Approaches to Networking
Chapter 11
By: Dr. Abbas Foroughi

11.1 Lans, Mans and WANs
Definition of communications network: a facility that interconnects a number of devices and provides a means for transmitting data from one attached device to another.
Classification of communications network on the basis of geographical scope:

1. Wide Area Networks
-Cover a large geographic area
-May require the crossing of public right-of-ways.
-May rely on circuits provided by a common carrier.
-Consist of a number of interconnected switching nodes. A transmission from any one device is routed through these internal nodes to the specified destination device.
Traditionally, WANs have provided only relatively modest capacity to subscribers - data rates of 64,000 bps or less.
Recently, ISDN provides circuit-switching and packet-switching services at rates up to 1.544 Mbps.
With optical fiber facilities, high-speed WANs now provide user connections in the 10s and 100s of Mbps., using frame relay and asynchronous transfer mode.

2. Local Area Networks
-Scope of the LAN is small, typically a single building or cluster of buildings.
-LANs are usually owned by the same organization that owns the attached devices. There may be a substantial capital investment for both purchase and maintenance. Use must do network management.
-Internal data rates of LANs are much greater than those of WANs.
-Traditionally, have provided data rates from about 1 to 20 Mbps,  inadequate.
-High-speed LANs are now being developed.

3. Metropolitan Area Networks
-Cover greater distances at higher data rates than LANs, although there is some overlap in geographical coverage.
-Primary market is the customer that has high-capacity needs in a metropolitan area.
-Provides required capacity at lower cost and greater efficiency than obtaining equivalent service from the telephone company.

11.2 Switching Techniques
For transmission of data beyond a local area, communication is typically achieved by transmitting data from source to destination through a network of intermediate switching nodes.
The switching nodes are not concerned with content of the data. They provide a switching facility that move the data from node to node until they reach their destination, or station.
Nodes - switching devices that provide communication They are connected to each other in  some topology by transmission links. Each station attaches to a node, and the collection of nodes is a communication network.
A simple network:
-some nodes connect only to other nodes. They handle the internal switching of info.
-other nodes have one of more stations attached as well. These nodes accept info from and deliver info to the attached stations.
-Node-station links are dedicated point-to-point links, usually multiplexed, using either frequency-division multiplexing or some form of time-division multiplexing.
-Usually, the network is not fully connected. There is not a direct link between every possible pair of nodes.

11.3 Circuit-Switching Networks
Basic Operation.
Dominant technology for both voice and data communications.
Dedicated communication path established between two devices through one or more intermediate switching nodes. Digital data are sent as a continuous stream of bits. Data rate is guaranteed, and delay is essentially limited to propagation time.

Three phases of communication via circuit switching:
1. Circuit establishment - before any signals can be transmitted, an end-to-end circuit must be established.
2. Information transfer - transmission may be analog voice, digitized voice, or binary data, depending on the nature of the network. Digital transmission is becoming more dominant.  Connections are usually full duplex, and signals may be transmitted in both direction simultaneously.
3. Circuit disconnect - after info transfer, connection is terminated, by one of the two stations.

Channel capacity must be reserved between each pair of nodes in the path and each node must have available internal switching capacity to handle the requested connection.
Circuit switching can be inefficient, because channel capacity is dedicated for the duration of a connection, even if no data are being transferred.
Info is transmitted at a fixed data rate with no delay other than the propagation delay through the transmission links. Delay at each node is small.
Public telephone networks
Used for voice and data.
Private branch exchange which interconnects telephones within a building or office.
Private networks

4 generic architectural components of circuit switching:
a. subscribers - devices that attach to the network.
b. Subscriber line - link between the subscriber and the network - using twisted pair wire.
c. Exchanges - intermediate switching nodes that support many subscribers.
d. Trunks - branches between exchanges. They carry multiple voice-frequency circuits using either FDM or synchronous TDM.
Circuit switching is good for voice transmission because it has little transmission delay and constant signal transmission signal rate.
Well suited to analog transmission of voice signals.
Key strengths - transparency - it appears like a direct connection to the two attached stations, no special networking logic is needed at the station.

Efficiency - minimization of the amount of equipment (switches and trunks) in the network.
Resilience - traffic on the network may surge about the normal level.
Routing strategy tries to maximize efficiency and resilience.
Traditionally, switches were organized into a tree structure or hierarchy, and additional high-usage trunks were added to connect exchanges with high volumes of traffic between them.
This approach was static.
Now, a dynamic approach is used. Routing decisions are influenced by current traffic conditions. Circuit-switching nodes have a peer relationship with each other rather than a hierarchical one. All nodes can perform the same function. More complex, but more flexible, because more alternative routes are available.
Alternate routing - the possible routes to be used between two end offices are predefined. Each originating switch selects the appropriate route for each call.  Each switch is given a set of preplanned routes for each destination, in order of preference.
Fixed-alternate routing scheme - when there is only one routing sequence defined for each source-destination pair.
Dynamic alternate routing scheme - a different set of preplanned routes is used for different time periods. Routing decision is based on current traffic status and historical traffic patterns.

Control Signaling:
Control signals are the means by which network is managed and by which calls are established, maintained, and terminated.
Signaling functions:
Supervisory  - binary character (true/false, on/off). Deal with availability of called subscriber and of the needed network resources.
Address - identify a subscriber.
Call information - signals that provide info to the subscriber about the status of a call.
Network management signals - used for maintenance, troubleshooting, and overall operation of the network.
Common channel signals - carry control signals independent of the voice channels. Can be configured with bandwidth required to carry control signals for a rich variety of functions.

