QUALITY OF SERVICE CONTROL IN MULTIPLE HOP WIRELESS COMMUNICATION ENVIRONMENTS [0001] This application claims the benefit of U.S. provisional application serial number 60/949,767 filed July 13, 2007 and U.S. provisional application serial number 61/033,067 filed March 3, 2008, the disclosures of which are incorporated herein by reference in their entireties. Cross-Reference to Related Applications [0002] This application is related to concurrently filed U.S. application serial number , entitled QUALITY OF SERVICE CONTROL IN MULTIPLE HOP WIRELESS COMMUNICATION ENVIRONMENTS, the disclosure of which is incorporated herein by reference in its entirety. Field of the Invention [0003] The present invention relates to wireless communications, and in particular to controlling quality of service in multiple hop wireless communication environments. Background of the Invention [0004] Wireless communications have become ubiquitous in modem society. Cellular networks have matured and now provide extensive coverage for voice communications, and are being increasingly used for data and media applications. However, data rates for cellular networks are relatively low, and thus are limited to those applications that do not require high data rates, such as basic Internet browsing, email, text messaging, and low resolution audio and video streaming. Although such applications are useful, consumers are demanding richer media experiences that require significantly higher data rates, such as those provided by broadband service providers. Broadband access is typically provided by cable and telephone service providers through hard-wired cable, digital subscriber line (DSL), T1, or T3 connections. Wireless access points may be coupled to the hard-wired connections to provide local wireless zones, or hotspots, in which mobile stations with complementary communication capabilities are afforded wireless broadband access. [0005] The Institute for Electrical and Electronics Engineers (IEEE) has set forth a widely used local wireless communication standard, which is referred to as the IEEE 802.11 standard or Wireless Fidelity standard (WiFi). Unfortunately, a WiFi access point has a very limited range of at most 100 to 300 feet, depending on environmental conditions. Given WiFi's limited range, continuous coverage throughout a large geographic area is impractical, if not impossible. As such, mobile users only get the benefit of wireless broadband access when they are within a WiFi hotspot, which is inherently limited in size. [0006] To address the limitations of WiFi and provide continuous broadband access over much larger areas in a fashion analogous to the coverage provided by cellular networks, the IEEE has set forth a next generation wireless communication standard, which is referred to as the IEEE 802.16 standard or wireless metropolitan area network standard (WiMAN). As the IEEE 802.16 standard has evolved, it has been referred to more frequently as the Worldwide Interoperability for Microwave Access standard (WiMAX). WiMAX promises to extend the wireless broadband access provided by a single access point up to 30 miles for fixed stations and three to ten miles for mobile stations. [0007] Given the extended range provided by WiMAX systems, the access points are generally referred to as base stations. Although these base stations provide broadband access over much larger areas, environmental conditions may limit access in certain areas within a given coverage area. For example, geographic elements, such as hills or valleys, may limit access within a coverage area. Buildings or other man-made structures may also affect access throughout a coverage area. Further, access within buildings or mass transit vehicles, such as buses, trains, boats, and the like, may be completely blocked, if notseverely limited. [0008] To address these areas of limited access within a coverage area of a base station, one or more relay stations may be employed to effectively extend the reach of the base station. Instead of the base station communicating directly with a mobile station or fixed station of an end user, the relay stations may act as liaisons between these stations and the base station. One or more relay stations may be provided between these stations and a given base station, depending on the needs of the communication environment. The base station and the relay stations use wireless communications to communicate with each other, and the last relay station in the relay path will communicate with the mobile or fixed stations. In addition to addressing dead spots in a given coverage area of a base station, relay stations may also be used to further extend the coverage area of a base station. In most instances, relay stations are less complex and expensive than base stations; therefore, using relay stations to extend the coverage area of a single base station is more economical than installing additional base stations and the infrastructure needed to connect the base stations to a core communication network. [0009] Relay stations may be fixed or mobile. For example, certain relay stations may be permanently affixed to or inside a building, whereas other relay stations may be mounted inside different cars of a subway train. To provide continuous coverage in a coverage area of a given base station, access provided to a mobile station may be transitioned from one relay station to another relay station, from the base station to a relay station, or from the base station to a relay station as the mobile station moves throughout the coverage area of the base station. Access may also be transitioned from one base station to another or from a relay station associated with a first base station to a relay station associated with a second base station as the mobile station moves from one location to another. Similarly, moving relay stations may transition from one base station to another as they move from one location to another. [0010] An issue arising from the use of relay stations is the inability to effectively control quality of service (QoS) for communications that are supported, at least in part, through one or more relay stations. QoS