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LTE - QoS

LTE - QoS [Under LTE]
» E-UTRA and E-UTRAN overall description - 36.300, Rel 8 «
» E-UTRAN access - 23.401, Rel 8 «
» Policy & Charging architecture - 23.203, Rel 8 «

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§

LTE - Signaling stacks MME « LTE Index » LTE - end-to-end UE initial attach