QCI stands for QoS Class Identifier. This is a special indentifier defining the quality of packet communication provided by LTE. The range of the class is from 1 to 9.  Each of this class is defined as in the following table (TS 23.203).

A QCI is only a number, and that is the point of it. 23.203 states that the standardized characteristics behind a QCI are not signalled on any interface. Each node is pre-configured with the treatment for each QCI, and only the number travels. So the table on this page is the contract, and the NAS message at the end only has to name a row of it. Let's look at the rows first, then at the ones added in later releases, and finally at the octet that carries the number to the UE.

• What does each standardized QCI promise ?
• Which QCIs have been added since Release 12 ?
• How does the UE learn its QCI ?
• Reference

What does each standardized QCI promise ?

Each row of the table below ties one QCI to four characteristics. The resource type says whether the bearer has a Guaranteed Bit Rate. The priority level, the Packet Delay Budget and the Packet Error Loss Rate then say how the bearer should be scheduled and protected. The last column gives example services only.

(3GPP -23.203 Table 6.1.7: Standardized QCI characteristics : Release 8)

Note : GBR stands for Guaranteed Bit Rate

• QCI 1 to 4 are GBR, and QCI 5 to 9 are Non-GBR. The GBR label sits in the middle of the first four rows, and the Non-GBR label in the middle of the last five.
• QCI 5, IMS Signalling, has priority 1, the highest priority in the table. Its delay budget is 100 ms and its loss rate 10^-6.
• QCI 1, Conversational Voice, accepts the highest loss rate, 10^-2, but it still gets a 100 ms delay budget and priority 2.
• QCI 8 and QCI 9 share one delay budget, one loss rate and one example cell. Only the priority separates them, 8 against 9.

From Rel 12, 4 additional QCIs(65,66,69,70) are defined mainly for Public Safety Group Communication as shown below.

(3GPP -23.203 Table 6.1.7: Standardized QCI characteristics : Release 12)

• The two red boxes mark the new rows. QCI 65 and 66 join the GBR group, and QCI 69 and 70 join the Non-GBR group.
• QCI 65, Mission Critical user plane Push To Talk voice, has priority 0.7 and a 75 ms delay budget. QCI 69, Mission Critical delay sensitive signalling, has priority 0.5 and a 60 ms delay budget.
• A priority below 1 puts both of them ahead of IMS Signalling on QCI 5.
• The delay budgets now carry NOTE references in the table itself. Those notes explain how much of each budget belongs to the radio interface.

The notes matter more than they look. For most QCIs, 23.203 NOTE 1 says that 20 ms of the delay budget belongs to the path between the PCEF and the radio base station. So the delay budget that applies to the radio interface is 20 ms smaller than the number in the table. For the Mission Critical QCIs, NOTE 7 assumes a PCEF close to the base station, and only 10 ms is subtracted.

The priority level works only when the delay budgets cannot all be met. 23.203 says that scheduling is primarily based on the delay budget. If the budget can no longer be met for bearers with sufficient radio channel quality, the scheduler meets the budget of priority level N before the budget of the next higher number. A lower number therefore means a higher priority. The delay budget itself is read as a maximum delay with a confidence level of 98 percent.

• The QCI is a pointer, not a parameter set : the characteristics are pre-configured in each node and are never signalled.
• The radio gets less than the full budget : 20 ms, or 10 ms for Mission Critical QCIs, is reserved for the core network path.
• Priority breaks ties under load : the delay budget drives scheduling first, and the priority level decides only when budgets cannot all be met.
• Mission Critical QCIs outrank IMS Signalling : QCI 69 and QCI 65 have priority levels 0.5 and 0.7, below the 1 of QCI 5.

Which QCIs have been added since Release 12 ?

The Release 12 table is no longer the whole list. 23.203 v20.0.0 has two tables, Part A and Part B, and together they define more than twenty standardized QCIs. The additions serve Mission Critical video, V2X, live uplink streaming, satellite access and low latency industrial traffic. The table below lists the Part A rows that the image above does not show.

< Based on 23.203 v20.0.0 Table 6.1.7-A, rows not in the Release 12 image >

QCI 10 is the one to notice. Its 1100 ms delay budget is there for satellite access. 23.203 notes a worst case one way propagation delay of about 270 ms for a GEO satellite, and QCI 10 can accommodate the worst case delay budget for that satellite type. The existing rows have changed too. QCI 3 now also lists V2X messages and two industrial examples, electricity distribution at medium voltage and process automation monitoring.

