1. Дайте объяснение header compression.
Header compression is a mechanism that compresses the header in a data packet before the packet is transmitted. Header compression reduces network overhead and speeds up the transmission of packets. Header compression also reduces the amount of bandwidth consumed when the packets are transmitted.
2. Какие виды header compression использует cisco.
TCP header compression
RTP header compression
3. Объясните принцип работы header compression в RTP
RTP header compression compresses the RTP header (that is, the combined IP, UDP, and RTP segments) in an RTP packet. RTP header compression identifies the RTP traffic and then compresses the IP header portion of the RTP packet. The IP header portion consists of an IP segment, a UDP segment, and an RTP segment. The minimal 20 bytes of the IP segment, combined with the 8 bytes of the UDP segment, and the 12 bytes of the RTP segment, create a 40-byte IP/UDP/RTP header. The RTP header portion is compressed from 40 bytes to approximately 5 bytes.
RTP header compression is supported on serial interfaces using Frame Relay, HDLC, or PPP encapsulation. It is also supported over ISDN interfaces.
4. Объясните работу Class-Based Header Compression в MQC
Class-based RTP and TCP header compression allows you to configure either RTP or TCP header compression for a specific class within a policy map (sometimes referred to as a traffic policy). You configure the class and the policy map by using the Modular Quality of Service (QoS) Command-Line Interface (CLI) (MQC). The MQC is a CLI that allows you to create classes within policy maps (traffic policies) and then attach the policy maps to interfaces (or subinterfaces). The policy maps are used to configure and apply specific QoS features (such as RTP or TCP header compression) to your network.
5. Назовите преимущества использования Class-Based Header Compression
Class-based header compression allows you to compress (and then decompress) a subset of the packets on your network. Class-based header compression acts as a filter; it allows you to specify at a much finer level the packets that you want to compress. For example, instead of compressing all RTP (or TCP) packets that traverse your network, you can configure RTP header compression to compress only those packets that meet certain criteria (for example, protocol type «ip» in a class called «voice).»
6. Для чего используется Voice Adaptive Traffic Shaping?
Voice Adaptive Traffic Shaping is only acailable with MQC-based Frame-Relay traffic shaping, and only with LLQ enabled in sharper-queue. The goal of VATS is to throttle the PVC’s sending rate once the system detected voice packets in the priority queue. The VATS feature is useful in the oversubscription scenaiio, where the customer sends traffic over the CIR. When the voice packet comes in the priority queue. the system will signal the shaper to slow its rate down to ensure router will never drop or delay any voice packets.
7. Для чего используется Voice Adaptive Fragmentation?
Frame Relay voice-adaptive fragmentation enables a router to fragment large data packets whenever
packets (usually voice) are detected in the low latency queueing priority queue or H.323 call setup
signaling packets are present. When there are no packets in the priority queue for a configured period of
time and signaling packets are not present, fragmentation is stopped. Frame Relay voice-adaptive fragmentation can be used in conjunction with or independent of
voice-adaptive traffic shaping.
To use voice-adaptive fragmentation, you must also have end-to-end fragmentation configured in a map
class or on the interface.