UMTS QoS Criteria for RBQ

3GPP has defined the QoS attributes of UMTS bearer service that are the essential criteria to evaluate distinct traffic class [17]. The UMTS core network bearer service attributes are shown in Table 3.1. Since UMTS background application services cover all non-time critical applications such as E-mail, SMS, and FTP. The preemptive behavior among UMTS background seems allowable and can be manageable.

Therefore, in the proposed queueing scheme, only background applications are chosen as the regulation targets. According to QoS bearer service attributes defined by 3GPP, the proposed RBQ scheme would follow those attributes to meet the QoS criteria required for UMTS background applications. Therefore, some core network bearer service attributes would be taken into consideration, such as maximum bitrate,

delivery order, maximum SDU (Service Data Unit) size, SDU error ratio, residual bit-error ratio, delivery of erroneous SDU and allocation/retention priority.

Table 3.1 UMTS bearer QoS attributes defined for each traffic class [17]

Traffic class Conversational Streaming Interactive Background

Maximum bitrate X X X X

Traffic handling priority X

Allocation/Retention priority X X X X

Sources statistics descriptor X X

Signaling indication X

Why these seven core network bearer service attributes would be considered to evaluate the performance of the proposed RBQ scheme? The reasons are described as the following:

• Maximum bitrate (kbps):

It is a maximum number of bits that a UMTS bearer can deliver into service access point (SAP) in a specified interval.

• Delivery order (y/n):

This attributes determines whether the bearer sequences SDUs in the correct order.

• Maximum SDU size (octets):

The maximum allowed SDU size in admission control and policing.

• SDU error ratio:

It is the fraction of SDUs. It is mainly used in UTRAN to configure protocols, algorithms, and error detection schemes.

• Residual bit error ratio:

It indicates the BER that is undetected. BER is specified for each subflow over the radio access bearer.

• Delivery of erroneous SDUs (y/n/-):

It indicates whether erroneous SDUs are delivered.

• Allocation/Retention priority:

It is used to discriminate between bearers when allocating or retaining scarce resources.

After reviewing the definitions of core network bearer service attributes, this study selects the “bit rate”, “allocation/retention priority”’ and “max SDU size” bearer service attributes as the evaluation criteria for the RBQ scheme. Since the RBQ scheme would operate in the IP layer within an UMTS core network, other core network bearer service attributes about packet deliver sequence control and error control would be handled by the upper layer protocol, TCP. Therefore, those core network bearer service attributes should not be considered as the evaluation criteria for the RBQ scheme. These attributes include “delivery order”, “residual BER”,

“delivery of erroneous SDU” and “SDU ratio”.

3.2 RBQ Scheme

3.2.1 Queueing Scheme Architecture

A queueing scheme can be divided logically into two parts, ENQUEUE and

DEQUEUE. The ENQUEUE would process the arriving packets from the upstream gateway and the DEQUEUE would take care of the departing packets that should forward to the next downstream gateway. The architecture of RBQ scheme would be also divided into two parts, the ENQUEUE and DEQUEUE. Figure 3.1 shows the architecture of RBQ scheme. The detailed components of RBQ scheme will be described at the next section.

Figure 3.1 Role Based Queueing Scheme Architecture

The ENQUEUE of RBQ scheme provides two functions, role selector and preemptive packet switcher. The role selector would examine the header of coming IP packet from the upstream gateway to identify a role that assigned to the packet. After examining the role identification of an IP packet, the role selector would base on the role identification to assign a QoS priority to the coming IP packet. Table 3.2 shows the mapping relationship between role identifications and QoS priority.

After a QoS priority setting in an UMTS background application packet, the ENQUEUE preemptive packet switcher would be activated to handle the coming

packets and enqueued packets into the logical queues. After a QoS priority setting in an

Table 3.2 A mapping between role identification and QoS priority Priority Level Role Identification

High Emergency Medium VIP

Low Member

UMTS background application packet, the preemptive packet switcher would depend on the QoS priority in the coming packet to enqueue the packet into the corresponding logical queue with the different QoS priority. The packets with the same QoS priority would be enqueued in the same logical queue. The logical queues for the different QoS priority are first-come-first-serve (FCFS) queues. Moreover, the preemptive packet switcher would perform a preemptive enqueue operation if the queue has no buffer size for a high QoS priority packet to enqueue the corresponding logical queue.

The packet switcher would base on QoS priority of the arrival packet to determine whether the arrival packet would have the privilege to release a buffer size from a logical queue with a lower QoS priority. If there is an enqueued packet in the lower QoS priority logic queue, the tail packet would be removed to release a buffer size for the arrival packet with a higher QoS priority. Otherwise, the arrival packet would be dropped. With the preemptive enqueue operating, high QoS priority packets could always have privileges to enqueue the logic queues. The detailed description of ENQUEUE scheduling is presented at section 3.2.3.

In the DEQUEUE part of BRQ scheme, a preemptive packet forwarder function is used to dequeue the packets form each logic queue. A round robin mechanism is adopted in the preemptive packet forwarder function. According to QoS priority

precedence, the packet forwarder function would forward the enqueued packets in the first QoS priority logic queue first. After finishing the packet forwarding in the first QoS priority logic queue, the packets in the second QoS priority logic queue would be forwarded. The packets in the third QoS priority logic queue would not be forwarded until there is no packet in the first QoS priority logic queue and the second QoS priority logic queue to be forwarded. With the preemptive packet forwarder function operating, the packets with high QoS priority always get precedence to be forwarded in the logical queue. Therefore, background applications with an emergent / important role could always receive better QoS.