Sealink

Sealink was a ferry company based in the United Kingdom from 1970 to 1984, operating services to France, Belgium, the Netherlands, Channel Islands, Isle of Wight and Ireland.

Ports served by the company included: Dover, Folkestone, Newhaven, Southampton and Harwich for services to the European continent; Holyhead, Fishguard, Heysham and Stranraer for services to Ireland and the Isle of Man; services to the Channel Islands from Weymouth and Portsmouth. The Isle of Wight was also served from Portsmouth and Lymington.

Sealink also operated the Steamer Passenger Ferry services on Windermere in Cumbria until privatisation when these were passed to the newly reformed Windermere Iron Steamboat Company (now Windermere Lake Cruises Ltd).

History

Sealink was originally the brand name for the ferry services of British Rail which ran shipping services in the United Kingdom and Ireland. Services to France, Belgium and the Netherlands were also run by Sealink UK as part of the Sealink consortium which also used ferries owned by French national railways (SNCF), the Belgian Maritime Transport Authority, Regie voor Maritiem Transport/Regie des transports maritimes (RMT/RTM) and the Dutch Stoomvaart Maatschappij Zeeland (Zeeland Steamship Company).

SEAlink

SEAlink is a file transfer protocol that is backward compatible with XMODEM but features a sliding window system for improved throughput. SEAlink was written in 1986 as a part of the SEAdog FidoNet mailer written by System Enhancement Associates, creators of the famous ARC program. It was licensed with a simple "give credit" requirement, but nevertheless was not very widely used except in FidoNet mailers. SEAlink, and most other XMODEM enhancements, were quickly displaced following the introduction of ZMODEM.

Description

XMODEM breaks down the file to be sent into a numbered series of packets of 128 bytes each. Packets are sent one at a time to the receiver, who either acknowledges the proper reception by sending an ACK character, or requests it to be resent by sending a NAK instead. The transfer then continues with another packet.

Although the time needed to send the ACK or NAK is almost zero, the propagation delay of the phone system itself introduces a latency. A latency of 0.1 seconds is not at all uncommon, and can be much longer on overseas links. That means that the maximum transfer rate of XMODEM is a function of the phone system more than the speed of the modems themselves. At 300 bit/s this is not a serious concern, as the packet will take about 3.5 seconds to send, the 0.1 delay being insignificant. However, at 9600 bit/s the packet only takes about 0.1 seconds to send, meaning that at least half of the potential throughput is lost while the sender waits for the ACK or NAK to arrive.