generally relates to metrics, such as delay, jitter, or data loss, that impact the quality of a given communication session or access in general. When a base station communicates directly with a mobile station over an air interface, it is relatively easy for the base station and the mobile station to cooperate with one another to both determine the communication conditions of the air interface and take steps to ensure a given level of QoS is maintained. However, the addition of one or more relay stations in the communication path significantly complicates QoS control, because there are two or more air interfaces between the base station and fixed or mobile stations, which are communicating with the relay stations. To further complicate matters, the conditions of these air interfaces may change dynamically, especially when moving relay stations are involved. [0011] The IEEE 802.16J standard addresses the use of relay stations and the control of communications over the multiple, wireless communication hops between a base station and a fixed or mobile station through one or more relay stations. However, IEEE 80.16j has not yet provided an effective and efficient way to provide QoS controls when relay stations are involved. As such, there is a need for a technique to provide QoS control when relay stations are used in wireless communication environments. Summary of the Invention [0012] According to one embodiment of the present invention, one or more relay stations may be employed along a wireless communication access path between a base station and a user terminal. The relay station directly serving the user temninal is an access relay station, and any relay stations between the access relay station and the base station are intermediate relay stations. A logical communication tunnel is established between the base station and the access relay station and through any intermediate relay stations to handle session flows of packet data units (PDUs) for downlink or uplink communications. A single tunnel may handle multiple session flows for the same or different user temriinals. For downlink communications, the base station is an ingress station and the access relay station is an egress station of the tunnel. For uplink communications, the access relay station is the ingress station and the base station is the egress station of the tunnel. [0013] Assuming the tunnel extends through at least one intermediate relay station, the ingress station will receive PDUs and schedule the PDUs for delivery to the first intermediate relay station of the tunnel. The PDUs are then delivered as scheduled via the tunnel to the first intermediate relay station in the tunnel. If the tunnel extends through multiple intermediate relay stations, the first intermediate relay station will receive the PDUs and schedule the PDUs for delivery to the next intermediate relay station of the tunnel. The PDUs are then delivered as scheduled via the tunnel to the next intemiediate relay station. The last intermediate relay station in the tunnel will receive the PDUs and schedule the PDUs for delivery to the egress station of the tunnel. The PDUs are then delivered as scheduled via the tunnel to the egress station. If the egress station is an access relay station, the PDUs are scheduled for delivery to the appropriate user terminals and then delivered as scheduled via the corresponding access connections. If the egress station is a base station, the PDUs are scheduled for delivery over the core network and then delivered as scheduled. [0014] As noted, the ingress station, egress station, and any intermediate relay stations may schedule the PDUs for delivery at different hops in the wireless communication path. This scheduling is preferably done to maintain appropriate QoS levels for the various session flows. However, the presence of the tunnel makes it difficult for the intemiediate relay stations, and in certain cases the egress stations, to properly schedule delivery of the PDUs, because these nodes do not have access to any scheduling or QoS related information for the PDUs. In one embodiment of the present Invention, the ingress station may add scheduling information to the PDUs before they are delivered to the intermediate relay stations or egress stations. The scheduling information is used by the intermediate relay stations to schedule the PDUs for delivery to the next intermediate relay station or the egress station, as the case may be. The scheduling information may also be used by the egress station to schedule the PDUs for delivery to the corresponding user terminals. The ingress station may add the scheduling infomiation to one or more headers or sub-headers of the PDU or in the body of each PDU. The PDUs may be media access control (MAC) or other protocol level PDUs. In one embodiment, the scheduling information added to the PDU by the ingress station bears on a QoS class associated with the PDU, aldeadline for the egress station to deliver the PDU to the corresponding user terminals, or a combination thereof. [0015] In one embodiment, when a PDU arrives, the ingress station will determine the arrival time for a PDU and determine a deadline for the egress station to deliver the PDU to the user terminal for the downlink, or over the core network for the uplink, based on QoS infonnation for the PDU. The QoS information may relate to the maximum latency, or delay, allowed for the PDU to reach the egress station. Based on the arrival time and the QoS information, the ingress station will calculate the deadline for the egress station to deliver the PDU to the user temiinal. Next, the ingress station will determine how long it will take the PDU to reach the egress station through the tunnel and schedule the PDU for delivery to the first intemnediate relay station in a manner ensuring that the PDU will reach the egress station prior to the deadline for the egress station to deliver the PDU to the user terminal for the downlink, or over the core network Tor the uplink. [0016] As noted, the ingress station may add the QoS class infonnation, a deadline for the egress station to deliver the PDU, or both to the PDU prior to