LTE中的UE數據傳輸——Bearer篇

LTE中的UE數據傳輸---Bearer篇

作者簡介：秋哥，資深協議工程師，多年來就職過國內兩大基站廠商，後負責各手機廠商modem支持工作，現負責某大廠modem端協議開發和維護。出於無私分享精神，特將多年來自己對modem端協議開發經驗進行總結，書寫成文供大家學習參考，並歡迎各位看官指正和垂詢。

在聊UE數據之前，要先弄清楚一個概念—Bearer。在無線通信中，Bearer會貫穿整個流程。

好，不廢話，來張圖。這是一個關於LTE中所有Bearer的架構，其中很多網絡側的，本文不做描述，這裡主要關注UE側與網絡交互的部分。而理解這部分，主要是為後續理解數據傳輸做前期準備。

圖1 – Beaer架構

1 Radio Bearer & E-RAB

· A data radio bearer transports the packets of an EPS bearer between a UE and one or more eNB(s). When a data radio bearer exists, there is a one-to-one mapping between this data radio bearer and the EPS bearer/E-RAB.

以上是協議中關於DRB（Data radio bearer）的描述，其主要是用來傳輸EPS bearer的（在UE與eNB之間）。但是到這裡依然沒弄懂這是個什麼東西？

圖2 – 控制面與用戶面

在詳細介紹RB概念之前，我們首先要有一個數據傳輸的基本思想：先建鏈、再傳輸。什麼意思？就是說在無線通信中（不限於LTE，任何無線通信）在傳輸的基本思想都是先建立信令鏈接（這其中會規劃各種參數的定義、協商等），這一步稱之為控制面的鏈接建立；之後再依據信令協商進行數據傳輸（當然，在傳輸過程中會一直有一些調度、功控等控制），此時稱之為用戶面（數據面）的過程。

1.1 什麼是Radio Bearer？

在協議中沒有Radio Bearer的詳細定義，但是從圖3可以給出一個定義：Radio Bearer是層2（L2，包括PDCP、RLC以及MAC）提供給層三（L3 ，NAS+RRC）的服務。也就是說，L3在向L2發送任何數據（包括信令）都是由RB（Radio Bearer）來承載（提供服務），說白了，RB就是L2提供給L3的一個接口。

圖3 – Radio Bearer的位置

1.2 RB（Radio Bearer）的分類

RB分為SRB（Signalling radio bearer）與DRB（Data radio bearer）。

"Signalling Radio Bearers" (SRBs) are defined as Radio Bearers (RB) that are used only for the transmission of RRC and NAS messages. More specifically, the following SRBs are defined:

· SRB0 is for RRC messages using the CCCH logical channel;

· SRB1 is for RRC messages (which may include a piggybacked NAS message) as well as for NAS messages prior to the establishment of SRB2, all using DCCH logical channel;

· For NB-IoT, SRB1bis is for RRC messages (which may include a piggybacked NAS message) as well as for NAS messages prior to the activation of security, all using DCCH logical channel;

· SRB2 is for RRC messages which include logged measurement information as well as for NAS messages, all using DCCH logical channel. SRB2 has a lower-priority than SRB1 and is always configured by E-UTRAN after security activation. SRB2 is not applicable for NB-IoT.

依據以上協議描述，SRB是用於傳輸信令的，只是不同階段使用不同的SRB，所以其對應的是協議的控制面。

同理，DRB是用於傳輸數據的，其用於用戶面。

1.3 E-RAB

· An E-RAB transports the packets of an EPS bearer between the UE and the EPC. When an E-RAB exists, there is a one-to-one mapping between this E-RAB and an EPS bearer.

· E-RAB: an E-RAB uniquely identifies the concatenation of an S1 Bearer and the corresponding Data Radio Bearer. When an E-RAB exists, there is a one-to-one mapping between this E-RAB and an EPS bearer of the Non Access Stratum as defined in [17].

綜上：E-RAB = RB + S1 Bearer（圖1已明確標識），S1 Bearer為eNB與S-GW間的Bearer，由網絡維護。本文主要關注RB，這裡不做贅述。

2 EPS Bearer

2.1 什麼是EPS Bearer？

在協議中沒有關於EPS Bearer比較明確的定義，我們可以理解為：EPS Bearer是一種由一條或多條SDFs (Service Data Flows )組成的邏輯集合，該集合在UE與PDN GW間傳輸。其中配置了很多關於傳輸數據時的一系類參數，這些參數由網絡（PDN GW）與UE間進行協商。

2.2 EPS Bearer的分類

2.21 Dedicated Bearer與Default Bearer

· Dedicated bearer: An EPS bearer that is associated with uplink packet filters in the UE and downlink packet filters in the PDN GW where the filters only match certain packets. Definition derived from 3GPP TS 23.401 [10].

