Robust Header Compression

Last updated: May 21, 2009

This section is only applicable to the lab applications.

The Robust Header Compression (ROHC) is one of the efficient header compression algorithms that can save bandwidth dramatically. For example, in case of GSM coded VOIP transmission, the payload is only 33 bytes (13.2 kbps x 20 ms) long and the header of IPv4 accounts for 55% of the packet. But with the help of ROHC, more than 90% of the size of these headers can be saved. For IPv6 this ratio is even larger.

This document describes the various ROHC negotiable parameters available for an active PDP context in the test set and when they takes effect.

ROHC Parameters

The figure below shows the configured ROHC parameters and status information for the active PDP contexts of the selected DUT IP address.

Configurable Parameters (shown as `ROHC Parameters')

The ROHC configuration is performed for each of the PDP contexts in the test set. Up to 4 DUT IP addresses are supported and for each address one primary and up to three secondary PDP contexts are available. See DUT PDP Setup for more information.

State:

If header compression state is set to on and the SNDCP header compression algorithm is ROHC, the State shows ON. Otherwise, it shows Off.

The GPIB command to set the header compression state for a primary/secondary PDP context is CALL:PPRocedure:SNDCp:IP[:ADDRess[1]|2|3|4][:CONText][:PRImary]:HEADer:COMPression[:STATe] and CALL:PPRocedure:SNDCp:IP:[:ADDRess[1]|2|3|4][:CONText]:SECondary<[1]|2|3>:HEADer:COMPression[:STATe] , respectively.

ROHC and RFC1144 are the two header compression algorithms supported by the test set. Note that the compression can be actually used only if both the test set and DUT support it.

The GPIB command to set the header compression algorithm for a primary/secondary PDP context is CALL:PPRocedure:SNDCp:IP[:ADDRess[1]|2|3|4][:CONText][:PRImary]:HEADer:COMPression:ALGorithm and CALL:PPRocedure:SNDCp:IP[:ADDRess[1]|2|3|4][:CONText]:SECondary<[1]|2|3>:HEADer:COMPression:ALGorithm , respectively.
ROHC Mode:

Over different types of links, ROHC operates in one of the following three modes:
1. U-mode(Unidirectional mode): is used for links without a return channel, packets are sent in one direction only: from compressor to decompressor.
2. O-mode(Bidirectional Optimistic mode): is similar to U-mode, but adds a feedback channel to send error recovery requests and (optionally) acknowledgments of significant context updates from decompressor to compressor.
3. R-mode(Bidirectional Reliable mode): uses the feedback channel more intensively by error correction and ACK check.
The GPIB command to set the ROHC mode for a primary/secondary PDP context is CALL:PPRocedure:SNDCp:IP[:ADDRess[1]|2|3|4][:CONText][:PRImary]:ROHC:MODe and CALL:PPRocedure:SNDCp:IP[:ADDRess[1]|2|3|4][:CONText]:SECondary<[1]|2|3>:ROHC:MODe , respectively.
Profiles:
The test set supports up to four ROHC profiles:

- Profile Identifier 0 (ROHC Uncompressed): packets are not compressed.

- Profile Identifier 1 (ROHC RTP): compresses packets with IP/UDP/RTP protocol headers.

- Profile Identifier 2 (ROHC UDP): compresses packets with IP/UDP protocol headers.

- Profile Identifier 3 (ROHC ESP): compresses packets with IP/ESP protocol headers.

If Profile x (x = 0, 1, 2, 3) is supported, the corresponding profile number `x' is displayed; otherwise, a `-' is displayed.

The GPIB command to set a ROHC profile state for a primary/secondary PDP context is CALL:PPRocedure:SNDCp:IP[:ADDRess[1]|2|3|4][:CONText][:PRImary]:ROHC:PROFile[0]|1|2|3[:STATe] and CALL:PPRocedure:SNDCp:IP[:ADDRess[1]|2|3|4][:CONText]:SECondary<[1]|2|3>:ROHC:PROFile[0]|1|2|3[:STATe] , respectively.
CID Mode:
ROHC has two different context identification (CID) modes, large(covering CID 0-16383) and small(covering CID 0 to 15).

The GPIB command to set a ROHC CID Mode for a primary/secondary PDP context is CALL:PPRocedure:SNDCp:IP[:ADDRess[1]|2|3|4][:CONText][:PRImary]:ROHC:CID:MODE and CALL:PPRocedure:SNDCp:IP[:ADDRess[1]|2|3|4][:CONText]:SECondary<[1]|2|3>:ROHC:CID:MODE , respectively.
Large MAX_CID:

The MAX_CID parameter indicates the highest compression context ID number to be used by the ROHC compressor. This setting indicates the value of the Large MAX_CID setting for the PDP context

The GPIB command to set a MAX_CID parameter for a primary/secondary PDP context when the CID mode is set to Large is: CALL:PPRocedure:SNDCp:IP[:ADDRess[1]|2|3|4][:CONText][:PRImary]:ROHC:CID:LMAXimum and CALL:PPRocedure:SNDCp:IP[:ADDRess[1]|2|3|4][:CONText]:SECondary<[1]|2|3>:ROHC:CID:LMAXimumGe , respectively.

