1. Introduction  
1.1. Background  
1.2. Related Work  
1.3. Overview of Paper Structure  
2. Basic Notations  
2.1. Inner-chain and Inter-chain  
2.2. Architecture Scalability  
2.3. Data Scalability  
2.4. Protocol Scalability  
3. Architecture Scalability  
3.1. Inner-chain Solutions  
3.2. Inter-chain Solutions  
4. Data Scalability  
4.1. Inner-chain Solutions  
4.2. Inter-chain Solutions  
5. Protocol Scalability  
5.1. Inner-chain Solutions  
5.2. Inter-chain Solutions  
6. Discussion  
6.1. Architecture Scalability  
6.2. Data Scalability  
6.3. Protocol Scalability  
7. Conclusion