Chapter 9: Introduction to Metropolitan Area Networks and Wide Area Networks

INTRODUCTION

Metropolitan area network (MAN) is a network that expands into a metropolitan area such as a city or a region and exhibits high data rates, high reliability and low data loss. Wide area network is a network that expands beyond a metropolitan area. They interconnect with huge numbers of workstations and can cover large geographic distances, including the entire earth.

METROPOLITAN AREA NETWORK BASICS

• MANs has many of the same technologies and communications protocols found in LANs and WANs
• Support high-speed disaster recovery systems, real-time transaction backup systems, interconnections between corporate data centers and internet service providers, support high speed connections among government, business, medical, and educational facilities
• almost exclusively fiber-optic networks and capable of supporting data rates into the tens of millions and hundreds of millions bits per second
• cover greater distance than LANS
• can recover very quickly from a link or switch/router failure
• MAN topologies are based on a ring
• the ability of a user to dynamically allocate more bandwidth on demand

1. SONET versus Ethernet

• Most MANs are SONET network built of multiple rings (for failover purposes)
• SONET is well-proven but complex, fairly expensive, and cannot be provisioned dynamically.
• Ethernet MANs generally have high failover times
• SONET is based upon T-1 rates and does not fit nicely into 1 Mbps, 10 Mbps, 100 Mbps, 1000 Mbps chunks, like Ethernet systems do.

• Metro Ethernet: is a data transfer service that can connect your business to another business using a standard Ethernet connection

• You may also connect your business with multiple businesses using a connection similar to a large local area network (Figure 9-4b)
• Thus, by simply sending out one packet, multiple companies may receive the data
• Neat thing about metro Ethernet is the way it seamlessly connects with a company’s internal Ethernet network(s)

WIDE AREA NETWORK BASICS

• WANs is a collection of computers and computer-related equipment interconnected to perform a given function typically using local and long-distance telecommunications systems
• Typically used to transfer bulk data between two endpoints and provide users with electronic mail services, access to database systems, and access to the internet
• WANs are very high speed with low error rates
• Usually follow a mesh topology

• A station is a device that interfaces a user to a network.
• A node is a device that allows one or more stations to access the physical network and is a transfer point for passing information through a network.
• A node is often a computer, a router, or a telephone switch.
• The sub-network or physical network is the underlying connection of nodes and telecommunication links.

Types of Network Structures

1. Circuit switched network:

• A network cloud in which a dedicated circuit is established between the sender and receiver, and all data passes over this circuit
• The telephone system is a common example.
• The connection is dedicated until one party or another terminates the connection.

2. Packet switched network:

• All data messages are transmitted using fixed-sized packages, called packets
• More efficient use of a telecommunications lie since packets from multiple sources can share the medium
• Datagram packet-switched network, each data packet can follow its own possible unique course through the cloud
• Virtual Circuit packet-switched network create a logical path through the subnet and all packets from one connection follow this path

3. Broadcast network

• typically found in LANs, but occasionally found in WANs
• A workstation transmits its data and all other workstations "connected to the network hear the data. Only the workstations with the proper address will accept the data

Connection-oriented Versus Connectionless Network Application

• Connection oriented network application: provides some guarantee that information traveling through the network will not be lost and that the information packets will be delivered to the intended receiver, which is called Reliable Service
• A connection-oriented application requires both sender and receiver to create a connection before any data is transferred.
• Applications such as large file transfers and sensitive transactions such as banking and business are typically connection-oriented.
• A connectionless application does not create a connection first but simply sends the data.

• A connection oriented application can operate over both a circuit switched network or a packet switched network
• A connectionless application can also operate over both a circuit switched network or a packet switched network but a packet switched network may be more efficient

ROUTING

• Each node in a WAN is a router that accepts an input packet, examines the destination address, and forwards the packet on to a particular telecommunications line.
• A router must select the one transmission line that will best provide a path to the destination and in an optimal manner.
• Often many possible routes exist between sender and receiver.

• The communications network with its nodes and telecommunication links is essentially a weighted network graph.
• The edges, or telecommunication links, between nodes, have a cost associated with them.
• The cost could be a delay cost, a queue size cost, a limiting speed, or simply a dollar amount for using that link.

• The routing method, or algorithm, chosen to move packets through a network should be:
• Optimal, so the least cost can be found
•  Fair, so all packets are treated equally
• Robust, in case link or node failures occur and the network has to reroute traffic.
• Not too robust so that the chosen paths do not oscillate too quickly between troubled spots.

1. Dijkstra's Least-cost Algorithm

• Is executed by each node and the results are stored at the node and sometimes shared with other nodes
• Calculation is time consuming so it is done on a periodic basis or when something is in the network changes
• By identifying all possible paths, it also identifies the lest cost path

2. Flooding Routing

• When a packet arrives at a node, the node sends a copy of the packet out every link except the link the packet arrived on.
• Traffic grows very quickly when every node floods the packet.
• To limit uncontrolled growth, each packet has a hop count.  Every time a packet hops, its hop count is incremented.  When a packet’s hop count equals a global hop limit, the packet is discarded

3. Centralized Routing

• One routing table is kept at a “central” node.
• Whenever a node needs a routing decision, the central node is consulted.
• To survive central node failure, the routing table should be kept at a backup location.
• The central node should be designed to support a high amount of traffic consisting of routing requests.

4. Distributed Routing

• Each node maintains its own routing table.
• No central site holds a global table.
• Somehow each node has to share information with other nodes so that the individual routing tables can be created.
• Possible problem with individual routing tables holding inaccurate information.

5. Adaptive Versus Fixed Routing

• With adaptive routing, routing tables can change to reflect changes in the network
• Static routing does not allow the routing tables to change.
• Static routing is simpler but does not adapt to network congestion or failures.

NETWORK CONGESTION

• When a network or a part of a network becomes so saturated with data packets that packet transfer is noticeably impeded, network congestion occurs.
• What can cause network congestion?  Node and link failures; high amounts of traffic; improper network planning.
• When serious congestion occurs buffers overflow and packets are lost.
• An application can observe its own traffic and notice if packets are disappearing.  If so, there may be congestion.  This is called implicit congestion control
• The network can inform its applications that congestion has occurred and the applications can take action.  This is called explicit congestion control
• Before making a connection, user requests how much bandwidth is needed, or if connection needs to be real-time
• Network checks to see if it can satisfy user request
• If user request can be satisfied, connection is established
• If a user does not need a high bandwidth or real-time, a simpler, cheaper connection is created
• This is often called connection admission control

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