SYST5030/4030 Design Requirements What are the nodes to be connected? What is the data traffic volume on each line (by application)? What are the peak traffic volume requirements? What is the response time required to the user, average/max? What is the error percentage? Compression ratio? Anticipated growth rate, yearly over next 3 to 5 years? Security issues? How reliable must the network be? Any new applications likely to come on stream?
SYST5030/4030 Network Design steps Message Analysis / Traffic Data Analysis Point to Point Traffic Table Study service options for each pair of nodes (e.g. Acunet [AT&T], leased line, VPN) Generate alternative network topologies Evaluate each alternative configuration in terms of cost, response time, reliability, etc. Select the best one.
SYST5030/4030 Computing Bits/sec on each link Start with average characters per day from link traffic table. Then add the effect of following additional factors: 1) Future growth 2) Protocol overhead 3) Transmission errors 4) Turnpike effect (usage is more than anticipated!) 5) Number of hours available per day 6) Periods of unusually high activity. Options for periods of peak activity: 1) Tolerate some slowdown. 2) Plan based on peak characters per day.
SYST5030/4030 Message Analysis Application Name: Hotel Reservation. Fields in the Input Message: Field Name Average Length Max Length Guest Name 15 chars30 chars Address50100 Arrival date66 Dep. date66 Payment mode44 Guaranteed11 Type of room Fields in reply message: Confirmation # 10 chars 10 chars
SYST5030/4030 Example Say, 10,000 transactions per hour and each transaction is 1000 bytes long. Characters per hour = 10,000,000 Account for Growth rate of 10% per year (5 year horizon): = 16,100,000 characters per hour (over 5 year horizon) Protocol overhead (10 bits per character): 16,100,000 X 10 = 161,000,000 bits per hour Transmission errors (1%): 161,000,000 X 1.01 = 162,600,000 bits per hour = 45,167 bits per sec. If line utilization is 67%, then we need a line speed = 45,167/0.67 = 67,413 bits per sec.
SYST5030/4030 Formulating Network Design Problems Minimize Communications cost such that: 1) Average response time < 1 secs 2) 90% response time < 1.5 secs. 3) Reliability requirements: e.g. there must be at least two paths between any 2 points, or any X link failures should not cause any disruption Formulation 2 Formulation 1 (simplified version) Minimize Total circuit mileage
SYST5030/4030 Network Topology MULTIPOINT: connect all pairs of nodes in a minimum spanning tree. Simple design, but not very reliable. Usually least expensive. Minimizes total mileage. RING: connect all nodes in a loop. STAR: connect all nodes to central computer. MESH: Multiple paths between nodes. Topology refers to arrangement of nodes in a network
SYST5030/4030 Multipoint Topology
SYST5030/4030 RING Topology
SYST5030/4030 Ring-Based WAN Design A ring-based WAN design connects all computers in a closed loop, with each computer linked to the next, usually with a series of point-to-point dedicated circuits. One disadvantage is of the ring topology is that messages can take a long time to travel from the sender to the receiver. In general, the failure of one circuit or computer in the network means that the network can continue to function. More reliable than multipoint.
SYST5030/4030 STAR Topology
SYST5030/4030 STAR Topology STAR: connect all nodes to central computer. Great for centrally controlled networks Easy to manage and fix problems Predictable performance Easy to expand. Failure prone at central point High circuit cost Not scalable Recommended for small networks,voice networks and switched networks accessing backbone networks.
MESH Topology MESH: Multiple paths between nodes. Resistant to nodal and link failures - highly reliable. Relatively easy to expand and modify However, more complex to design and manage. Recommended for host-to-host connectivity, large multi- application networks. Employed by banks, financial and insurance companies, retail operations.
Centralized versus Distributed Network design Centralized network: One central computer. Where to locate the central computer? Location of concentrators? Multi-point line layout? Distributed network: 1. Develop network topology. 2. Assign traffic flows over network links. 3. Size the line capacities based on topology and traffic flows. Need to consider: How many computers? Where? Routing Paths? Data allocation to nodes? Distributed network design is much harder than centralized design.
