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WAN and VLAN Solutions

Network design



Desirable properties of computer networks


  1. General scope – It is preferable to have computer network designs that satisfy a wider range of requirements and support a wider range of hardware and software. Designs become less scalable as they become more specific to a particular problem or requirement.
  2. Scalability – Computer networks should be able to handle small and large number of nodes without a significant reduction in efficiency or performance.
  3. Robustness – The operation of a computer network should minimise the impact from problems and errors within the network and from the nodes.
    • Networks should assist administrators and users in the detection and identification of problems and errors.
    • Networks should have safety barriers to reduce the spread of problems from one part of the network to other parts of the network.
    • Networks should continue operating in a stable manner after a problem has been identified and removed or fixed.
    • Networks should maintain sufficient operating information to assist the analysis of nodes and systems that function incorrectly instead of ceasing to operate (Byzantine failure).
  4. Automatic configuration – Configuration changes due to new or replacement hardware and software should require minimal administrative or user intervention.
  5. Predictability – The operational characteristics of a computer network should be sufficiently deterministic such that similar designs should have similar operations. Predictable networks are easier to analyse and repair.
  6. Incremental design – Incremental changes to parts of a computer network should be possible to allow the integration of new hardware or software.




Design issues common to networking devices






Evolution of network design themes






Attributes of network services provided to users


Making decisions about the network services to provide requires a mix of the following attributes. It often is not practical or economical to maximise or optimise every attribute!


These attributes may be defined in a service level agreement (SLA).




Relative comparison of latency and bandwidth


An improvement in latency often improves capacity, but, an improvement in capacity often makes latency worse.

Capacity (bandwidth):

Latency (delay):

Source: Patterson, D. (2004, Oct). Latency lags bandwidth. Communications of the ACM, 71-75.




Availability measured in allowed downtime per week






Client server network flow model


This is the most commonly recognised model since it is often found in both enterprise (internal to an organisation) and Internet (accessible to the public) applications. Flows tend to be directional and asymmetric, but, often with predictable patterns to the network flows.





Hierarchical client server network flow model


This model is becoming more widespread as more applications move to the cloud computing architecture. Flows tend to be directional and asymmetric, but, often with different patterns for each tier of the hierarchy.





Peer to peer network flow model


Nodes tend to operate at the same level of the network hierarchy and typically share a common service profile.





Distributed processing network flow model


This is a specialised model which often require high performance requirements from the network design.





Hierarchy and diversity are critical to network design


Hierarchy is the number of tiers of interconnection nodes and the amount of concentration of network traffic at the interconnection nodes. It determines the structure and scalability of the network.

Diversity is the amount of choices within each tier of the network. It affects the redundancy of the network.

Network design requires making trade-offs between hierarchy and diversity in order to achieve the system requirements.




Core/distribution/access architectural model


This architectural model partitions the network into 3 conceptual tiers:
  1. Core tier
    • Very high speed throughput, usually at Layer 3
    • No policies that would slow traffic down, e.g. ACLs or filters
    • Provides redundancy and scalability
  2. Distribution tier
    • Aggregation point for access switches, using both Layer 2 and Layer 3 switching
    • Apply policies for quality of service (QoS) and security
    • Provides high availability and load balancing
  3. Access (edge) tier
    • Provides access to end-users and devices
    • High port density and low cost per switch port
    • Increase network convergence by supporting a wider range of devices and traffic





Key steps in network design


  1. Collect information and generate system requirements
  2. Develop the physical and logical architecture
  3. Develop implementation and verification plans, including equipment provisions