Typically,　these　are　concern　with　the　establishment　of(66)the　network　and　with　the　control　　of　the　flow　of　messages　across　this　channel.　The　provision　of　such　facilities　is　only　part　of　the　network　requirements,　however,　since　in　many　applications　the　communicating　computers　may　be　of　different　types.　This　means　that　they　may　use　different　programming　languages　and,　more　importantly,　　different　forms　of(67)interface　between　user(application)program,　normally　referred　to,　　as　application　processes　or　APs,　and　the　underlying　communication　services　may　be(68)For　example,　one　computer　may　be　a　small　single　-user　computer,　while　another　may　be　a　large(69)system.　In　the　earlier　days　of　computer　communication,　these　issues　meantthat　　only　closed　communities　of　computers(　that　is,　from　the　same　manufacturer)could　communicate　　with　each　other　in　a　meaningful　way.　IBM’s　Systems　Network.

(76)

(74)

(73)

(75)

(71)

One　of　the　functions　of　the(71)is　to　store　the　applications　programs　{　or　the　various　secondaries.　　The(72)may　have　similar　or　dissimilar　functions.　The　same　control　code　received　from　two　different,　secondaries　may　warrant　two　different　actions　on　the　part　of　the　host　computer.　If　the　system　so　requires,　the　host　computer　must　also　provide(73)for　the　centralized　data　and　the　software　for　database　management.　Data　information　is　normally　stored　in　auxiliary　memories　to　which　the　computer　has　ready　access.　The(74)peripherals　connected　to　the　host　computer　allow　for　human　entry　and　intervention.　As　needs　change,　application　programs　may　require　additions,　　deletions,　or　changes.　Data　link　problems　also　arise　which　are(75)the　range　of　the　computer's　programmed　capabilities.　In　such　cases,　human　entry　is　required　to　make　the　necessary　changes　to　resolve　the　existing　problems.

(70)

(68)

(69)

(66)