Note:the actually used ROHC MAX_CID for a PDP context is the result of the negotiated between the test set and the DUT.
Small MAX_CID:

The MAX_CID parameter indicates the highest compression context ID number to be used by the ROHC compressor. This setting indicates the value of the Small MAX_CID setting for the PDP context

The GPIB command to set a MAX_CID parameter for a primary/secondary PDP context when the CID mode is set to Small is: CALL:PPRocedure:SNDCp:IP[:ADDRess[1]|2|3|4][:CONText][:PRImary]:ROHC:CID:SMAXimum and CALL:PPRocedure:SNDCp:IP[:ADDRess[1]|2|3|4][:CONText]:SECondary<[1]|2|3>:ROHC:CID:SMAXimum , respectively.

Note:the actually used ROHC MAX_CID for a PDP context is the result of the negotiation between the test set and the DUT.
TBCompression:
ROHC Timer-based compression of RTP timestamp is an algorithm developed to compress the 32-bit RTP Timestamp (TS) field for audio/video RTP packets.

The GPIB command to turn on/off this timer-based compression for a primary/secondary PDP context is CALL:PPRocedure:SNDCp:IP[:ADDRess[1]|2|3|4][:CONText][:PRImary]:ROHC:TBCompress[:STATe] and CALL:PPRocedure:SNDCp:IP[:ADDRess[1]|2|3|4][:CONText]:SECondary<[1]|2|3>:ROHC:TBCompress[:STATe] , respectively.

Negotiated parameters (shown as `ROHC Status')

Since the negotiation between the test set and the DUT may be different from what are configured in the test set, the negotiated parameters indicates the actual ROHC status. The corresponding information is shown as following:

State:
If ROHC is in use, it shows On. Otherwise, it shows Off.

The GPIB command to query the state for a primary/secondary PDP context is CALL:STATus:PPRocedure:SNDCp:IP:ADDRess[1]|2|3|4[:CONText][:PRImary]:ROHC[:STATe]? and CALL:STATus:PPRocedure:SNDCp:IP:ADDRess[1]|2|3|4[:CONText]:SECondary<[1]|2|3>:ROHC[:STATe]? , respectively.
Entity Number:It shows the ROHC entity number assigned to a primary or secondary PDP context. If ROHC is not supported, it displays `----'.
The GPIB command to query the Entity Number for a primary/secondary PDP context is CALL:STATus:PPRocedure:SNDCp:IP:ADDRess[1]|2|3|4[:CONText][:PRImary]:ROHC:ENTity? and CALL:STATus:PPRocedure:SNDCp:IP:ADDRess[1]|2|3|4[:CONText]:SECondary<[1]|2|3>:ROHC:ENTity? , respectively.
Profiles:
A list of ROHC profile supported by both the test set and the DUT as the result of the XID negotiation for the PDP context. 0, 1, 2, and 3 refer to UCOMP, RTP, UDP, and ESP respectively. If profile x (x = 0, 1, 2, 3) is supported, the correspondent profile number `x' will be displayed; If profile are not supported, `----' is displayed.

The GPIB command to query Profiles for a primary/secondary PDP context is CALL:STATus:PPRocedure:SNDCp:IP:ADDRess[1]|2|3|4[:CONText][:PRImary]:ROHC:PROFile[0]|1|2|3[:STATe]? and CALL:STATus:PPRocedure:SNDCp:IP:ADDRess[1]|2|3|4:CONText:SECondary<[1]|2|3>:ROHC:PROFile[0]|1|2|3[:STATe]? , respectively.
MAX_CID:
Indicates the value of the negotiated MAX_CID parameter for the PDP context. If the ROHC is not in use, displays `----'.

The GPIB command to query the MAX_CID for a primary/secondary PDP context is CALL:STATus:PPRocedure:SNDCp:P:ADDRess[1]|2|3|4:CONText:PRImary:ROHC:CID:MAXimum? and CALL:STATus:PPRocedure:SNDCp:IP:ADDRess[1]|2|3|4:CONText:SECondary<[1]|2|3>:ROHC:CID:MAXimum? , respectively.

How to set the ROHC parameters?

The following procedure details how to access and configure the Header Compression parameters.

Press CALL SETUP .
Press the More key on the lower left corner to select the Protocol Control(2 of 4).
Press the More key then select SNDCP ( F2 ) key to display the SNDCP softkey menu. Use the knob to select the parameter that you want to set.
Press theHeader Compression ( F1 ) key to display the Header Compression softkey menu.
Select the parameter that you want to set and set the value.

Operating Considerations

The ROHC mode is only available in Active Cell(GPRS/EGPRS) operating mode.

The measured IP data throughput may increase for the application over IP/UDP/RTP, IP/UDP/RTP or IP/ESP.

When a PDP context is active and an external handover occurs on two boxes, the SNDCP XID parameters will be re-negotiated with the ROHC parameters for the new box. This will have an impact on throughput even if both boxes have the same parameters.