Exercise 11.3 asked the reader to write a series of single statements. Actually, these statements form the core of an important type of file-processing program, namely, a file matching program. In commercial data processing, it’s common to have several files in each system.

In an accounts receivable system, for example, there is generally a master file containing detailed information about each customer such as the customer’s name, address, telephone number, outstanding balance, credit limit, discount terms, contract arrangements and possibly a condensed history of recent purchases and cash payments.

As transactions occur (i.e., sales are made and cash payments arrive in the mail), they are entered into a file. At the end of each business period (i.e., a month for some companies, a week for others and a day in some cases) the file of transactions (called "trans.dat" in Exercise 11.3) is applied to the master file (called "oldmast.dat" in Exercise 11.3), thus updating each account's record of purchases and payments. After each of these updatings run, the master file is rewritten as a new file ("newmast.dat"), which is then used at the end of the next business period to begin the updating process again.

File-matching programs must deal with certain problems that do not exist in single-file programs. For example, a match does not always occur. A customer on the master file might not have made any purchases or cash payments in the current business period, and therefore no record for this customer will appear on the transaction file. Similarly, a customer who did make some purchases or cash payments might have just moved to this community, and the company may not have

had a chance to create a master record for this customer.

Use the statements written in Exercise 11.3 as the basis for a complete file-matching accounts receivable program. Use the account number on each file as the record key for matching purposes. Assume that each file is a sequential file with records stored in increasing account number order. When a match occurs (i.e., records with the same account number appear on both the master file and the transaction file), add the dollar amount on the transaction file to the current balance on the master file and write the "newmast.dat" record. (Assume that purchases are indicated by positive amounts on the transaction file, and that payments are indicated by negative amounts.)

Suppose that between a sending host and receiving host there are three packet routers. The transmission rates between the sending host and the router, between routers and between the third router and the receiving host are TR1, TR2, TR3 and TR4, respectively. The links between the sending host and router R1; and router R3 and the receiving host are very slow. Other links are very fast. Assuming that the routers use store-and-forward packet switching, the links are congested (there are queuing delays) what is the total end-to-end delay to send a packet of length L?