The Cyclades computer network is made up of three layers: Data Transmission
Layer, Transport Layer, and an Application Layer. The developers of Cyclades gave network design the highest
priority. The principle of a layered architecture and the independence of each
layer was the foundation on which they built the network. This layering makes
the Cyclades network very similar to today's networks (although today’s
networks have many more layers).
Figure:
Cyclades Functional Layout [Pouzin
1982]
Catenet - An abstract packet switching
network (PSN) resulting from the juxtaposition of several PSNs.
Concentrator - Specialized version of a host on which only
relevant parts of the network protocols have been implemented.
Host - any computer connected to a
network that resides some distance away from a router. Multiple routers can
connect to a host, allowing traffic distribution across multiple lines.
Liason - term used to describe a connection between two ports.
Node - In Cyclades documents, the term "node" is equivalent to the
present-day term: "router".
Packet Switching Network (PSN) - A set of communications
computers (nodes) connected via physical circuits that carry bit strings using a
store and forward technique.
ST - abbreviation for "transfer station", which is
the host software that host computers communicate with.
The Data Transmission Layer is the bottom-most layer. In
Cyclades, this
layer refers to Cigale, a packet-switching network. Cigale is a simple network
that provides its users with a datagram service whose primary goal is to
forward packets. Cyclades engineers designed the architecture with the
intention of interoperability with any packet switching network, such as
ARPANET.
In the context of another network, Cigale itself is a router. The foresight of network evolution and the recursive
definition of networks inspired the very simple and basic design of the data
transmission layer. Cyclades demonstrates "complexity at the networks
edge", meaning that the work takes
place within intelligent end-systems (the hosts). This strategy is in sharp
contrast with the Arpanet. With Cyclades, routing a message is a distributed
activity, and routing to the local user occurs within the end-network. Addressing and routing procedures
treat interconnected networks as simply routers of a super-network.
The common agreements required of all networks in order to interconnect are the following: an addressing plan to
designate networks, basic header format, maximum packet size, a plain packet
delivery service, and a set of accounting practices.
The transport layer is the middle layer of the overall Cyclades
architecture. It is the interprocess communication facility.
The transport layer resides on top
of the data transmission boxes (Cigale), and consists of transport entities called Transport Stations. Transport
stations are pieces of software running on the hosts to provide transport
services to the higher layers through transport accesses. In order to provide
the transport service, the transport stations cooperate according to a transport
protocol. A transport protocol is a set of rules which define procedures and
message formats for exchanging information between two transport stations.
Ports are defined as a common name space for addressable entities such as
resources, user processes, and terminals. Additionally, they provide a means to exchange messages and
establish connections for private conversations between two ports, called a
liaison.
The Virtual Device Control Layer focuses on terminal access to the network. The need for this layer
stems from the fact that the construction of terminals at that time did not allow for operation through a network. Instead they interacted very intimately with
a computer through a physical line. This layer hides the heterogeneity of
terminal devices by translating specific terminals to a virtual terminal
specification (the virtual terminal protocol).
