Category: VoIP – Voice over IP

If you know what delay is, jitter is simply the difference in packet delay. In other words, jitter is measuring the time difference in packet inter-arrival time.

It is a specific phenomenon that normally exists in bigger packet-switched networks. As a time-shift phenomenon, it usually does not cause any communication problems. Actually, TCP/IP is responsible for dealing with the jitter impact on communication.

On the other hand, when we speak about Voice traffic and VoIP network environment this can be an issue. When someone is sending VoIP communication at a normal interval, (let’s say one frame every 10 ms), those packets could have stuck somewhere in-between the network and not arrive at expected regular pace to the destined station. It is not usual, but the packets could take different routes or get load-balanced through two similar paths where one of those is congested in that moment.

That’s the whole jitter phenomenon. We can look at it as the anomaly in tempo, with which packet is expected to come and the time he was late to really get there.

In the image above, you can notice that the time it takes for packets to be sent is not the same as the time in which they will arrive at the receiver side. One of the packets encounters some delay on its way and it is received a little later than it was expected. Jitter buffers are entering the story. They will try to remedy packet delay if required and if possible. VoIP packets in networks have very changeable packet inter-arrival intervals because they are usually smaller than normal data packets, and are therefore more numerous, with a bigger chance to get some delay.

We can protect our voice network with simple Auxiliary VLAN but sometimes to be more secure Auxiliary VLAN are not enough. In this case we can use Security appliances such firewalls or VPN termination devices or both.

Firewall maybe seems like very clean and simple mechanism to protect RTP protocols transmitted voice packets but there’s a problem. Protecting voice networks with a firewall is not so simple because we are not sure what UDP port will be used by the RTP voice packets flow.

If we look at some Cisco network architecture and Cisco device environment, a UDP port for an RTP stream is an random port selected from the pool of 16,384 to 32,767. We surely don’t want to open all those ports on firewall just to be sure that the VoIP will function well. So many open ports may be seen from other side like a bunch of security holes.

Ways to ensure the voice network security are many. In this article we will se the first of them that must be configured in every serious network. Implementing Auxiliary VLANs will make VoIP Networks more secure using separated VLANS for data traffic and voice.

Voice and data traffic will be transferred in the same way across the same cable and same switch by default. That means that calls and all other network traffic will be transferred in the same time in the same way and every user on the network will be able to see that data using some network sniffing tool like Wireshark. This default network setting may be used to capture call packets that are crossing the network and attacker can reproduce the call in .mp3 or some other sound format. We need to separate voice network from data network completely in order to make impossible to sniff call packets from user computer. 

We mentioned earlier all different attacks that can be focused to our VoIP network. Now is the time to see how we can apply different protection methods to avoid VoIP quality mitigation that can be started by all those attacks.

Separating voice traffic from data traffic with voice VLANs or by use of VPNs and firewall to protect voice traffic can easily prevent basically all most common attacks. One more thing that is important and often ignored is the security of voice endpoints and servers that can be increased with some simple configuration changes.

This article will list all well known and famous “voice protocols”. And, also we will shed some light on various things that those protocols are doing that make up a voice over IP network functional.
 

VoIP Protocols

VoIP networks are very popular these days. In order to support communication between traditional PBXs, Cisco IP phones, analog PSTN, and the analog telephones, all over IP network, quite a number of protocols are needed. Few protocols are indicating protocols (for instance, MGCP, H.323, SIP, H.248, and SCCP) used to position, sustain, and bring down a call. Other protocols are marked at the real voice packets (for example, SRTP, RTCP, and RTP) relatively indicating information. Few of the most common VoIP protocols are shown and described here.