inspection (no inspect skinny), the issue disappeared.

The IOS version was 12.4(21a). I didn’t have the time to check for any reported bug. So if you ran into this issue, remote skinny inspection and give it a try.

H.323 Terminology

It is very clear to network architects that video is going to be the killer application for enterprises and the Internet. However a lot of network architects came from pure networking background and thus may not have the knowledge of the video protocols and standards.

The two major standards that tell us how to run video are SIP and H.323. I will be discussing H.323 today.

I found that the H.323 tutorial written by Intel hosted on IEC website is an excellence paper to help network architect to understand the basics of H.323.

The following are some H.323 terminologies what network architect should know:

Terminals are simply the end video hosts. It could be a laptop, video phone, large screen TV, etc. A H.323 must supports G.711 audio codec, all other codecs optional. Of course your terminals will need to be able to support one of the video codes in order to act as an video endpoint.

A H.323 gateway is a device the connect the H.323 network to some other non-H.323 networks. For example, if you want to have a video conference call between a terminal on a h.323 network and a terminal on a SIP network, you will need a H.323 gateway. Gateway provides call setup/release as well as media conversion services. Gateway is not needed if the terminals are all on the same H.323 network.

A zone is a collection of H.323 gateways, terminals and MCU and gatekeeper. A zone can only have one gatekeeper. A zone can cross subnet boundary.

Although you can have H.323 terminals talking to each other without gatekeeper, an H.323 network without gatekeepers are very limited in its functionality. Also, if a gatekeeper is present, the terminals must use the gatekeeper. Gatekeepers is the brain of the H.323 network. It provides addressing, authentication, authorization, call-routing, accounting, bandwidth management and other features. Gatekeepers do not process the actual video streams.

You can use a Cisco Router as an H.323 gatekeeper, make sure the IOS has an “x” on its name. i.e.: c2600-ix-mz.122-11.

Multipoint Control Units (MCU)
You will need a MCU if you want to run video conference with more then 2 terminals. The job of a MCU is to determine CODEC for the terminals and manage video conference resources. Video streams could be routed and processed by the MCU in some setup.

Gateway, gatekeeper and MCU can be deployed in the same physical hardware, or each could be deployed as a dedicated device on an H.323 network.

H.225 consists of call signaling and RAS (Registration, Admission, and Status). When a terminal needs to call another terminal, or want to join a video conference, it uses H.225 to communicate with the gatekeeper, or other terminals.

H.245 is a control procotol. It is used to exchange terminal capabilities and creation of media channels.

Real-Time Transport Protocol (RTP)
The payloads of video stream are carried by RTP packets, and then encapsulated by UDP. RTP provides payload-type identification, sequence numbering, timestamping, and delivery monitoring.

Real-Time Transport Control Protocol (RTCP)
RTCP is the control protocol for RTP. It provides feedback on the quality of the video streams, among other functions.