Network
Using Wan and Network Services
The
major objective of network design is to select the network service and to
determine the transmission speed for the system. Following are the typical
examples of network using WAN and network services:
Host to
Terminal Connection
The host
to terminal connection is a conventional type of connection between a mainframe and dumb terminals. This connection is widely used
for routine work. Fixed message length, for example, makes estimation of traffic
easy. As terminals are relatively slow, telephone network, low speed leased
line, packet switching network, ISDN and so on are mainly used. A terminal
controller (TC) may be used to integrate two or more terminals for connection
with a high-speed line. The TC shown is used to integrate two or more terminals
for connection with a single WAN line.
LAN to
LAN Connection
The
configuration is used to connect LANs that are remote from one another. There
is a large difference in transmission speed between LAN and WAN. In addition,
using a high speed line can become substantially expensive. Therefore, proper
arrangements must often be made to reduce traffic within the WAN. Leased line,
ISDN, and frame relay are mainly used for this connection.
Remote LAN Connection
The
configuration is mainly used to connect a remote LAN and PC. Due to the limited
number of terminals to be connected and also due to restrictions on allowable
expense, the types of WAN that can be used are also limited. In general ISDN is
frequently used. Some servers are exclusively designed for the access purpose.
LAN to
LAN Connection
Router
Concepts
A router
has two functions as follows:
·
Forwarding Function It is a function that allows selection of the appropriate
route based on IP header information and sends packets through this
route.
·
Filtering Function It is a function that allows dumping of invalid
packets for a specific network instead of forwarding.
Forwarding
Function
The most
important processing in forwarding is to determine the route. In general,
routing refers to the processing used for determining the next hop (IP address
of next router). Here, we consider that the processing for sending packets to
the host in the same network is also performed as an integral part of routing.
Routing in the same network may be termed as local routing.
Filtering
Function
The
second function available with a router is that it can filter packets to
determine whether they can pass through it. For example, consider that there
are two networks connected via router.
Now, we
impose the following conditions:
1. Hosts
A and C cannot communicate using UDP packets
2. Host
B cannot use FTP for communication with host D
3. Host
C cannot communicate with host A
4. Host
D can communicate with host B only using TELNET.
In this
case the network administrator sets the router as given below:
Exclusion List
(Source
host == A, Destination host == C, Protocol == UDP)
(Source
host = C, Destination host = A, Protocol = UDP)
(Source
host = B, Destination host = D, Port = FTP)
(Source
host == D, Destination host = A)
Inclusion List
(Source
host = D, Destination host = B, Protocol = TELNET)
Routing
Method
Routing
plays a major role in the forwarding function. This can be divided into
following two methods:
·
Static routing: Routing information is manually specified. It
provides fixed route information to each router. If there is no change in
route, it is made manually.
·
Dynamic routing: Routing information is automatically updated by
routers when changes are made to the network configuration. It is convenient,
as it does not involve human intervention in case of changes to the network
configuration. Its disadvantage, however, is that the overhead required to send
configuration change information can be a heavy burden.
Local
Routing
If a
packet sent from the source host is addressed to a host in the same subnet, the
IP layer of the source host must obtain the MAC address of the destination
host.
If a
packet is addressed to a host outside the subnet of the originating host, the
IP layer must search for a router to relay the packet and obtain the MAC
address of that router to send the packet. Therefore, processing required for
transferring a packet from the source host to the destination host or to the
relaying router is called local routing.
When host A communicates’ with host B over the same LAN,
addresses are specified as shown below:
Originating host IP address = A (ip), Originating host
MAC address = A (mac)
Originating host IP address = B (ip), Originating host
MAC address = B (mac)
On the other hand, when host A needs to communicate with
the host C via router R, communication is. achieved in 2 stages, first from
host A to router R, and then router R to host C.
Host A Router R
Source host IP address = A (ip), Source host MAC address
= A (mac)
Destination host IP address = C (ip), Destination host
MAC address = R (mac)
Router A Host C
Source host IP address = A (ip), Source host MAC address
= R (mac)
Destination host IP address = C (ip), Destination host
MAC address = C (mac)
It is important to note that MAC addresses change while
IP addresses remain the same.
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