Part B follows different rules, which is why 23.203 keeps it in a separate table. Its Packet Error Loss Rate also counts packets that arrive later than the delay budget, and each row adds a Maximum Data Burst Volume and a Data Rate Averaging Window of 2000 ms. The delay budget then applies only to bursts no larger than that volume.

< Based on 23.203 v20.0.0 Table 6.1.7-B >

The value 1354 bytes is not arbitrary. 23.203 sets it to avoid IP fragmentation on an IPv6 based, IPSec protected GTP tunnel to the eNB, with 4 bytes left for a GTP-U extension header.

• QCI values now run well beyond 9 : 23.203 v20.0.0 defines QCIs from 1 to 10 and from 65 to 85, with gaps.
• Part B counts late packets as lost : QCI 82 to 85 include packets delivered after the delay budget in the loss rate.
• QCI 10 is the satellite QCI : its 1100 ms delay budget covers a GEO satellite link.
• QCI 80 has the shortest Part A budget : 10 ms, for low latency eMBB and Augmented Reality, with only 2 ms reserved for the core network.

How does the UE learn its QCI ?

The number reaches the UE in NAS, inside the EPS quality of service IE. The two captures below come from the two ESM messages that set up a bearer. The default bearer carries only a QCI, and the dedicated bearer carries a QCI and four bit rates.

The specific QCI value is allocated for each UE and is informed to UE via 'Activate default EPS bearer context request' message as shown below. (Followings are just a couple of examples.)

Capture : EPS quality of service in Activate default EPS bearer context request, shown as a decoder tree. The values come from one recorded exchange.

Activate default EPS bearer context request ::= DIVISION
.    ...
    EPS quality of service
        Length: 1
        Quality of Service Class Identifier (QCI): QCI 9 (9)

Capture : EPS quality of service in Activate dedicated EPS bearer context request, shown as a decoder tree. The values come from one recorded exchange.

Activate dedicated EPS bearer context request ::= DIVISION
.    ...
    EPS quality of service
        Length: 5
        Quality of Service Class Identifier (QCI): QCI 1 (1)
        Maximum bit rate for uplink : 1 kbps
        Maximum bit rate for downlink : 1 kbps
        Guaranteed bit rate for uplink : 1 kbps
        Guaranteed bit rate for downlink : 1 kbps

• The default bearer shows Length: 1 and QCI 9 (9). One octet of content is the QCI alone, which matches NOTE 6 of 23.203, where QCI 9 is typically used for the default bearer of non privileged subscribers.
• The dedicated bearer shows Length: 5 and QCI 1 (1). Five octets are the QCI followed by four bit rate octets, maximum and guaranteed, uplink and downlink.
• All four bit rates are 1 kbps. That is far below what a voice codec needs, so these look like placeholder values from a test setup rather than a real voice bearer.

24.301 clause 9.9.4.3 codes the QCI as a plain binary number in octet 3. Octets 1 and 2 are the IEI and the length, so the whole IE is 3, 7, 11 or 15 octets long. The first capture is the 3 octet form, and the second is the 7 octet form. Octets 8 to 15 extend the bit rates, first beyond 8640 kbps and then beyond 256 Mbps.

Some values of octet 3 are not QCIs at all. From the network to the UE, 0 is reserved. From 128 to 254 the values are operator-specific QCIs, and 255 is reserved. If the UE receives a QCI it does not understand, it picks a QCI of its own version for internal use. It picks a GBR QCI when the IE carries a guaranteed and a maximum bit rate, and a Non-GBR QCI when it carries neither. In later NAS signalling the UE still uses the QCI it received.

One more rule explains the default bearer capture. For all Non-GBR QCIs, the maximum and guaranteed bit rates in this IE shall be ignored. So a Non-GBR bearer gains nothing from the four extra octets, and the network usually sends the short form.

• Only the QCI number crosses the air : octet 3 of the EPS quality of service IE carries it as a binary value.
• The IE length tells you the bearer type : 3 octets means a QCI alone, and 7 or more means bit rates follow.
• 128 to 254 belong to the operator : those QCIs have no standardized characteristics.
• A UE never rejects an unknown QCI outright : it maps the value to a known GBR or Non-GBR QCI for internal use.

Reference

• 23.203 : 3GPP - Policy and charging control architecture, v20.0.0. Clause 6.1.7.2 with Table 6.1.7-A, Table 6.1.7-B and their notes.
• 24.301 : 3GPP - Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3, v20.0.0. Clause 9.9.4.3, EPS quality of service.