(44)使得一系列不同的操作具有相同的名字，用户可以发送一个通用的消息，面实现的细节则由接收对象自行决定。

(78)

(77)

(76)

(75)

In　the　open　systems　interconnection (OSI) reference　model,　"layer"　means　one　of　seven　conceptually　complete,(71) arranged　groups　of　services,　functions,　and　protocols,　that　extend　across　all　open　systems.Application　layer　provides　means　for　the　application(72) to　access　the　OSI　environment.　Presentation　layer　provides　for　the　selection　of　a　common　syntax　for　representing　data.(73) layer　provides　the　means　necessary　for　cooperating　presentation　entities　to　organize　and　synchronize　their　dialog　and　to　manage　their　data　exchange.　Transport　layer　provides　a　reliable　end-to-end　data　transfer　service.　Network　layer　provides　for　the　entities　in　the　transport　layer　the　means　for　transferring　blocks　of　data,　by(74) and　switching　through　the　network　between　the　open　systems　in　which　those　entities　reside.　Data　link　layer　provides　services　to　transfer　data　between　network　layer　entities.　Physical　layer　provides　the　mechanical,　electrical,　functional,　and　procedural　means　to　establish,　maintain　and(75) physical　connections　for　transfer　of　bits　over　a　transmission　medium.

(73)

(71)

(72)

Digital　computer　is　controlled　by　internally　stored　programs　and(66) of　using　common　storage　for　all　or　part　of　a　program　and　also　for　all　or　part　of　the　data　necessary　for　the　execution　of　the　programs;　executing　user-written　or　user-designated　programs;　performing　user-designated　manipulation　of　digitally　represented　discrete　data,　including.(67) operations　and　logic　operations;　and　executing　programs　that(68) themselves　during　their　execution.Analog　computer's　operations　are　analogous　to　the(69)　of　another　system　and　that　accepts,　processes,　and　produces　analog　data.(70) computer　integrates　analog　computer　components　and　digital　computer　components　by　interconnection　of　digital-to-analog　converters　and　analog-to-digital　converters.

(70)