[re-post] Seamless and lossless handovers in LTE

Posted: August 8, 2012 in technical
Tags: , , ,

The first post is about handovers in LTE. I will not describe the LTE architecture because anyone who might be interested in the subject must already knows that. This post is not about what happens in the core of the network when a handover is decided, but what happens in the access area. Between two eNBs that are connected though an X2 link, as far as I understand, there are two types of handovers: seamless and lossless. The names are not very intuitive, but the idea is that seamless handover is used for bearers transporting traffic that is very sensitive to delay and jitter and would rather accept retransmissions than delay, like VoIP, while the lossless handover is used for bearers that transport traffic that does not care too much about delay but is sensitive to retransmissions, like FTP and HTTP.

The decision for one of the HO types or the other is made by the source eNB. The major difference between the two HO types is that when using lossless HO besides the information sent by the source eNB to the target eNB in the Handover Request message, the source eNB also sends on the X2 link the downlink packets it received for the UE and which have not been sent or acknowledged yet. This way all the packets are delivered and no retransmission should occur. When thinking about this one must take into consideration that in a VoIP conversation the packets used are very small while for HTTP an FTP large packets are used. The retransmission of such large packets would use valuable bandwidth and TCP would protocol to slow down the conversation. On the other hand for VoIP the packets used are very small (retransmission of a few packets is not very expensive in terms of bandwidth), and it is very important that the packets arrive with the smallest delay possible.

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