Fs with better than best effort service such as bandwidth-intensive file transfer. 4. Best effort (BE) for best-effort traffic. For uplink services such as UGS (unsolicited grant service), there are no explicit bandwidth requests issued by subscriber station. The use of any contention services is prohibited and there is no unicast request opportunity provided. The UGS may include a grant management (GM) subheader containing: Slip indicator telling us that there is a backlog in the buffer due to clock skew or loss of MAPs. Poll-me bit: indicates that the terminal needs to be polled (allows for not polling terminals with UGS-only services). Uplink services-rtPS is intended for rt-VBR-like service flows such as MPEG video. It is prohibited from using any contention request, and the terminals are polled frequently enough to meet the delay requirements of the SFs (service flows). Bandwidth (bit rate capacity) is requested with BW request messages (a special MAC PDU header). It may use a grant management subheader. A new message request may be piggybacked with each transmitted PDU. The uplink service-nrtPS is intended for non-real-time service flows with better than best effort service, typically bit rate capacity intensive file transfer. It works like rt-polling except that polls are issued less frequently. The use of contention requests are allowed in this case, and it may use the Grant Management subheader. A new request may be piggy-backed with each transmitted PDU. The uplink service-best effort (BE) serves generic data such as HTTP, SMTP, and SO on. Here there are no QoS guarantees and contention requests are allowed. It may also use grant management subheader and, as above, a new request may be piggybacked with each transmitted PDU. The IEEE 802.16 protocol has a request/grant scheme that is self-correcting. There is no acknowledgment, and all errors are handled in the same way with periodical aggregate requests. Bandwidth (bit rate capacity) requests are always per connection. Grants are either per connection o