Most  computer  processing  units  (CPUs)  are  able  to execute  instructions  and  basic  decision-making  steps  at a rate of several million instructions per second. Data can  be  transferred  between  the  computer’s  memory  and the cpu at these same rates of speed. The ideal network could keep up with the high speed of the cpu and be able to transfer data between the stations of the network at rates close to the rates that data is moved around within the cpu and memory. However, this is just not possible with a telephone line linked system, which is limited in the range of frequencies it can carry. When high- frequency signals are carried by wire such as twisted- pair, all sorts of electrical effects come into play. It’s not sufficient  to  simply  link  computer  systems  with common wire. Considerable thought must be given to the  electrical  characteristics  of  the  connection.  The cable selection must be made during the design phase of the network to ensure that the decision is not left to be made during the installation of the network. NETWORK OPERATING SYSTEM A network operating system (NOS) is a software package that makes it possible to implement and control a network and enables users to use the resources and services on that network. A NOS’s tasks include: l l l l l l Providing access to files and resources; Providing electronic mail (e-mail) services; Enabling nodes on the network to communicate with each other; Enabling  processes  on  the  network  to communicate with each other; Responding to requests from applications and users on the network; and Mapping requests and paths to the appropriate places on the network. A NOS may be server-based or peer-based. Server based  NOSs  are  considerably  more  complex  and powerful than NOSs for peer-to-peer networks. In a server-based network, the NOS and the server run the show, and the workstations will generally run a network shell. By contrast, in a peer-to-peer network any station can function as file server or as a client for network services. Operating systems which have built-in networking capabilities include the following: l UNIX® . Windows NT® l Novell® DOS 7 In most of these cases, the operating system’s networking  capabilities  can  be  greatly  enhanced through  the  use  of  utilities  or  other  third-party programs.  To  learn  more  about  these  utilities  or programs,  check  the  manuals  that  come  with  the operating  system. FIREWALLS Firewalls can be used for securing a local area network  from  a  public  network  like  the  Internet. Firewalls are always a part of a much larger security plan. Choosing a firewall starts with a clear definition of the security goals. This includes decisions on what logging and alarms are needed, what authentication is acceptable and where security barriers are needed. Once the policy, philosophy, and service goals are defined, often only a few products on the market really fit  these  needs. There are several types of firewalls that can be divided into packet filtering and application layer firewalls. Packet Filters Packet  filters  operate  at  a  lower  level  than application  layer  firewalls.  Packet  filters  decide whether to forward an IP packet based on the source or destination  address  found  at  the  network  layer.  Routers typically  implement  this  type  of  filtering,  but  since packets containing bogus IP addresses can easily be created, it’s not too hard to gain access through even the most elaborate set of IP address filters. Although the router on an Internet link can filter packets, it probably wasn’t designed to provide the level of control that a firewall product can. A router examines one packet at a time and forwards the packet. Application Layer Firewall Application layer firewalls, on the other hand, are designed specifically to control unwarranted access to your network. They can also deal with some of the trickier protocols. Application layer firewalls gain more insight into the data conversations that traverse an Internet  link  because  they  examine  packets  and protocols at and above the transport layer, which 1-18