Quality of Service

2.4 Quality of Service

The network associates a certain quality of service (QoS) with each data transmission in GPRS packet mode. The appropriate QoS is characterized according to a number of attributes negotiated between the MS and the network. Figure 2.2 characterizes the application in terms of error tolerance and delay requirements.

Figure 2.2: Applications in terms of QoS requirements. ( From - [1].)

A first list of attributes is defined in Release 97/98 of the 3GPP recommendations. It was replaced in the release 99 by new attributes.

2.4.1 Attributes in Release 97/98

In Release 97/98 of the 3GPP recommendations, QoS is defined according to the following attributes:

• Precedence class. This indicates the packet transfer priority under abnormal conditions, as for example during a network congestion load.

• Reliability class. This indicates the transmission characteristics; it defines the probability of data loss, data delivered out of sequence, duplicate data delivery, and corrupted data. This parameter enables the configuration of layer 2 protocols in acknowledged or unacknowledged modes.

• Peak throughput class. This indicates the expected maximum data transfer rate across the network for a specific access to an external packet switching network (from 8 to 2,048 Kbps).

• Mean throughput class. This indicates the average data transfer rate across the network during the remaining lifetime of a specific access to an external packet switching network (best effort, from 0.22 bps to 111 Kbps).

• Delay class. This defines the end-to-end transfer delay for the transmission of service data units (SDUs) through the GPRS network. The SDU represents the data unit accepted by the upper layer of GPRS and conveyed through the GPRS network. Table 2.1 shows the delay classes.

  Table 2.1: Delay Classes

  	Delay (Maximum Values)
  	SDU size : 128 octets		SDU size: 1,024 octets
  Delay Class	Mean Transfer Delay (s)	95 Percentile Delay (s)	Mean Transfer Delay (s)	95 Percentile Delay (s)
  1. (Predictive)	< 0.5	< 1.5	< 2	< 7
  2. (Predictive)	< 5	< 25	< 15	< 75
  3. (Predictive)	< 50	< 250	< 75	< 375
  4. (Best Effort)	Unspecified
  From: [2].

The delay class for data transfer gives some information about the number of resources that have to be allocated for a given service. Predictive value in delay class means that the network is able to ensure an end-to-end delay time for the transmission of SDUs; best effort means that the network is not able to ensure a value for an end-to-end transfer delay; in this case transmission of SDUs depends on network load.

2.4.2 Attributes in Release 99

The attributes of GPRS QoS were modified in Release 99 of the 3GPP recommendations in order to be identical to the ones defined for UMTS. The attributes described below apply to both GPRS and UMTS standards. Table 2.2 gives the characteristics of the different classes.

Table 2.2: Traffic Classes

Traffic Class	Real-Time Conversational	Real-Time Streaming	Interactive Best Effort	Background Best Effort
Fundamental Characteristics	No transfer delay variation between the sender and the receiver; stringent and low delay transfer	No transfer delay variation between the sender and the receiver	Request response pattern; preserve pattern content	No time constraint; preserve pattern content
Example of Applications	Conversational voice and videophone	One-way video, audio streaming, still image, and bulk data	Web browsing, voice messaging and dictation, server access, and e-commerce	E-mail, SMS, and fax

Four classes of traffic have been defined for QoS:

1. Conversational class. These services are dedicated to bidirectional communication in real time (e.g., voice over IP and videoconferencing).

2. Streaming class. These services are dedicated to unidirectional data transfer in real time (e.g., audio streaming, one-way video).

3. Interactive class. These services are dedicated to the transport of human or machine interaction with remote equipment (e.g., Web browsing, access to a server, access to a database).

4. Background class. These services are dedicated to machine-to-machine communication that is not delay sensitive (e.g., e-mail and SMS).

Table 2.3 lists the expected performance for conversational services.

