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Resilient Cloud Network Mapping with Virtualized BBU Placement for Cloud-RAN Carlos Colman-Meixner† ,Gustavo B. Figueiredo∗ ,Matteo Fiorani§ ,Massimo Tornatore†‡ ,and Biswanath Mukherjee† † University of California, Davis, USA, Email:[cecolmanmeixner, bmukherjee, mtornator]@ucdavis.edu ∗ Federal University of Bahia, Salvador, Brazil, Email:
[email protected] § KTH Royal Institute of Technology Stockholm, Sweden, Email:
[email protected] ‡ Politecnico di Milano, Italy, Email:
[email protected]
Abstract— Cloud Radio Access Network (C-RAN) will improve mobile radio coordination and resource efficiency by allowing baseband processing unit (BBU) functions to be virtualized and centralized, i.e., deployed in a BBU hotel. We consider a BBU hoteling scheme based on the concept of access cloud network (ACN). An ACN consists of virtualized BBUs (vBBUs) placed in metro cloud data centers (metro DCs). A vBBU is connected to a set of remote radio heads (RRHs). ACN resiliency against network and processing failures is critical for C-RAN deployments. Hence, in this study, we propose three protection approaches: 1+1 ACN protection, 1+1 ACN and vBBU protection, and partial ACN protection. Simulation results show that both 1+1 ACN and 1+1 ACN and vBBU protection requires large capacity for backup to provide 100% survivability for singlelink and single-DC failures. As a result, we suggest a partial ACN protection approach which provides degraded services with only 8% additional network resources. Index Terms— Cloud radio access network, Virtualized baseband processing unit, Cloud network, Resiliency.
I. I NTRODUCTION Cloud Radio Access Network (C-RAN) is a new architecture for 5G mobile access networks that enables sharing and virtualization of baseband processing unit (BBU) functions (i.e., BBU hoteling) to achieve efficient resources utilization, improved radio coordination, and cost savings. While the use of cloud-based infrastructure to offer cloud BBU services is an attractive solution [1], [2], the resiliency of cloud BBU hotelling [3] is a key requirement given that a single network and/or processing unit failure might produce large disruptions [4]. In this paper, we define and solve the resilient Cloud Network (CN)1 mapping [5] with virtualized BBU (vBBU) placement problem. In our scenario, we define CNs connecting vBBUs placed in metro data centers (metro DC) with sets of remote radio heads (RRHs) from different wireless areas. To solve this problem, we propose three approaches – i) 1+1 ACN protection; ii) 1+1 ACN and vBBU protection; and iii) Partial ACN protection – to reduce the backup cost of “1+1” based approaches by exploiting the bandwidth adjustment in the affected RRHs. Such approaches are based on dedicated virtual link protection [6] and resilient VM placement [4]. We compare our approaches in term of resource usage. In the rest of this paper, we introduce the cloud BBU hoteling scheme. Then, we introduce an integer linear program (ILP) model for our approaches, provide illustrative numerical examples, and then conclude our work. 1 Cloud
Network (CN) consists of a set of virtual machines (VMs) connected by virtual links to provide virtualized processing and virtualized communication for a tenant [4].
II. C LOUD BBU H OTELING We consider a metropolitan WDM optical network that provides low-delay and high-bandwidth connections between metro DCs and central offices (COs) (Fig. 1(a)). A fronthaul network (based on, e.g., WDM-PON or TWDM-PON [7], [8]) provides flexible and low-delay connectivity between the CO an a set of RRHs, i.e., cells with a given wireless coverage area (Fig. 1(b)). Metro DCs are placed inside an area defined by the maximum round-trip delay tolerated by the wireless technology [1]. Each virtualized BBU (vBBU) consists of a virtual machine (VM) hosting a BBU placed in a metro DC and connected by virtual links to a set of RRHs (i.e., a set of cells). An ACN is a set of vBBUs under a single management (Fig. 1(c)) (e.g., belonging to the same virtual network operator). Each virtual link carries common public radio interface (CPRI) traffic generated by RRHs first to the optical line terminal (OLT) in the CO and then to the vBBU in the metro DC. An example of non-survivable ACN mapping is introduced in Fig 1(d) where a failure in a link between SW 1 and CO 1 will disrupt 100% the BBU services of Area 1. Hence, we define and study the resilient cloud network mapping with vBBU placement (RCM-vBBU) problem.
