Page 300 - 5G Basics - Core Network Aspects

1 Core network aspects

path transmission proxy (i.e., multi-path transmission enhanced service control function (MPT-enhanced
SCF)) so that overall network performance improvement can be achieved.

The following metrics are used to evaluate the performance of access networks:
– network throughput;
– response time;
– network bandwidth capacity;
– PLR.

In addition, for the MUEs/UEs without multi-path transmission capability, network-based methods can be
used to support the functionality.

7.3 Description of the mechanism
7.3.1 Packet delivery mechanism
In multi-path transmission, packets can be routed over multiple paths simultaneously to increase the network
utilization by balancing the traffic load over the networks. Different numbers of packets from the same
service may appear in different paths, and so the packet sequence number cannot be used for transmission
control of each path. The multi-path transmission control is not only ensures in-order data delivery, but also
guarantees the reliability of the path transmission.

In the multi-path transmission control, the following aspects of service flow separation are required to be
considered:

1) In-order delivery
To ensure in-order delivery, a connection level receive buffer is required to be maintained at the
receiving end, where packets of a service are placed until they are in order and can be read by the
application. Hence, a data sequence number is necessary for traffic aggregation. For instance,
stream control transmission protocol (SCTP) uses stream sequence number (SSN), and multi-path
TCP (MPTCP) uses connection-level sequence for in-order delivery.

2) Reliable delivery
To ensure reliable delivery, multi-path transmission control is required to detect and retransmit any
missed data, if necessary. In order to identify the packets on a specific path that may be
retransmitted, a sub-transmission flow sequence number is necessary for reliable delivery. For
instance, SCTP uses transmission sequence number (TSN), and MPTCP uses sub-transmission flow-
level sequence number for reliable delivery.
7.3.2 Path selection and management mechanism

7.3.2.1 Path selection mechanism
Different types of services often have different requirements for transmission performance. Path selection is
required to take both the performance parameters of each path (e.g., bandwidth, latency, reliability, energy
consumption) and the services' requirements into consideration. MUEs with multi-path transmission
capability can choose one or more paths for service transmission. In multi-path transmission control, path
selection can be divided into the following two cases:
1) Select multi-path transmission
Generally, services with high bandwidth requirements can be split and routed over multiple paths
simultaneously. A multi-path transmission can increase the network utilization by balancing the
traffic load over the networks, which removes the restriction of a single network not satisfying the
high bandwidth requirement.

For a service with high interactivity and small throughput, it would be preferable to use the lowest
latency path to transfer traffic and retain one or more additional paths for resilience and fault
recovery purposes. For other services, the reserved path can also be used to balance traffic load and
traffic retransmission.

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