Telecom Tutorials by Samir Amberkar

LTE - QoS [Under LTE]
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In LTE, end-to-end data flow is based on following bearer architecture.

....  UE  EPS bearer  PGW  E-RAB  SGW  S5/S8 bearer Radio bearer  eNB  S1 bearer ....

There is one-to-one mapping between EPS bearer, ERAB, (Data) Radio bearer, S1 bearer, and S5/S8 bearer. Concerned nodes hold this mapping. For instance, eNB and SGW holds the mapping between EPS bearer and S1 bearer; eNB also has to maintain an important mapping between ERAB, (Data) Radio bearer, and S1 bearer. Refer diagram below:

UE  PDN(IP) address UL TFT EPS Bearer ID Radio Bearer ID MAC LC ID

UE Context at eNB  EPS Bearer ID ERAB ID S1AP ID GTPU TEID Radio Bearer ID MAC LC ID

UE Context at MME  EPS Bearer ID ERAB ID S1AP ID

UE Context at SGW  EPS Bearer ID S1 GTPU TEID S5/S8 GTPU TEID

UE Context at PGW  PDN(IP) address DL TFT EPS Bearer ID S5/S8 GTPU TEID

EPS Bearer capabilities are defined in terms of following QoS paramters:

Common   QCI - QoS Class Identifier   ARP - Allocation and Retention Priority

GBR   GBR - Guaranteed Bit Rate   MBR - Maximum Bit Rate

Non-GBR   APN-AMBR - per UE per APN Aggregate Maximum Bit Rate   UE-AMBR - per UE Aggregate Maximum Bit Rate

A bearer is considered as of GBR type if it supports guaranteed bit rate. Otherwise, it would be of Non-GBR type. As mentioned in article on initial attach, during PDN connectivity procedure, a "default bearer" is established. This is always of Non-GBR type. With the same PDN, if another bearer is established, it is called "dedicated bearer". This could be of GBR type or of Non-GBR type. For Non-GBR bearers, per UE and per PDN/APN specific aggregate bit rate parameters apply as shown above.

Most important QoS parameter is QCI; at top level it tells the kind of service that can be supported by the bearer and at implementation level, it maps to measurable values of so called "standardised QCI characteristics". For example, QCI 1 is suitable for voice call and at implementation level, it maps to {priority=2, packet delay budget=100ms, packet error loss rate=0.01}. QCI range from 1 through 9 as shown below:

Type	QCI	Priority	Packet delay (ms)	Packet error rate	Service example
GBR	1	2	100	0.01	Voice call (CBR)
	2	4	150	0.001	Video call (CBR)
	3	3	50	0.001	Real time Gaming
	4	5	300	0.000001	Video on demand
Non-GBR	5	1	100	0.000001	IMS Signaling
	6	6	300	0.000001	Voice/video over IP, Chat, DL Video streaming, Internet, E-mail etc.
	7	7	100	0.001
	8	8	300	0.000001
	9	9	300	0.000001

All above QoS parameters are known to UE except ARP. ARP stands for "Priority for Allocation and Retention". As the name suggest, ARP is meant to be used during two procedures: during Admission or allocation (setup signaling) of the bearer and during lifetime of the bearer, when new bearer is to be admitted/allocated (ARP parameters of both bearers are compared to see if existing bearer can be deallocated to make space for new bearer). Handover is another scenario where ARP decides which bearer can be dropped from bearers allocated to a UE. ARP has three sub-parameters: priority level (1 thru 15), preemption capability (capable to preempt/not capable to preempt), and preemption vulnerability (preemtable/not preemtable).

References: 3G Evolution: HSPA and LTE by Dahlman, Parkvall, Sköld, and Beming, LTE - From Theory to Practice, Edited by Sesia, Toufik, and Baker, and SAE and the Evolved Packet Core: Driving the Mobile Broadband Revolution by Olsson, Sultana, Rommer, Frid, and Mulligan.

Copyright © Samir Amberkar 2010-11

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