Page 297 - 5G Basics - Core Network Aspects

Core network aspects 1

of data. In multi-path transmission control, based on the requirements in [ITU-T Y.2251], the following
requirements are to be considered:
1) Congestion control and traffic transmission adjustment
Congestion avoidance and reliability can be achieved directly at the transport layer. Changes in the
congestion window (Cwnd) can reflect the degree of congestion on each path. As each path has its
own features, it can avoid or relieve congestion occurrences using multi-path transmission. In multi-
path transmission, the available paths can be treated as independent paths with independent
congestion control decisions, or all the paths can be coupled to each other to perform congestion
control.
As the total session Cwnd equals the Cwnd summation of each available path, it is required to
consider balancing the rapid increase or decrease of the total Cwnd.
For example, Alice is connected to WLAN and 3G access networks. The WLAN access is used for web
browsing, and the 3G access is used for downloading high definition movies. A voice call is received
leading to congestion on the 3G path. To enhance the user experience, the download flow can be
adjusted to go through the WLAN path.
2) Retransmission scheme
In multi-path transmission, there may be several available paths for retransmission. The continuous
retransmission using the initial path may be inefficient. Therefore, the transmission may be
enhanced using non-congested paths during the duration of the session.
If several non-congested paths are available, an appropriate path may be chosen according to the
service. For example, a service requiring low delay is required to choose a minimum delay path,
likewise a service requiring large throughput or low packet loss rate (PLR) is required to choose
appropriate path(s). However, if the user is only capable of connecting with two access networks,
retransmission can be done using the non-congested path.
In a dynamic scenario, if the user is connected to the WLAN and 3G access networks, and the services
transmitted through WLAN retransmit frequently, the retransmission packets may be adjusted to
utilize the 3G path.
3) Service flow separation
Just as a file can be split into a number of small file blocks, likewise a real-time service flow may be
divided into several sub-transmission flows to be transmitted using different paths; these sub-
transmission flows in turn may be reassembled into a complete service flow at the receiving end.
Since the original numbered packets are separated and transmitted through multiple paths, the sub-
transmission flow sequence numbers are required to be added to each sub-transmission flow. For
example, in a certain path, a sub-transmission flow sequence number is used for transmission
control, and a data sequence number is used for data recovery at the receiving end.

The simultaneous use of multiple paths in data transmission can enhance the overall bandwidth,
and ensure a high transmission rate, low PLR and low transmission delay for a mobile user. And the
sub-transmission flows can be transmitted through different paths for better quality of service (QoS)
and achieving sharing resources.
For example, when a user is watching a high-definition video, several sub-transmission flows
transmitting through different paths can improve the user experience. When the user is maintaining
multiple connections and downloading a file with a large volume of data, to improve the
transmission rate and to balance the data downloading, the data file can be separated and
transmitted through several paths.

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