Review of Topology and s / Concepts BSAD 141 Dave Novak Sources: Network+ Guide to Networks, Dean 2013 Overview Three base wired topologies Bus, star, ring Two wireless topologies Ad-hoc, infrastructure Three basic access techniques CSMA/CD, CSMA/CA, and token passing CSMA and delay versus circuit switching Architecture Basic level any topology will support any architecture Star topology may support client server (CS) or peer-to-peer (P2P) architecture Likewise, P2P or CS architecture may be implemented using star, bus, or ring topology Architecture vs- Topology Topology physical or logical design or layout of the network What is the difference between a physical star and logical star topology? Architecture describes the functionality and administrative structure of the network How devices interact Whether there are servers and exactly what these devices do The access technique describes how the devices that are using a particular networking technology (such as Wi-Fi or Ethernet) share or coordinate the use of, the communication medium This is relevant because the vast majority of networking technologies and implementations involve the sharing of the communications medium CSMA/CD? CSMA/CA? 1
Token Passing? Distance limitation and LANs Distance is a fundamental consideration in the design of any network particularly when it comes to popular LAN technologies such as Wi-Fi and Ethernet The length of an Ethernet network cannot be continually increased by adding new segments using switches, hubs or repeaters Ethernet and Delay CSMA/CD (and CSMA/CA for that matter) designed to function with limited latency Scheme fails if length is too long or too many segments are connected Serious performance problems if you string a bunch of Ethernet segments together with repeaters Discussion In data networks, the data are broken up into smaller packets as opposed to transmitting large streams of data Packets promote equity Enables many devices to share the medium Data networks not designed to transfer data as a stream of continuous bits Data are divided into small generic blocks called packets Dividing data into packets helps determine which blocks have errors and which do not Think about the concept of encapsulation What happens when data are encapsulated? What is the difference between a packet and a frame? 2
Determines how connections are established between different nodes on the network Focus on 2 types of switching 1) Type of switching Why is this an issue? 2) A connection-based transmission technology where 2 nodes establish a dedicated connection for the duration of the communication session Requires that the sending node contact receiving node to establish the connection Based on the concept of streaming large amounts of data What is a potential drawback to a dedicated connection? Some amount of bandwidth on circuit/channel is dedicated and remains available until connection is terminated Conceptually a reserved path from sender to receiver is created that provides an open road for non-stop transfer Not shared While nodes are connected, all data follow same path Data arrive in order 1. Pick up the phone and listen for a dial tone. This lets you know that you have a connection to the local office of your telephone carrier 2. Dial the number of the party you wish to talk to 3. Your call is routed through the switch at your local carrier to the party you are calling 4. A connection is made between your phone and the other party's phone using several interconnected switches along the way 5. The phone at the other end of the connection rings, and someone answers the call. 6. The connection opens a dedicated circuit for the duration of the call 7. You talk for a period of time and then hang up your phone 8. When you hang up, the communication circuit or channel is closed, freeing the bandwidth along the circuit in between the caller and the receiver Source: http://computer.howstuffworks.com/ip-telephony2.htm Advantages of Circuit Dedicated connection between 2 hosts full use of communication channel for duration of communication session Highly reliable (with respect to path, variation in delay, and constant bit rate) Requires little packet overhead during transmission because path / communication circuit is dedicated and known Makes optimal use of available bandwidth (ratio of overhead to data is very small) during transmission but not if channel is idle 3
Disadvantages of Circuit Inflexible with respect to many internet-based applications (data are bursty) Involves a lot of communication overhead and time with respect to ESTABLISHING the connection on the communication channel Can waste a lot of bandwidth if data are not continually being sent (inefficient if idle) In event of failure, data are not packetized and do not seek an alternate path path may be lost and all data may be lost A digital networking communication method where data are broken into small packets and each packet can be transmitted independently Delivers variable rate data streams During transmission packets are buffered and queued which leads to variable delay and throughput No communication is needed to explicitly establish a connection with receiving node ahead of time Two modes 1) Connectionless (datagram) 2) Connection-based (virtual circuit) 1) Connectionless mode (datagram) Asynchronous communication no coordination Implication: modern, data-oriented, digital, packet-switched networks can take advantage of both connectionless and connectionbased transfer Each packet much include complete routing information because each packet can be routed individually Out-of-order delivery, different paths 2) Connection-based mode (virtual circuit) Protocols establish a virtual connection between sender and receiver This connection is established via software rather than physically A temporary dedicated channel is established Connection set up and torn down as needed Packets include connection identifier Packets are typically delivered in order Advantages of Packet switching Bandwidth from end-to-end is optimized Unused circuits are torn down and bandwidth is reallocated Supports variable data transfer rates In event of failure, packets can reroute Designed to support bursty data transfer 4
Disadvantages of Packet switching Variable delay Packets can be lost or corrupt Sophisticated protocols are needed to provide some level of reliability during transfer Not good for certain types of applications that require constant bit rates, limited variation in delay, or no data loss (e.g. real-time video) Summary Three base wired topologies Bus, star, ring Two wireless topologies Ad-hoc, infrastructure Three basic access techniques CSMA/CD, CSMA/CA, and token passing versus circuit switching 5