Fig. 1. Cloud-BBU hotelling infrastructure and an ACN. a) Metro DCs and a metro network. b) CO connecting RRHs using TWDM-PON. c) An ACN of two vBBUs (V 1 and V 2) connecting RRHs R1,R2,R3,R4 of Areas 1 and 2 connected to the COs 1 and 2. d) Non-survivable ACN mapping.
III. R ESILIENT C LOUD N ETWORK M APPING WITH V IRTUALIZED BBU P LACEMENT (RCM- V BBU) A. RCM-vBBU Problem Statment Input: metro-access network and ACN requests. Output: survivable ACN mapping and vBBUs placement. Goal: minimize resource usage.
2
• • • • •
ˆ set of candidate virtual links e =< r, p > between a given E: RRH r ∈ Vr and a DC p ∈ Vd . ˆ Aˆ >: ACN request consisting of a subset of RRHs γ =< R, ˆ ∈ Vr , and a subset of areas Aˆ ∈ Va . R Γ: set of ACN requests, i.e., γ ∈ Γ. F : maximum number of RRH connections supported by any vBBU. M : a large number used for binarization purposes.
2) Variables: • • • • •
Fig. 2. Examples of protection approaches used in ACN mapping with vBBU placement. a) A non-survivable ACN mapping, where a failure disrupts all users of the Area. b) Using 1+1-ACP approach, the working virtual links will fail but the vBBU will keep connected to their RRHs. c) 1+1-vBBU which places a backup vBBU provides and the same survivability. d) PartACP approach allows RRHs R1 and R2 to share virtual links with RRHs R3 and R4 in case of failure.
B. Proposed Approaches To solve the RCM-vBBU problem, we consider the integration of two resiliency techniques used for cloud networks [4] in three different approaches that can be used in CloudRAN. The resiliency techniques are: i) Dedicated virtual link protection [6] based on 1+1 dedicated path protection (DPP), and ii) Resilient VM placement (RVMP) [4] based on 1+1 VM replication for vBBU resiliency. Our three proposed approaches are as follows: i) 1+1 ACN Protection (1+1-ACP) consists in the mapping of dedicated backup virtual links [6] to provide 100% resiliency from RRHs to vBBUs against any single-link failure in the network (Fig. 2(b)). ii) 1+1 ACN and vBBU protection (1+1-vBBU) adds 100% resiliency against any single metro DC failure by placing the backup vBBU in a different metro DC (Fig. 2(c)). However, 1+1 approaches require more than 100% additional capacity, hence we suggest a partial protection scheme. iii) Partial ACN protection (Part-ACP) allows only half of virtual links to share the same link to avoid 100% disconnection in case of a single-link failure. In Fig. 2(d), Part-ACP maps two virtual links in the link [SW 1 - CO 1] and two virtual links in the links [SW 1 - SW 2] and [SW 2 - CO 1]. Thus, the failure of link [SW 1 - CO 1] will disrupt only 2 of 4 virtual links. For example, we can assume that RRHs can reduce their CPRI traffic from 9.8 Gbps (8 × 8 MIMO) to 2.4 Gbps (4 × 4 MIMO) without losing cell coverage, but decreasing to 50% their available bandwidth.
• • •
e,a Wi,j : 1 if virtual link e connecting a RRH from area a, is mapped in the optical link (i, j) ∈ E; otherwise 0. e,a Mi,j : 1 if backup virtual link e connecting a RRH from area a is mapped in optical link (i, j) ∈ E; otherwise 0. γ,a Cr,p : 1 if a virtual link connects a RRH r from area a with the metro DC p for ACN request γ; otherwise 0. γ,a Br,b : 1 if a backup virtual link connects a RRH r from area a with metro DC b for ACN request γ; otherwise 0. γ Xp,a : 1 if γ uses metro DC p to place a vBBU for a set RRHs from area a and for an ACN request γ; otherwise 0. γ Yp,b : 1 if γ uses metro DC b to place a backup vBBU for a set of RRHs from area a and for an ACN request γ; otherwise 0. Zp : 1 if the metro DC p is used for any working vBBU; otherwise 0. Up : 1 if the metro DC p is used for any backup vBBU; otherwise 0.
