Abstract
The 3rd Generation Partnership Project (3GPP) is investing a notable effort to mitigate the endogenous stack and protocol delays (e.g., introducing new numerology, through preemptive scheduling or providing uplink granted free transmission) to attain to the heterogeneous Quality of Service (QoS) latency requirements for which the fifth generation technology standard for broadband cellular networks (5G) is envisioned. However, 3GPP’s goals may become futile if exogenous delays generated by the transport layer (e.g., bufferbloat) and the Radio Link Control (RLC) sublayer segmentation/reassembly procedure are not targeted. On the one hand, the bufferbloat specifically occurs at the Radio Access Network (RAN) since the data path bottleneck is located at the radio link, and contemporary RANs are deployed with large buffers to avoid squandering scarce wireless resources. On the other hand, a Resource Block (RB) scheduling that dismisses 5G’s packet-switched network nature, unnecessarily triggers the segmentation procedure at sender’s RLC sublayer, which adds extra delay as receiver’s RLC sublayer cannot forward the packets to higher sublayers until they are reassembled. Consequently, the exogenously generated queuing delays can surpass 5G’s stack and protocol endogenous delays, neutralizing 3GPP’s attempt to reduce the latency. We address RLC’s related buffer delays and present two solutions: (i) we enhance the 3GPP standard and propose a bufferbloat avoidance algorithm, and (ii) we propose a RB scheduler for circumventing the added sojourn time caused by the packet segmentation/reassembly procedure. Both solutions are implemented and extensively evaluated along with other state-of-the-art proposals in a testbed to verify their suitability and effectiveness under realistic conditions of use (i.e., by considering Modulation and Coding Scheme (MCS) variations, slices, different traffic patterns and off-the-shelf equipment). The results reveal current 3GPP deficits in its QoS model to address the bufferbloat and the contribution of the segmentation/reassembly procedure to the total delay.
Highlights
Ultra-reliable low-latency communications (URLLCs) are intended to address two orthogonal weaknesses faced in contemporary cellular networks: reliability and low-latency.On the one hand, as Shannon proved in his information theory founding paper [1], given a noisy discrete channel and a transmission rate smaller than the channel capacity, there exists an encoding scheme capable of generating an equivocation rate ( ) arbitrarily small
Packet Delay Budget (PDB) indicates the upper bound for the permissible delay, measured from the N6 interface (i.e., from the moment that the packet arrives to the User Plane Function (UPF)) until the packet is received by the UE, while Packet Error Rate (PER) is defined as the amount of packets received in the UE’s Packet Data Convergence Protocol (PDCP) sublayer divided by the amount of packets forwarded by the Radio Link Control (RLC) sublayer of the 5G Access Network (5G-AN)
We finish this section showing the effect of Elastic Quantum Partition (EQP) on two UEs with 50% of the total Resource Block (RB) each, that belong to two different mobility patterns MCS
Summary
Exogenous 5G stack latency causes (i.e., latencies not directly induced by the 5G stack such as the bufferbloat or the RLC packet segmentation/reassembly procedure) can become the main contributors to the delay. 5G simplifies this process as the packet starting position information is byte aligned and, it can be directly accessed, sacrificing some throughput to reduce the latency This latency reduction may not play an important role if another phenomenon that contributes to augment the delay in 5G is ignored: the segmentation/reassembly procedure. Even though the segmentation/reassembly procedure depends on 5G stack exogenous causes (i.e., packet sizes, radio link conditions and MAC scheduler algorithm1), it has a non-negligible contribution in the delay that a packet suffers, as demonstrated in this paper.
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