Rete FDDI

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CSITE received in 1991 a special funding (750.000.000 italian lire) in the framework of the EUREKA research project PROMETHEUS. The special funding, which has been requested by Prof.Andrisano, has been employed to install an FDDI network which will connect several DIGITAL work stations located in the different buildings of the Department.

Network initial structure

The structure in which the actual network has been mounted was a monolithic Ethernet backbone, going through the old Faculty building and the DEIS main building and connected to the Computing Centre. The output towards the outer world was, and still is, granted by a dedicated link, with a line at 2 Mbit/s, to the CINECA.

The new data network connects the Faculty buildings using a unique high speed backbone (100 Mbit/s). The used solution foresees the riconfiguration in three isles located in the three buildings and the connections towards the outer world.

By standard, the Ethernet network consists of a bus topology, with a speed of 10Mbit/s and a collision detection access algorithm. This system has the advantage of keeping installation, management and new clients insertion at low cost, but is limited by a drop in performance when the the number of accesses increases, and by the difficult location of failures. This characteristic gave various problems within the Faculty, with the result of a temporary block of the whole network in case of failures.
The three buildings are connected using three bridges for protocol and transmission speed (100 Mbit/s - 10 MBit/s) conversion, therefore they are indipendent. In the FDDI net, data is trasmitted with a speed ten times higher. In this way, an high quality client/server structure has been realized.

Updates and division in isles

The increases of the number of clients on the net, of the physic dimension of the net, of the number of services offered and required, and the needing of connecting the new building (Ex-scuderie) required a global system revision. The direction has been to create separated isles, all connected with an high speed and highly reliable net. In this way possible problems, which could previously block the entire backbone, are now limited to an isle, making it easier to locate them. From the net load point of view the isles are separated and therefore the local traffic does not slow down the entire backbone; furthermore the traffic between two distinct isles does not involve all the others. Every isle consists of Ethernet bone, to assure a complete compatibility with the prevoius cables and interfaces. The connection between the isles is ensured by a double counter- rotating ring, realized in optical fiber and managed with the FDDI protocol. The real speed of this network is standardized at 100 Mbit/s, the access protocol is token passing and the communication is usually on one ring. The interfaces are sufficently intelligent to detect failures an in such a case they automatically use the second ring, excluding the part where the failure occured. This mechanism is able to exclude more than one system, even creating disconnected subrings The token passing access assures a finite time for packet delivery, avoiding the traffic problems that come with collision detection systems, hence it is well suited for a strategic and reliable connection between different areas. The link between Ethernet and FDDI is accomplished by three bridges, one each isle, which execute protocol translation, local packet filtering and physic quantities conversion (electrical towards optical). The filtering is necessary to mantain local packet inside an isle and to avoid unusable packets to reach the isle. This is done at level 2 (ISO-OSI), where the peripheral address is known but there are no routing capabilities. It is also possible to connect directly to the FDDI ring a station with an optical interface. This link can be accomplished either by a double connection (DAS) or by a single one (SAS). The former interfaces directly to the double ring whereas single connection devices form a subring interfaced to the double ring by a device called concentrator.

New network structure

The order to Digital was 8 workstations with optical interface with a single attack, 3 bridges and 3 optical concentrators. Other two Ethernet-linked diskless station have been delivered. Once that the three isles (Ex-Scuderie building, DEIS main building and Faculty old building - Telcommunications) have been selected, these have been connected using the three bridges; it is however possible an increase in isle number to decongest the traffic within an isle. It is to be noted that the double ring is only a virtual ring; actually it is a star topology easily reconfigurable. The star centre is in the CCIB (Centro di calcolo della Facolta' - Faculty Computing Centre) which stands in a baricentric position.

For each building the optical termination has been inserted into racks inside which the communication ring passes, on attestation panels and optical fiber permutation in the following way:

Rack A (CCIB area):

is the central node in which the three FDDI branches merge. His function is entirely passive since it only has to mantain the physical continuity of the branches for the logic realization of the ring using permutation connection link. Furthermore it contains a FDDI 10/100 bridge which interfaces the entire FDDI network to the old Faculty Ethernet backbone.

Rack B (old TLC building):

consists of an FDDI concentrator which gives four connections, two of which are occupied by SAS stations (called promet2 e promet3) and the third is used by a 10/100 bridge for the local Ethernet isle interface.

Rack C (area CIOC):

consists of a concentrator with three ports occupied by SAS stations (promet5, promet6, promet7) and one used by a 10/100 bridge which interfaces the local Ethernet network through a DELNI device (DEC Ethernet Local Network Interconnect: a tranceiver for network connection of eight units) and a DEMPR (DEC MultiPort repeater: Thin-Wire Ethernet multiport repeater for the connection to a baseband cable) which distributes the internal network through eight Thin-Wire cable branches on two floors of the building.

Rack D (area DEIS):

consists of an FDDI concentrator which connects two SAS stations (promet1, promet4) and the 10/100 bridge which is physically located in the Armadio A in the CCIB.

The previous picture shows the racks and a detailed view of the thin-wire Ethernet network connected to Armadio C, whereas the following picture shows a floorplan of the network within the Faculty. The previous picture shows the racks and a detailed view of the thin-wire Ethernet network connected to Armadio C, whereas the following picture shows a floorplan of the network within the Faculty.

Future developments

The structure is modular and easily expandible. The present and future traffic is to be moved to the FDDI backbone, to optimze the area functions. The system expansions are to be thougth in function of the traffic and connection requirements. It is believed that the Ethernet lines should be kept, considering their low cost and high diffusion.

Division in IP subnets

This term is intended to distinguish logically (but not physically) different networks. Actually there are 5 subnets, with the following IP:

  • 137.204.57
  • 137.204.56
  • 137.204.59
  • 137.204.58
  • 137.204.212

The 137.204 address identifies the ALMANET network. The promet network essentially consists of station linked in optical fibers, but there are also two Ethernet-connected diskless stations. To go through different networks a commutation is required, and it is realized by a router able to operate at level 3 (ISO-OSI). This function is realized by CISCO of CINECA, even if this solution can be really time consuming since it involves various ALMANET circuits for essentially internal communications. It is reccomended to define internal routers following ALMANET policy or make each station a router for itself for the various subnets.

 

Pontecchio site

In the Pontecchio site an Ethernet backbone for the local connection of machines and printers has been installed.The previous picture shows the connection system between the Pontecchio LAN and the outer world. The only link, before the reconfiguration, was granted by a dedicated line (ISPT) towards CINECA at 9600 bit/s which, through a statistic concentrator, gives availability of three lines at 4800 bit/s and two lines at 2400 bit/s for a simple terminal emulation connection. A direct link to the Faculty was found to be necessary and was achieved through the activation of a Numeric Direct Circuit( Circuito Diretto Numerico - CDN), initially offered from SIP, and successively granted by ALMANET.

The link using CDN

CDN are connections for data trasmission, at medium and high speed, used to connect two or more sites. Their principal characteristic is to use exclusively numeric transmission media. With this service dedicated links are possible, used exclusively for 24 hours a day or used partially at definite times, for the trasmission of data between treminals in a point-to-point configuration to directly connect two sites or multipoint to connect a central sites with various peripheral sites. The rent of a CDN at 19,2 Kbit/s, has a cost lower than 15 millions/year; with a CDN at 64Kbit/s, the cost would be 23 millions/year. After having taken a series of measures of the connection via the CDN, the conclusion has been that it appears a valid solution for elementary network services, frequently used inside the department. During the sperimentation, the connection has been mainly used for electronic mail, Internet sites connection and file transfer.