· Default bearer: An EPS bearer that gets established with every new PDN connection. Its context remains established throughout the lifetime of that PDN connection. A default EPS bearer is a non-GBR bearer. Definition derived from 3GPP TS 23.401 [10].

按照性質區分，可以分為上述兩種EPS Bearer。二者的區別與聯繫可以參照下表：

表1 – Default Bearer與Dedicated Bearer的區別與聯繫

2.22 GBR Bearer與Non-GBR Bearer

· GBR bearer: An EPS bearer that uses dedicated network resources related to a guaranteed bit rate (GBR) value, which are permanently allocated at EPS bearer establishment/modification. Definition derived from 3GPP TS 23.401 [10].

· Non-GBR bearer: An EPS bearer that uses network resources that are not related to a guaranteed bit rate (GBR) value. Definition derived from 3GPP TS 23.401 [10].

按照資源調度模式劃分，可以劃分為GBR Bearer和Non-GBR Bearer。這裡就比較難理解了，涉及到了資源調度問題，那在理解這兩個Bearer之前，需要理解一個概念：QCI。

GBR與Non-GBR對應不同的QCI，不同的QCI對應不同的資源調度：優先級、時延、丟包率等等。

· QoS Class Identifier (QCI): scalar that is used as a reference to access node-specific parameters that control bearer level packet forwarding treatment (e.g. scheduling weights, admission thresholds, queue management thresholds, link layer protocol configuration, etc.), and that have been pre-configured by the operator owning the eNodeB. A one-to-one mapping of standardized QCI values to standardized characteristics is captured in

依據上述，QCI是一個固定值，不同的QCI值規定了不同的配置。詳細配置如下表。

表2 – QCI（協議3GPP23203）

最新協議版本（R15）規定了如上的QCI，詳細的描述可以參考3GPP23203。所以，又可以說根據不同的QCI可分為GBR與Non-GBR Bearer。針對不同業務，網絡會配置不同的Bearer，同時攜帶不同的QCI。

由此，GBR類承載，用於對實時性要求較高的業務，需要網絡對該類承載保證最低的比特速率，比如典型的QCI1~4、QCI65~67以及QCI75。之後這是規定了最低的傳輸比特速率，協議也規定了此時的最高傳輸比特速率—MBR（Maximum Bit Rate，最大比特速率）參數定義了GBR承載在RB資源充足的條件下，能夠達到的速率上限。MBR的值大於或等於GBR的值。協議原文如下：

Each GBR bearer is additionally associated with the following bearer level QoS parameter:

· Guaranteed Bit Rate (GBR): the bit rate that can be expected to be provided by a GBR bearer;

· Maximum Bit Rate (MBR): the maximum bit rate that can be expected to be provided by a GBR bearer. MBR can be greater or equal to the GBR.

而Non-GBR用於對實時性要求不高的業務，不需要網絡對該類承載保證最低的比特速率，其典型的QCI範圍是QCI5~9。在網絡擁擠的情況下，業務需要承受降低速率的要求。對於Non-GBR，使用UE-AMBR（Aggregate Maximum Bit Rate，聚合最大比特速率）來限制所有Non-GBR承載的最大速率（注意，不是限制某個Non-GBR承載，而是針對UE級別的。）

2.23 兩種分類的關係

首先先看下協議的描述：

· An EPS bearer is referred to as a GBR bearer if dedicated network resources related to a Guaranteed Bit Rate (GBR) value that is associated with the EPS bearer are permanently allocated (e.g. by an admission control function in the eNodeB) at bearer establishment/modification. Otherwise, an EPS bearer is referred to as a Non-GBR bearer.

NOTE 11:Admission control can be performed at establishment / modification of a Non-GBR bearer even though a Non-GBR bearer is not associated with a GBR value.

· A dedicated bearer can either be a GBR or a Non-GBR bearer. A default bearer shall be a Non-GBR bearer.

下圖很直觀的反映了Bearer分類間以及各參數的關係。

圖4 –QCI、GBR、Non-GBR、MBR、AMBR之間的關係