11.4 ISDN
Definition: Worldwide telecomm service that uses digital transmission and switching technology to support voice and digital data communications.
Implemented as a set of digital switches and paths supporting a broad range of traffic types and providing value-added processing services.
Principles of ISDN:
a. support of voice and nonvoice applications using a limited set of standardized facilities.
b. Support for switched and nonswitched applications.
c. Reliance on 64-kbps connections. Provides circuit-switching and packet-switching connections.
d. Intelligence in the network
e. Layered protocol architecture
f. Variety of configurations
ISDN User Interface - user has access to the ISDN via a local interface to a digital pipe of a certain bit rate. A user may access circuit-switching and packet-switching services in a dynamic mix of signal types and bit rates.
ISDN Transmission Structure:
a. B channel (64 kbps) - basic use channel.
b. D channel  (16 or 64 kbpa) - carries signaling info to control circuit-switching calls on associated B channels at the user interace, and may be used for packet-switching or low-speed telemetry when no signaling info is waiting.
c. H channels (384, 1536, or 1920 kbps) for user info at high bit rates.
These channel types are grouped into transmission structures that are offered as a package to the user.
a. Basic access -meets needs of most individual users.  Allows simultaneous use of voice and several data applications, such as packet-switching access, link to a central alarm service, fax. .
b. Primary access - intended for users with greater capacity requirements, such as offices with a digital PBX or a local network.
Role of ISDN - market forces have driven telecomm technology, products, and services beyond the slow pace of ISDN standardization, making ISDN a special service, not a universal service.

11.5 Packet-Switching Networks
Definition a method of transmitting message through a communication network, in which long messages are sub-divided into short packets. Each packet is passed from source to destination through intermediary nodes. At each node, the entire message is received, stored, briefly, and then passed on to the next node.
It remains one of the few effective technologies for long-distance data communications.
The advantages of packet switching are:
Resource sharing

S nodes are distributed, there is a time delay between a change in status in one portion of the network and the knowledge of that change elsewhere.
Overhead involved in communicating status info.
Can never perform perfectly, and elaborate algorithms are used to cope with the time delay and overhead penalties of network operation.
Basic operation of Packet-Switching:
Data is transmitted in short blocks, called packets, of 1000 octets.
Each packet contains a portion of the user's data plus some control information.
Control info includes the info that the network requires to be able to route the packet through the network and deliver it to the intended destination.
At each node, the packet is received, stored briefly, and passed on to the next node.
Advantages of packet-switching over circuit-switching:
a. line efficiency is greater, because a single node-to-node link can be dynamically shared by many packets over time.
b. can carry out data-rate conversion.
c. Packets are still accepted when traffic becomes heavy, but delivery delay increases.
d. Priorities can be used - if a node has a number of packets queued for transmission, it can transmit the higher-priority packets first.
Disadvantages of packet switching relative to circuit switching:
a. each time a packet passes through a packet-switching node, it incurs a delay not present in circuit switching.
b. Jitter - because packets between a given source and destination may vary in length, may take different routes, and may be subject to varying delay in the switches they encounter, the overall packet delay can vary substantially.
c. To route packets, overhead info, must be added to each packet, which reduces the communication capacity available for carrying user data.
d. More processing is involved in the transfer of info using packet switching.
Switching Techniques:
a. datagram approach - packets are treated independently, with no reference to packets that have gone before. Self-c0ntained packets carry info sufficient for routing from the originating data terminal equipment to the destination without relying on earlier exchanges between the DTE and the network.
b. Virtual-circuit approach - preplanned route is established before any packets are sent. Then all packets between a pair of communicating parties follow this same route through the network.
Routing in packet-switching:
Adaptive routing is used. Principal conditions that influence routing decisions are:
a. failure - when a node or trunk fails, it can no longer be used as part of a route.
b. Congestion - when a particular portion of the network is heavily congested, it is desirable to route packets around rather than through the area of congestion.
For adaptive routing to work, info about the state of the net must be exchanged among the nodes.
Congestion Control in packet-switching:
Maintains the number of packets within the network below the level at which queuing delays become excessive. When line for which packets are queuing becomes more than 80% utilized, the queue length grows at an alarming rate.

11.6 X.25
Interface between attached devices and the network.
X.25 specifies an interface between a host system and a packet-switching network.
Functionality of x.25 is specified on three levels:
a. physical level - physical interface between an attached station and the link that attaches that station to the packet-switching node.
b. Link level - provides reliable transfer of data across the physical link.
c. Packet level - provides a virtual circuit service that enables any subscriber to the network to set up logical connections called virtual circuits, to other subscribers.

11.7 Traditional Wide Area Network Alternatives
Value-added network - a privately owned packet-switched network whose services are sold to the public.
Public Data Network - a packet-switched network that is publicly available to subscribers. Usually connotes government control or national monopoly.
Wider Area Networks for Voice:
Packet switching does not satisfy the voice communication demands for natural two-way conversation, and delays are too variable with packet-switching to make it useful for voice communication.
Wide Area Telecommunications services use circuit switching.
Wide Area Networks for Data:
a. public packet-switching networks
b. private packet-switching networks
c. private leased lines
d. public circuit-switching networks
e. private circuit-switching networks
Cost/Performance Considerations:
Stream traffic - lengthy, continuous transmission (file transfer, telemetry)
Bursty traffic - short, sporadic transmissions. (interactive client-server traffic, data entry, time-sharing)
Public circuit-switching is inefficient for bursty traffic, but good for stream traffic.
Packet switching is best for bursty traffic.
Other considerations:
Strategic control - designing and implementing the network to meet the organization's unique requirements. Possible with dedicated lines of private packet-switching network.
Growth control - allows users to plan for network expansion and modifications arising as their needs change. Private packet-switching networks allows this.
Day-to-day operation - user is concerned with accommodating peaks of traffic and quickly diagnosing and repairing faults.