ENTERPRISE NETWORKS II: WIDE AREA NETWORKS
transmitting the ASCII code and the companion receiver only able to receive EBCDIC.
Such compatibility MAY extend through all seven OSI layers. For example, frame relay
is based ONLY on OSI layers 1 and 2. It is the responsibility of the frame relay user to
provide the necessary compatibility of the upper OSI layers.
THE CONCEPT OF PACKET DATA COMMUNICATIONS
The concept of a packet-switched network is based on the idea that the network switching
nodes will have multiple choices for routing of data packets. If a particular route becomes
congested or has degraded operation, a node can send a packet on another route, and if
that route becomes congested, possibly a third route will be available to forward the
packet to its destination.
At a data source, a file is segmented into comparatively short data packets, each of the
same length and each with its own header and trailer. As we mentioned, these packets
may take diverse routes through various nodes to their destination. The destination node
is responsible for data message reassembly in its proper order.
This is a short description of an idealized packet-switched network. We will run into
some terminology typical of packet handling and switching of data. For example:
A. There is connection-oriented and connectionless service.
B. Packets may be acknowledged or unacknowledged.
C. We can have PVCs or SVCs (permanent virtual circuits or switched virtual circuits).
Our POTS is a typical connection-oriented service. In other words a fixed or virtual
connection is set up at the beginning of a telephone call, is maintained in that condition
throughout the call, and is taken down (terminated) at the end of the call. The circuit is then
returned to the pool of idle circuits awaiting the next call assignment. Connection-oriented
is sometimes called circuit-switched service.
In ideal packet-switched data service the service is connectionless. The packet is
released by the originator and placed on an idle or "free" outgoing data link to the
next node. Each node ideally maintains a dynamically updated routing table. The rout-
ing table will provide information on the appropriate outgoing port to direct the packet
toward its destination and will reflect information regarding congestion and/or degrada-
tion or outage on all possible outgoing routes. In the real world this dynamic updating is
seldom achievable because we will tend to load circuits up with service messages rather
than revenue-bearing traffic.
A more common method is the virtual connection where a logical connection is set up
in advance before any packets are sent. The packet originator sends a call request to its
serving node, which sets up a route in advance to the desired destination. All packets of a
particular message traverse this route, and each packet of the message contains a virtual
circuit identifier (logical channel number) along with the packet data. At any given time
each station can have more than one virtual circuit to any other station in the network.
With virtual circuits, routing decisions are made in advance.
Another routing method is called the datagram. Datagram service used optimal routing
on a packet-by-packet basis, usually over diverse routes. It is that ideal packet switching
service described above. With the datagram approach ad hoc decisions are made for
each packet at each node. There is no call-setup phase with datagrams as there is with
virtual connections. Virtual connections are advantageous for high community-of-interest