As discussed earlier in Chapter 2, IEEE 802.11s uses HWMP for mesh path selection.
As a hybrid protocol, HWMP aims at merging advantages of both the proactive and the reactive path selection mechanisms. It can be configured to operate either in on-demand reactive mode or in tree-based proactive mode. The standard defines ALM as the path selection metric for each link. ALM (C) is defined as follows,
C=
Oca+Op+ Bt
r 1
1−ef (3.12)
WhereOca andOp are constants named as channel access overhead and protocol overhead respectively. Btis the test frame size. The input parametersr andef are bit rate in Mbps and the frame error rate for the test frame size Bt.
3.5.1 Limitations of HWMP
Scheduling and path selection are interdependent in a directional antenna based mesh network. The elements of a mesh path selection protocol should configure the end-to- end path so that it provides maximum efficiency, while at the same time minimizes the contention among communicating beams. The scheduling algorithm should use this information to schedule the beam-forming to further improve network performance.
IEEE 802.11s integrates HWMP for efficient mesh path selection based on MAC layer information. However, HWMP is designed for omni-directional MAC, and does not consider existing scheduling information while allocating path for a new flow.
HWMP uses ALM as the path selection metric. However, the link cost for this metric is calculated using a test frame. Lets consider the scenario given in Fig. 3.6. For the ease of presentation a single interface is considered at every mesh STA. Similar situation can arise with multiple interface scenario based on the interference graph. The dotted lines denote the connection links between the mesh STAs. There is an ongoing flow between I2 andI3. Because there is a single ongoing flow, based on the scheduling policy given in section 3.4, the link between I2 and I3 will be fully utilized. In this scenario, a new flow between I5 and I8 needs to be scheduled. The sub-flows I2 → I3 and I6 → I7 interfere with each other. When I7 sends a test frame to calculate airtime cost between I6 and I7, the cost will be very high because the two links are contending, and at that time, link
1 3 4
5 6 7 8
9 10 11 12
2
Link with existing flow Best Path
Path selected through HWMP
Figure 3.6: Two Flow Scenario
I2 → I3 utilizes the complete capacity based on the proposed scheduling. Thus HWMP may select an alternate pathI5 → I9→ I10→ I11→ I12→ I8. However, if the capacity is equally divided between the contending flows, then the path I5 → I6 → I7 → I8 may give minimum airtime cost. This is not possible using test frame based per-hop airtime link cost calculation. The next subsection provides an amendment over HWMP based on the scheduling mechanism given in section 3.4 for efficient path selection in a direction mesh network.
3.5.2 HWMP Path Metric: Scheduling Based ALM
The ALM, when calculated using a test frame, may provide stale information because the actual transmission rate based on the scheduling policy is not known. However, when a new flow is admitted in a stable network, its actual transmission rate can be predicted based on current scheduling constraints. Let a newly generated flow has to be scheduled at linkIs → It. First, the current condition of the link need to be predicted. The link is called to be“maximally loaded”if the remaining bandwidth in that link is less than the required bandwidth for an arbitrary minimum priority flow. Note that the proposed scheduling mechanism initially allocate the transmission rates based on their priority requirements, as given in equation (3.9). Formally“maximally loaded” link can be defined as follows:
Definition 3.7. A link Is → It is called to be maximally loaded, if the following condition holds,
η− X
F ∈C(Is)
λ(F)≤ξ(Fminp,Is) (3.13)
whereFminpis an arbitrary flow with the minimum priority, andξ(Fminp,Is)is calculated
according to equation (3.9).
Based on this definition, following theorem can be derived.
Theorem 3.7. If a linkIs→ Itis maximally loaded, then based on the proposed scheduling policy, a newly admitted sub-flow Fnew(Is,It) can achieve at most ξ(Fnew,Is) amount of transmission rate.
Proof. Let there arennumber of sub-flows through the linkIs→ It, denoted asF1(Is,It), F2(Is,It), ..., Fn(Is,It) corresponding to the flows F1,F2, ...,Fn. Further assume that based on flow priority, the proposed scheduling mechanism allocates ξ(Fnew,Is) amount of transmission rate initially for newly admitted flow Fnew. Note that,
ξ(Fnew,Is)≥ξ(Fminp,Is) Let equation (3.13) is satisfied. This implies,
η− X
F ∈C(Is)
λ(F)≤ξ(Fnew,Is)
So, if ξ(Fnew,Is) amount of transmission rate needs to be allocated to sub-flow Fnew(Is,It), then the transmission rate of other flows have to be reduced. Consider any arbitrary flowFi. Based on Problem 3.1, Fi is already in the minimum of the maximum achievable transmission rates of all its sub-flows. If the transmission rate for sub-flow Fi(Is,It) is reduced to allocateξ(Fnew,Is), thenλ(Fi) would be the reduced transmission rate for sub-flow Fi(Is,It). This is true for all the flows. Thus the maximum achievable transmission rate for sub-flow Fnew(Is,It) is the initially allocated rate based on flow priority, which is ξ(Fnew,Is).
When a new flowFnew is going to be scheduled in a link Is → It, there can be two possibilities,
1. The link is maximally loaded. Then the flow can get at mostξ(Fnew,Is) amount of bandwidth through that link.
2. Otherwise, the flow can get at mostη− P
F ∈C(Is)
λ(F) amount of bandwidth through that link.
In the proposed modifications over HWMP, the PREQ message contains a field called
“effective data rate” (EDR) which is used as the data rate value (r) for airtime link cost
calculation, as given in equation (3.12). Based on this observation, EDR at everyIs for a newly admitted flow Fnew (EDRFnew(Is)) is calculated as follows:
EDRFnew(Is) = max
η− X
F ∈C(Is)
λ(F)
, ξ(Fnew,Is)
(3.14) The HWMP protocol is augmented as follows. The source interface initializes EDR to∞and airtime link cost to zero, and broadcasts PREQ through all its interfaces. When an interfacesreceives the PREQ message, it calculates EDR as follows:
EDRFnew = min(EDRFnew, EDRFnew(Is)) (3.15) Where EDRFnew is the EDR value received through the PREQ packet. Then the Schedule based ALM (S-ALM) is calculated as follows:
CS =
Oca+Op+ Bt
EDRFnew
1
1−ef (3.16)
If the interface is not the final destination, then it broadcasts the updated PREQ through all its interfaces. The updated PREQ contains the updated EDR and updated metric value. The rest of the HWMP protocol works similar to the standard.