Networking: switching

Basic Concepts

Circuit Switching Systems

A circuit switched system is one where a dedicated connection must be set up between two nodes before they may communicate. For the duration of the communication, that connection may only be used by the same two nodes, and when the communication has ceased, the connection must be explicitly cancelled. A good example of this is the early telephone exchange systems, where one caller would ask the operator to connect them to a receiver, where the end result was a physical electrical connection between the subscriber's telephones for the duration of the call. The primary characteristics of a circuit switched network are fixed-bandwidth and low transmission delay once a connection has established. Also it can be quite expensive as when traffic is low on a virtual circuit unused transmission capacity is wasted i.e. during international or long distance calls the charges will add up until the call is ended, even when either party are not speaking. When data is sent it must arrive in sequencing order and at a constant arrival rate.

References: Introduction to Networking, WestNet Learning Technologies

Packet Switching Systems

Packet-Switched networks, this networks divides all messages on the LAN (local area network) into small chunks called packets and attaches information to the front of the packet that identifies the recipient. The packets from all the machines on the local area network are placed on a high-bandwidth cable running through all the machines on the network. As packets move around the network, each machine analyses the header to see if the packet is for it. If not, it is sent further on. While packet switching is a more flexible approach than circuit switching, it does have a few problems. The primary problem is network traffic. As the number of nodes on the network increases, the network traffic increases too, sometimes reaching the network limit. Another problem with packet switching is that there is no guarantee of packets getting from source to destination, which is one of the strong points of circuit switching.

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References: Introduction to Networking, WestNet Learning Technologies

http://www.privateline.com/Switching/packet.html

Cell Switching Systems (ATM)

ATM (asynchronous transfer mode) is an international standard for a high-speed connection oriented, cell-switching technology. Cell switching closely resembles packet switching in that it breaks a data stream into packets which are then placed on lines that are shared by several streams. One major difference is that cells have a fixed size, each being 53 bytes while packets can have different sizes.

References: telecom.tbi.net/switching.htm

General Routing and Congestion Control Algorithms

Routing is the act of moving information across an internetwork from a source to a destination. Along ...

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References: Introduction to Networking, WestNet Learning Technologies

http://www.privateline.com/Switching/packet.html

Cell Switching Systems (ATM)

References: telecom.tbi.net/switching.htm

General Routing and Congestion Control Algorithms

Routing is the act of moving information across an internetwork from a source to a destination. Along the way, at least one intermediate node typically is encountered. Routing involves two basic activities: determining optimal routing paths and transporting information groups through an internetwork. Routing protocols use metrics to evaluate what path will be the best for a packet to travel. A metric is a standard of measurement, such as path length, reliability and bandwidth, which is used by routing algorithms to determine the optimal path to a destination. To aid the process of path determination, routing algorithms initialize and maintain routing tables, which contain route information. Route information varies depending on the routing algorithm used. Routing algorithms have used many different metrics to determine the best route. Routing algorithms can base route selection on multiple metrics, combining them in a single (hybrid) metric. All the following metrics have been used:

Path length
Reliability
Delay
Bandwidth
Load
Communication cost

Path length is the most common routing metric. Some routing protocols allow network administrators to assign arbitrary costs to each network link.

Reliability, in the context of routing algorithms, refers to the dependability (usually described in terms of the bit-error rate) of each network link.

Routing delay refers to the length of time required to move a packet from source to destination through the internetwork. Delay depends on many factors, including the bandwidth of intermediate network links, the port queues at each router along the way, network congestion on all intermediate network links, and the physical distance to be traveled.

Bandwidth refers to the available traffic capacity of a link. All other things being equal, a 10-Mbps Ethernet link would be preferable to a 64-kbps leased line. Although bandwidth is a rating of the maximum attainable throughput on a link, routes through links with greater bandwidth do not necessarily provide better routes than routes through slower links.

Load refers to the degree to which a network resource, such as a router, is busy. Load can be calculated in a variety of ways, including CPU utilization and packets processed per second. Monitoring these parameters on a continual basis can be resource-intensive itself.

Communication cost is another important metric, especially because some companies may not care about performance as much as they care about operating expenditures. Although line delay may be longer, they will send packets over their own lines rather than through the public lines that cost money for usage time.

Congestion control is a technique for monitoring network utilization and manipulating transmission or forwarding rates for data frames to keep traffic levels from overwhelming the network medium. It gets its name because it avoids "network traffic jams." Congestion control algorithms prevent the network from entering congestive collapse. Congestive collapse is a situation where, although the network links are being heavily utilized, very little is being done.

References :(ebook - PDF) Sybex - CCNA 2.0 Study Guide (640-507)

Handout “fundamentals of routing”

Internet Protocol Addressing and Routing

IP addresses have 4 bytes, each of which is called an octet. Since each byte in the address has 8 bits, an IP address is 32 bits long. IP addresses are usually displayed in decimal format where the value of each byte is converted from binary to decimal. This makes them easier to remember. Each computer on a TCP/IP network has to have a unique, numeric IP address. The IP address is similar to a mailing address; some of the bits represent the network segment that the computer is on. That's like the street name of a mailing address. Other bits represent the particular host on the segment; that's like the house number. A host is any system that has an IP address; this can be devices like network-attached printers as well as individual computers. When the Internet first started, all IP addresses fell into one of three pre-defined address classes. The address class tells you which bits in the IP address are used for the network address, and which are used for host addresses. These three categories got translated into three different IP address classes: Class A, B, and C. Only the largest companies received Class A network addresses, while small companies were limited to Class C addresses. Each class uses a different part of the 32-bit IP address space as the network portion of the address, leaving the remaining bits for use as the host address.

http://www.computerguru.net/tech/netw64.htm

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