GTP Loss and Impact of Transport Bandwidth

Above is practical result of Transport Bandwidth Change impact and its difficult to isolate throgh OSS KPIs , It needs GTP analysis along with field testing 

1. Site was already congested and >95% Radio Capacity was used 

2. By Changing Transport BW from 200Mbps to 400Mbps site comsumes more 20% traffic with 110% Radio capacity used and better performance .

3. There is no impact seen in OSS KPIs but field test shows >85% throghput degradation.

4. Packet loss and Latency was all across reason for user experiance degradation.

5. One Major thumb rule that i came across that 

                          Transport BW  > (Sum of Cell throughput of site ) * 118%

What is GTP  and GTP Loss ?

1. GPRS Tunneling protocol is an important IP based protocol used in GSM, UMTS and LTE core networks. It is used to encapsulate user data when passing through core network and also carries bearer specific signaling traffic between various core network entities.

2. It provides mobility. When UE is mobile, the IP address remains same and packets are still forwarded since tunneling is provided between PGW and eNB via SGW.

3. Multiple tunnels (bearers)can be used by same UE to obtain different network QoS

4. Main IP remains hidden so it provides security as well

5. As shown above GTP tunnel is formed between eNB and PGW via SGW but in case of Samsung SGW & PGW both are at same location (called SAEGW). So we will always check GTP loss between SGW and eNB

6. It is only calculated on bearer down link(SAEGW --> eNB direction).

7. GTP Loss can happen only by user traffic but cannot occur by ping or traceroute between SAEGW and eNB.

8. GTP tunnel is formed only between SAE-GW and eNB so GTP loss has no relation with UE RF conditions

9. GTP Loss means packet drops between SAEGW and eNB ,when UE tries to download any data then TCP session is established between UE and server.When this data comes to SAE-GW from server then It encapsulates this data using GTP, UDP, IP, L2 and L1 headers and forwards this data to eNB through GTP tunnel.GTP header consists of Sequence no. & tunnel end point Id (TE ID) which is assigned to each packet.This encapsulated data transfers through GTP tunnel with Sequence no. 1,2,3,4,5….. and so on and reaches to eNB.If after 2nd packet eNB receives 4th packet instead of 3rd packet then this event would be counted as loss in GTP loss counter but after receiving 3rd packet this event would be counted as Out of Sequence packet

Why GTP Loss happens?

There are basically two reasons for GTP loss (packet drop) in backhaul-

—Buffer Overflow [ Will explain below ]

—Physical drops    [ Physical issues like SFP module problem, cable problem, loosed connection, line card problem, duplex mismatch, etc..]

What is Buffer in a network device?

Small amount of memory used for temporary storage of data, usually to compensate for differences in between receiving and transmitting speeds on a network device. It serves as a reservoir in which the higher speed ingress data to be dumped which is then 'trickled' to the slower one.

What is Buffer overflow?

-When server is sending data packets with very high speed and receiving node cannot forward it with the same speed due to bottleneck in network and it has small buffer to store packets so it will start discarding excess packets once its buffer gets full . This situation is called Buffer overflow.

Not enough buffer in the bottleneck (Buffer size < 512KBytes and bottleneck bandwidth <= 250Mbps) causes low Throughput