D. ILP formulation Equation (1) minimizes the network (Net) resources usage and Eq. (2) minimizes the usage of metro DCs (Proc) to map e,a the ACN. In Part-ACP approach, all Mi,j = 0. X
min
1) Given: •
G(V, E): graph representing the infrastructure composed of V = (Vd ∪ Vo ∪ Vr ∪ Va ) nodes and E = (Em ∪ Ea ) edges where Vd is a set of metro DCs, Vo is a set of metro network nodes, Vr is a set of RRHs, Va is a set areas, Em is a set of metro network links, and Ea is a set of access network links.
ˆ e∈E
(i,j)∈E
min
a∈Va
X
e,a e,a Wi,j + Mi,j
(1)
γ∈Γ
(Zp + Up )
(2)
p∈Vd
Equations (3) and (4) place the working vBBUs and map virtual links connecting them with a set of RRHs. X
γ,a ˆ γ∈Γ Cr,p ≤ F, ∀ p ∈ Vd , a ∈ A,
(3)
ˆ r∈R γ Xp,a ≥
X γ,a 1 X γ,a γ Cr,p , Xp,a ≤ Cr,p M ˆ r∈R
(4)
ˆ r∈R
ˆ γ∈Γ ∀ p ∈ Vd , a ∈ A,
Equation (5) maps backup virtual links. Backup vBBUs placement is defined by a similar equation to Eq. (4) which defines γ γ,a γ,a Yb,a by replacing the variable Cr,p for the variable Br,p . X X
γ,a γ,a ˆ γ∈Γ Br,b ≤ Cr,p ∀ p ∈ Vd , a ∈ A,
(5)
ˆ b∈Vd r∈R
Equation (6) ensures flow conservation of working virtual links. Note that, to ensure flow conservation of backup virtual e,a links, we must use the same equation, but with variables Mi,j γ,a e,a γ,a and Bse ,de instead of variables Wi,j and Cse ,de . X
e,a Wi,s − e
(i,se )∈E
C. Variable and Symbols
X X X
X
(k,j)∈E
Wse,a = −Csγ,a e ,j e ,de
(6a)
Wde,a = Csγ,a e ,j e ,de
(6b)
(se ,j)∈E e,a Wi,d − e
(i,de )∈E
X
X
e,a Wk,j −
X (de ,j)∈E
X
e,a ˆ k ∈ V − {se , de } , Wi,k = 0, ∀e ∈ E,
(i,k)∈E
ˆ se ∈ R, ˆ sd ∈ Vd , γ ∈ Γ a ∈ A, (6c)
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Equation (7) enforces 100% survivability against single-link failure. For Part-ACP approach, F becomes F2 . This constraint limits to 50% the sharing of each link by virtual links connecting the same vBBU. This ensures 50% protection. X
X
e,a e,a ˆ γ∈Γ ≤ F ∀ a ∈ A, + Mi,j Wi,j
(7)
ˆ (i,j)∈Em e∈E
Equation (8) determines the metro DC used for working vBBUs. Similar equation determines the metro DC used for γ backup vBBUs (Up ) by using the variable Yp,a instead of the γ Xp,a . Zp ≥
X γ 1 X γ Xp,a , Zp ≤ Xp,a , ∀ a ∈ Va , γ ∈ Γ M p∈V p∈V d
(8)
d
Additional constraint used in 1+1-ACP approach (Eq. (9)) to enforce working and backup virtual links connectivity to the same vBBU. Zp − Ub = 0 ∀ b, p ∈ Vd
(9)
Additional constraint used in 1+1-vBBU approach (Eq. (10)) to enforce backup vBBUs placement in different metro DC. Zp − Ub