delivering it to the first intermediate relay station. Upon receiving the PDU from the ingress station, the intemnediate relay station may access any available QoS information or deadline information provided in the PDU. The first intermediate relay station may then determine how long it will take the PDU to reach the egress station through the remaining portion of the tunnel and schedule the PDU for delivery to the next intermediate relay station or the egress station, as the case may be, in a manner ensuring that the PDU will reach the egress station prior to the deadline for the egress station to deliver the PDU to the user terminal for the downlink, or over the core networi< for the uplink. The PDU may be processed in the same manner by each intermediate relay station until the PDU reaches the egress station. The egress station may use the delay information in the PDU for scheduling the PDU for delivery to the user terminal. The egress station will deliver the PDU to the user terminal prior to the deadline for the egress station to deliver the PDU to the user terminal for the downlink, or over the core network for the uplink. Notably, the QoS class infonnation may be used to break scheduling ties where multiple PDUs are scheduled for delivery by an ingress station, intermediate relay station, or egress station at the same time. Preferably, the PDUs associated with a higher class of service are delivered before those with a lower class of service. Further, the scheduling or delivery deadlines may be based on a particular frame or time. [0017] Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures. Brief Description of the Drawing Figures [0018] The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the invention, and together with the description serve to explain the principles of the invention. [0019] FIGURE 1 is a communication environment according to one embodiment of the present invention. [0020] FIGURE 2 is a block representation of a wireless communication path according to one embodiment of the present invention. [0021] FIGURES 3A-3C provide a communication flow for downlink communications according to one embodiment of the present invention. [0022] FIGURE 4 is a block representation illustrating the use of a link log in association with downlink communications according to one embodiment of the present invention. [0023] FIGURES 5A-5C provide a communication flow for uplink communications according to one embodiment of thepresentinvention.. [0024] FIGURE 6 is a block representation Illustrating the use of a linlt lo^-in-- association with uplink communications according to one embodiment of the present invention. [0025] FIGURE 7 is a block representation of a base station according to one embodiment of the present invention. [0026] FIGURE 8 is a block representation of a user temninal according to one embodiment of the present invention. [0027] FIGURE 9 is a block representation of a relay station, such as an access relay station or Intermediate relay station, according to one embodiment of the present invention. Detailed Description of the Preferred Embodiments [0028] The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the invention and illustrate the best mode of practicing the invention. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the invention and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims. [0029] With reference to Figure 1, a wireless communication environment 10 is illustrated according to one embodiment of the present invention. As depicted, various user tenninals (UT) 12 may communicate over a core network 14 through a corresponding base station controller (BSC) 16, base station (BS) 18, and one or more relay stations. Depending on the location and function of the relay stations, the relay stations may be considered intermediate relay stations (IR) 20 or access relay stations (AR) 22. The user tenninals 12 may represent mobile or fixed terminals that are capable of supporting wireless communications with one or more of the base stations 18 and access relay stations 22. The intermediate relay stations 20 and access relay stations 22 also support wireless communications. In particular, the access relay stations 22 will support wireless communications with the user tenninals 12 as well as with intemnediate relay stations 20 or base stations 18. One or more intennediate relay stations 20 will reside between a base station 18 and an access relay station 22, and will facilitate wireless communications with the base station 18, with the access relay station 22, or both. 10030] Accordingly, a user terminal 12 may communicate directly with a base station 18 or an access relay station 22. As illustrated, user terminal 12A is served directly by a base station 18. User terminals 12B, 12C, and 12D are served by different access relay stations 22. The access relay station 22 that serves user terminal 12B is directly served by a base station 18. User temriinal 12C is served by an access relay station 22 that is linked to a base station 18 through a single intermediate relay station 20. User temninal 12D is served by an access relay station 22 that is coupled to a base station 18 through two intermediate relay stations 20. As such, user terminals 12 may be served by base stations 18 or access relay stations 22, and any number of intermediate relay stations 20 may be provided to wirelessly connect a base station 18 with a given access relay station 22. [0031] Preferably, the user terminals 12 are able to move about the communication environment 10, and thus be served by different access relay stations 22 and base stations 18, depending on their location. Further, the access relay stations 22 may be mobile or fixed. Accordingly, the access relay stations 22 may transition from being served directly by one base station 18 to another base station 18, or an intermediate relay station 20. Mobile access relay stations 22 may also transition from one intermediate relay station 20 to another. [0032] Communications between the base stations 18, intermediate relay stations 20, access relay stations 22, and the user temninals 12 are provided via wireless communication linl