Quality of Service (QoS)-Part8
Compression and LFI
Types of compression:
1.Payload Compression
2.Header Compression
Payload Compression
1. Traditional Compression
STAC=Sacrifice processor load on router to get more bandwidth on the link
PRED=Predicture=Sacrifice routers memory on router to get more bandwidth on the link
MPPC= Microsoft Point-to-Point Compression-For microsoft enable devices. Like in old
DOS systems
2. We also have compression for audio codec for example
G.711-Primary=64 kbps for the voice
G.723=6.3 kbps for the voice. It's being phased out.
G.729=8 kbps for the voice
Header Compression
Primary, for the voice like audio codec compression technique like G.729, sometimes, header size is larger than payload itself. So, by compressing the heading we gaining a lot with RTP. This shouldn't be called header compression but header suppression.
TCP Header Compression
It is true header compression. TCP generally sends packets with large payload.So, IN TCP header, payload is typically 1500 bytes of size. This way, by compression the header, generally, we do not save much bandwidth but adds on router processing. But in some conditions, we can enable TCP header compression, For example, Telnet have a packet size of 1 byte and rest is the header.
RTP Header Compression (cRTP)
RTP Header Compression is not a true compression. During the single session, for e.g. when we pick-up the phone and start talking, we generally do not change IP addressing, port numbers, DLC or, MAC. RTP header suppression, suppress the header, instead of compressing it. It cache the header information which is being repeated and keeps session opened based on session sequence. It can keep track of incoming packets on the basis of particular session number they belong to. This way, It does not require to process full header every time. On average, It takes a 40 byte header and compress/suppress it to 2-3 bytes header.
Link Fragmentation and Interleaving (LFI)
Serialization Delay
Link Fragmentation
Interleaving
Serialization Delay
It is the time taken in mili-seconds by interface to put packet on the wire to be transmitted on the wire. For example a 56kbps link, serialization delay is considered as 214ms. It means it will take 214 ms for the interface to move a byte of information from router to the physical line itself.This delay is called serialization delay. This adds into the delay between one point to another point for given speed.
Cisco recommends a serialization delay should be no more than 10-15 mili seconds.
Link Fragmentation
Cisco also recommends not to enable LFI for the links faster than 768 kbps because we do not generally need it. We might need this tool in slow WAN connections where we need to give priority to small voice/video packets before large data packets. Queuing cannot help in this case because it is only useful where data packet is not yet put on to the wire for the transmission whereas when a large data packet already started to transmit on the wire, LFI can be an useful tool.
Generally, it takes around 15 msecs serialization delay for the router to put 1500 Bytes information onto the wire which is idle. If you enable link fragmentation for the higher capacity connections, you are adding that extra header information to each small packet which can hurt in long run. It is two way swords because, if you fragmenting a 1500 bytes packet into 3 small packets, it will also add 3 additional headers between 2 end points and this means more traffic. This way you are sacrificing the bandwidth. So Link Fragmentation and Interleaving is basically meant for slow and temporary congested WAN connections. For long run, it is advisable to upgrade the bandwidth where this technique is being implemented.
Interleaving
Interleaving is nothing but to put small voice packet in between to enable it to be transmitted before the large data packet.
LFI feature is only application for FR and PPP links, ATM links have inbuilt Fragmentation feature supported by their 53bytes cells.
Compression and LFI
Types of compression:
1.Payload Compression
2.Header Compression
Payload Compression
1. Traditional Compression
STAC=Sacrifice processor load on router to get more bandwidth on the link
PRED=Predicture=Sacrifice routers memory on router to get more bandwidth on the link
MPPC= Microsoft Point-to-Point Compression-For microsoft enable devices. Like in old
DOS systems
2. We also have compression for audio codec for example
G.711-Primary=64 kbps for the voice
G.723=6.3 kbps for the voice. It's being phased out.
G.729=8 kbps for the voice
Header Compression
Primary, for the voice like audio codec compression technique like G.729, sometimes, header size is larger than payload itself. So, by compressing the heading we gaining a lot with RTP. This shouldn't be called header compression but header suppression.
TCP Header Compression
It is true header compression. TCP generally sends packets with large payload.So, IN TCP header, payload is typically 1500 bytes of size. This way, by compression the header, generally, we do not save much bandwidth but adds on router processing. But in some conditions, we can enable TCP header compression, For example, Telnet have a packet size of 1 byte and rest is the header.
RTP Header Compression (cRTP)
RTP Header Compression is not a true compression. During the single session, for e.g. when we pick-up the phone and start talking, we generally do not change IP addressing, port numbers, DLC or, MAC. RTP header suppression, suppress the header, instead of compressing it. It cache the header information which is being repeated and keeps session opened based on session sequence. It can keep track of incoming packets on the basis of particular session number they belong to. This way, It does not require to process full header every time. On average, It takes a 40 byte header and compress/suppress it to 2-3 bytes header.
Link Fragmentation and Interleaving (LFI)
Serialization Delay
Link Fragmentation
Interleaving
Serialization Delay
It is the time taken in mili-seconds by interface to put packet on the wire to be transmitted on the wire. For example a 56kbps link, serialization delay is considered as 214ms. It means it will take 214 ms for the interface to move a byte of information from router to the physical line itself.This delay is called serialization delay. This adds into the delay between one point to another point for given speed.
Cisco recommends a serialization delay should be no more than 10-15 mili seconds.
Link Fragmentation
Cisco also recommends not to enable LFI for the links faster than 768 kbps because we do not generally need it. We might need this tool in slow WAN connections where we need to give priority to small voice/video packets before large data packets. Queuing cannot help in this case because it is only useful where data packet is not yet put on to the wire for the transmission whereas when a large data packet already started to transmit on the wire, LFI can be an useful tool.
Interleaving
Interleaving is nothing but to put small voice packet in between to enable it to be transmitted before the large data packet.
LFI feature is only application for FR and PPP links, ATM links have inbuilt Fragmentation feature supported by their 53bytes cells.
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