RFWiMAX QoS Classes: UGS, rtPS, ertPS, nrtPS, and BE
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The WiMAX standard (802.16) defines five Quality of Service (QoS) classes to cater to various application needs. These classes prioritize different aspects like bandwidth, latency, and jitter, ensuring optimal performance for diverse traffic types. Let’s explore each of these classes in detail:
- Unsolicited Grant Service (UGS)
- Extended Real Time Polling Service (ertPS)
- Real Time Polling Service (rtPS)
- Non Real Time Polling Service (nrtPS)
- Best Effort Service (BE)
Each WiMAX QoS class possesses specific parameters, including bandwidth request methods, minimum throughput requirements, and limitations concerning delay or jitter.
Unsolicited Grant Service (UGS)
UGS is designed for Constant Bit Rate (CBR) real-time traffic that requires a fixed bandwidth allocated periodically. Once the connection is established, no further requests are needed. Think of applications like T1/E1 connections.
QoS parameters crucial for UGS include:
- Maximum Sustained Traffic Rate (MST)
- Tolerated Jitter
- Maximum Latency
Refer to Table 2 for a comparison of UGS with other WiMAX QoS types.
Extended Real Time Polling Service (ertPS)
ertPS aims to support Voice over IP (VoIP) applications, especially those with silence suppression mechanisms. During periods of silence, no traffic is transmitted, optimizing bandwidth usage.
QoS parameters for ertPS mirror those of UGS. Applications suitable for ertPS are listed in Table 1.
ertPS shares similarities with UGS in that the base station (BS) assigns MST during active mode. However, unlike UGS, no bandwidth is allocated during silent periods. The BS needs to poll the mobile subscriber (MS) to determine if the silent period has ended or is still ongoing.
Real Time Polling Service (rtPS)
rtPS caters to Variable Bit Rate (VBR) real-time traffic, exemplified by MPEG video streaming. Unlike UGS, where the BS makes fixed allocations, rtPS involves the BS regularly polling the MS to ascertain its bandwidth requirements.
Bandwidth allocation in rtPS is need-based and adaptive. For this QoS type, both the Minimum Reserved Traffic Rate and MST must be explicitly specified. In contrast, for UGS and ertPS, the Minimum Reserved Traffic Rate and MST are typically identical.
The QoS parameters for rtPS align with those of UGS.
Non Real Time Polling Service (nrtPS)
nrtPS is suitable for non-real-time VBR traffic that doesn’t demand strict delay guarantees but requires a minimum guaranteed rate. File Transfer Protocol (FTP) falls under this QoS category.
Best Effort Service (BE)
In the BE class, bandwidth is granted to the MS only if there’s bandwidth leftover from other QoS classes.
This QoS class guarantees neither delay nor throughput.
It allows for the specification of both the Minimum Reserved Traffic Rate and the Maximum Sustained Traffic Rate.
WiMAX QoS Type Comparison
Below are tables summarizing and comparing the various WiMAX QoS classes.
Table 1
| WiMAX QoS type | Applications/Uses | Scheduling | Bandwidth requests/ BW REQ types | Parameters/ specifications |
|---|---|---|---|---|
| UGS | CBR real time traffic(periodic), for example T1/E1 connection | Static allocation type, grant equal to MST(Maximum Sustained Traffic Rate) | Reserves bandwidth during setup time, poll-me bit for unicast polling, no piggyback requests, no bandwidth stealing. No other kinds of polling, GM can be used for BW REQ in case of rate mismatch compensation | MST equal to Min. reserved traffic rate, max. latency, tolerated jitter, uplink grant scheduling type, unsolicited grant interval |
| ertPS | VOIP(silence suppression), video conference(real time variable size data,periodic) | Dynamic allocation | reserves bandwidth during setup, allows piggyback requests, allows bandwidth stealing. allows all kinds of poling | MST is equal to min. reserved traffic rate, max. latency and unsolicited grant interval |
| rtPS | Real time video(real time variable size data on periodic basis) | Dynamic allocation | allows piggyback,bandwidth stealing and unicast polling | Min.reserved traffic rate, max. sustained traffic rate, max. latency and uplink grant scheduling type |
| nrtPS | FTP, variable size data | Dynamic allocation | allows piggyback,bandwidth stealing and all kinds of polling | Min. reserved traffic rate, max. sustained traffic rate, traffic priority and uplink grant scheduling type |
| BE | Web Traffic | Dynamic Allocation | Allows piggyback, bandwidth stealing and all kinds of polling | Min. reserved traffic rate, Max. sustained traffic rate, traffic priority and uplink grant scheduling type |
Table 2
| WiMAX QoS | PROS | CONS |
|---|---|---|
| UGS | No overhead, meet guaranteed latency of Mobile Subscriber requests for real time service | Bandwidth may not be utilized fully as they are granted irrespective of need |
| ertPS | Optimal latency and data overhead efficiency | Need to use polling and mechanism to inform BS when the traffic starts during silence. |
| rtPS | Optimal data transport efficiency | requires overhead of BW REQ and polling latency |
| nrtPS | provides efficient service for non real time traffic with min. reserved rate | Not Applicable |
| BE | Provides efficient service for BE(Best Effort) Traffic | No service guarantee, some connections may stay for long time period |