SYST5030/4030 Backbone networks Backbone networks consists of high speed links and nodes that consolidate traffic from smaller access networks. Networks can be structured in three ways: Private access networks connecting to private backbone networks. Private access networks connecting to public backbone networks. Combination of the above. (SMDS [Switched Multimegabit Data Service] and ATM are good public backbone options. Frame relay is a good option for both public and private backbones.) Private backbones integrate the company's subnetworks and provide a reliable, secure and high speed means of transmission for distributed voice, data and time-sensitive applications. Public backbones are also becoming popular with VPNs (or virtual private networks).
SYST5030/4030 Network design techniques Examples of network design techniques: 1. Minimum spanning tree to minimize circuit mileage 2. Queuing models for response time computations 3. ADD heuristic for solving location problems 4. Linear programming techniques 5. Other heuristics Medium or large size problems cannot be solved optimally. Therefore, we often use heuristics. Sometimes, the problem must be simplified to make it tractable.
SYST5030/4030 Minimum Spanning Tree Prim's Algorithm : 1. Start with any single site or node in the tree and add nodes one at a time. 2. At each stage, add the node nearest to the whole tree (in case of a tie choose either one randomly). 3. Stop when all nodes are included. Problem: Connect several nodes to a central node such that total length of links is minimized.
SYST5030/4030 Minimum Spanning Tree Kruskal's Algorithm : 1. Sort all inter-node distances in ascending order. 2. Check each potential link starting with the shortest. 3. If that link can be added without forming a circuit, add it. 4. Stop when all nodes are connected in the tree.
SYST5030/4030 Cost Components in Pricing Mileage charge for IXC (inter exchange channel) Mileage charge for intra LATA channels at each end of circuit Cost of channel options: C type or D type conditioning Central office or end office connection charge at each end of the IXC (i.e. at each point where IXC connects to intra lata lines) Access coordination for intra-Lata channels.
SYST5030/4030 AT&T’s V and H (Vertical and Horizontal) coordinate system was devised in 1957 by Jay K. Donald for the easy computation of distances between telephone switching centers or network elements. The system is based on the Donald Elliptic Projection, a two-point equidistant projection covering the land masses of the continental United States and Canada. The result is a basic 10,000 X 10,000 “V&H grid,” as shown above. The system is based on units of the square-root of one-tenth of a mile. A V and H coordinate does not have an uniquely defined unit name, but one “coordinate” is approximately one-third of a mile. V and H Coordinate System Source: Mileage = where v1, v2, h1, and h2 are vertical and horizontal coordinates respectively
SYST5030/4030 Calculating mileage Each central office (or end office) has a vertical and horizontal coordinate Mileage = Tulsa V1 = 7707H1 = 4173 SFO V2 = 8492H2 = 8719 Mileage = (Note: Always round the fractional part of the mileage up)
SYST5030/4030 Cost Comparison Circuit from to Monthly Installation Cost for T-1 circuit Cost for 512 K circuit Cost for 256 K circuit Cost for 56K circuit Cost for 9.6K circuit (source: OLD Information provided by MCI)
SYST5030/4030 Automated Network Design Tools Requirements analysis: collecting data on line types and costs, node types and costs, sources and traffic volumes. Topological design: Using heuristics and other techniques to produce network topology. Performance analysis: Assessing cost, reliability and delay for each design. There are several tools available in the market. Prices range from few $1000 to $100,000. Examples: Autonet Advisor, Performance-3,WinMIND, etc.
SYST5030/4030 Summary Network design is both an art and a science. Need to combine several techniques. Tools of increasing sophistication available in the market at reasonable price. Hard to do optimal design for more than 50 node network. Bottom line: Good design can save a company millions of dollars per year in cost and also increase revenues.