Table 2.3: End User Performance Expectations-Conversational/Real-Time Services

				Key Performance Parameters and Target Values
Medium	Application	Degree of Symmetry	Data Rate	End-to-End One-Way Delay	Delay Variation Within a Call	Information Loss
Audio	Conversational voice	Two-way	4-25 Kbps	<150 ms preferred <400 ms limit Note 1	< 1 ms	< 3% of frame error rate
Video	Videophone	Two-way	32-384 Kbps	< 150 ms preferred < 400 ms limit Lip-synch: <100 ms		< 1% of frame error rate
Data	Telemetry - two-way control	Two-way	<28.8 Kbps	< 250 ms	N/A	Zero
Data	Interactive games	Two-way	< 1 KB	< 250 ms	N/A	Zero
Data	Telnet	Two-way (asymmetric)	< 1 KB	< 250 ms	N/A	Zero
From: [1].

Table 2.4 lists the expected performance for streaming services.

Table 2.4: End User Performance Expectations-Streaming Services

				Key Performance Parameters and Target Values
Medium	Application	Degree of Symmetry	Data Rate	One-Way Delay	Delay Variation	Information Loss
Audio	High-quality streaming audio	Primarily oneway	32-128 Kbps	< 10 s	< 1 ms	<1% FER
Video	One-way	One-way	32-384 Kbps	< 10 s		<1% FER
Data	Bulk data transfer/ retrieval	Primarily oneway		< 10 s	N/A	Zero
Data	Still image	One-way		< 10 s	N/A	Zero
Data	Telemetry-monitoring	One-way	<28.8 Kbps	< 10 s	N/A	Zero
From: [1].

Table 2.5 lists the expected performance for interactive services.

Table 2.5: End User Performance Expectations-Interactive Services

				Key Performance Parameters and Target Values
Medium	Application	Degree of Symmetry	Data Rate	One-Way Delay	Delay Variation	Information Loss
Audio	Voice messaging	Primarily no way	4-13 Kbps	< 1 sec for playback < 2 sec for record	< 1 ms	< 3% FER
Data	Web browsing-HTML	Primarily oneway		< 4 sec/page	N/A	Zero
Data	Transaction services - high priority (e.g., e-commerce and ATM)	Two-way		< 4 sec	N/A	Zero
Data	E-mail (server access)	Primarily oneway		< 4 sec	N/A	Zero
From: [1].

The Release 99 of 3GPP recommendations defines attributes for QoS such as traffic class, delivery order, SDU format information, SDU error ratio, maximum SDU size, maximum bit rate for uplink, maximum bit rate for downlink, residual bit error ratio, transfer delay, traffic-handling priority, allocation/retention priority, and guaranteed bit rate for uplink and guaranteed bit rate for downlink.

• Traffic class indicates the application type (conversational, streaming, interactive, background).

• Delivery order indicates if there is in-sequence SDU delivery or not.

• Delivery of erroneous SDUs indicates if erroneous SDUs are delivered or discarded.

• SDU format information indicates the possible exact sizes of SDUs.

• SDU error ratio indicates the maximum allowed fraction of SDUs lost or detected as erroneous.

• Maximum SDU size indicates the maximum allowed SDU size (from 10 octets to 1,520 octets).

• Maximum bit rate for uplink indicates the maximum number of bits delivered to the network within a period of time (from 0 to 8,640 Kbps).

• Maximum bit rate for downlink indicates the maximum number of bits delivered by the network within a period of time (from 0 to 8,640 Kbps).

• Residual bit error ratio indicates the undetected bit error ratio for each subflow in the delivered SDUs.

• Transfer delay indicates the maximum time of SDU transfer for 95th percentile of the distribution of delay for all delivered SDUs.

• Traffic-handling priority indicates the relative importance of all SDUs belonging to a specific GPRS bearer compared with all SDUs of other GPRS bearers .

• Allocation/retention priority indicates the relative importance of resource allocation and resource retention for the data flow related to a specific GPRS bearer compared with the data flows of other GPRS bearers (useful when resources are scarce ).

• Guaranteed bit rate for uplink indicates the guaranteed number of bits delivered to the network within a period of time (from 0 to 8,640 Kbps).

• Guaranteed bit rate for downlink indicates the guaranteed number of bits delivered to the network within a period of time (from 0 to 8,640 Kbps).