We will talk about LTE QoS this time.

QoS functions provided by any network, whether wired or wireless, are all based on standards (IETF RFC, IEEE 802, 3GPP TS, etc.). They may work differently using different standards depending on whether the network is wired (Ethernet/IP/MPLS) or wireless (LTE/WiBro/Wi-Fi). But, basically what the QoS is about is that traffic quality is guaranteed (i) if you pay more, or (ii) for high-priority traffic (e.g. voice or video traffic that is more sensitive to delay in its nature than Internet traffic). 

Practically, (i) does not sound very likely because no network operator offers a service plan that guarantees certain level of QoS to those who pay more. But, (ii) sounds like a more practical and sensible reason for most network operators to have QoS functions. In a wired network, the most common usage of QoS would be for VoIP or IPTV services. I've been using KT IPTV.  KT provides a higher QoS level for its IPTV (Live & VoD) traffic than for its Internet traffic (with differentiated treatment, e.g. 802.1p for L2, DSCP for IP, and EXP field of MPLS header for MPLS), guaranteeing the quality of the IPTV traffic even when there is very high Internet traffic. So, I can watch PSY dancing without any service interruption, which makes me a very satisfied subscriber of KT.

Now, we will look into QoS in LTE, a wireless network. We will go over the basic features of the LTE QoS only this time, and will revisit it for a more in-depth description in the later posts. 

 
As you may recall, when a UE attaches to an LTE network, an EPS bearer connecting from the UE to a PGW (UE - eNB - S-GW - P-GW) is created as a combination of one logical channel and two GTP tunnels. Each UE can have more than one EPS bearer depending on the services in use (e.g. three if using Internet, IPTV and VoIP. The number of bearers will be determined according to the policy of the network operator.). There are two types of EPS bearers, default and dedicated, depending on when they are created. 

Default EPS Bearer

• When a UE attaches to an LTE network, at least one EPS bearer has to be created. This EPS bearer is called the "default EPS bearer", and remains activated until the UE detaches from the network.
• The QoS of the default EPS bearer is Non-Guaranteed Bit Rate (Non-GBR), which means support of best effort (no guaranteed quality) delivery. 

 
Dedicated EPS Bearer

• Additional EPS bearers that may be created after the default one are called "dedicated EPS bearers". Generally, Internet and voice services are separately provided through two different PDNs. However, we will use an example of using one PDN for both services here. When a UE attaches to an LTE network, only a default EPS bearer for Internet service is created. Later when the UE attempts to use a voice (VoIP) service, a higher QoS level than the Internet service is required. Since the default EPS bearer set for the Internet service cannot meet the QoS level required for the voice service, a dedicated EPS bearer for voice service can be created on demand.
• The dedicated EPS bearers can be either GBR or Non-GBR. If for the voice service as in the foregoing example, it has to be GBR for guaranteed QoS.

An EPS bearer is a pipe (delivery path) connecting from a UE to a P-GW. Through this pipe (i.e. EPS bearer), various types of traffic classified by 5-tuple are delivered. These types of traffic are called IP flows, and each IP flow is classified by the 5-tuple (Source IP, Destination IP, Protocol ID, Source Port, and Destination Port). For example, when a UE connects to Google, it would have a 5-tuple, which would be defined as IP flow, as follows:

• Source IP = UE IP address
• Destination IP = Google server IP address
• Protocol ID = 6 (refers to TCP)
• Source Port = Random number (Ephemeral port number)
• Destination Port = 80 (refers to WWW)

IP Flow and Service Data Flow (SDF)

So, depending on how many applications/services a UE is using (or depending on the incoming and outgoing traffic), there can be quite a lot of IP flows (e.g., Google, Yahoo, chatting, games, VoIP, YouTube, etc.). These IP flows are mapped to Service Data Flow (SDF) by the classifier based on 5-tuple in a P-GW. The classifier is called as ACL in conventional IP router, and as SDF Template in LTE. Once they are mapped to SDF, the P-GW processes QoS at SDF level (Detailed QoS process will be discussed next time) so that the SDF can be mapped to the EPS bearer and delivered to the UE.

Once the P-GW processes QoS at SDF level and has each SDF mapped to the EPS bearer, it processes QoS at EPS bearer level from the P-GW  to the UE where SDF remains undisclosed.