‫ﺑﻪ ﻧﺎم ﭘﺮوردﮔﺎر ﻣﻬﺮﺑﺎن‪ -‬ﺗﻤﺮﻳﻨﺎت ﺳﺮي اول ﺷﺒﻜﻪ ﻫﺎي ﻛﺎﻣﭙﻴﻮﺗﺮي‬
‫داﻧﺸﮕﺎه ﺻﻨﻌﺘﻲ ﺷﺮﻳﻒ – ﻣﺪرس اﺣﺴﺎن ﻣﻠﻜﻴﺎن – ﻧﻴﻤﺴﺎل اول ﺳﺎل ﺗﺤﺼﻴﻠﻲ ‪87-86‬‬
‫ﺑﺎ ﺗﻮﺟﻪ ﺑﻪ آﻧﻜﻪ ﻓﻬﻢ دﻗﻴﻖ واژهﻫﺎ و ﺗﻌﺎرﻳﻒ ﻓﺼﻞ اول‪ ،‬زﻳﺮﺑﻨﺎي ﻳﺎدﮔﻴﺮي ﻓﺼﻮل ﺑﻌﺪي اﺳﺖ ﻟﺬا ﺗﻮﺻﻴﻪ ﻣﻲﺷﻮد در ﭘﺎﺳﺨﮕﻮﻳﻲ ﺑﻪ ﺳﻮاﻻت دﻗﺖ و‬
‫وﺳﻮاس ﺑﻪ ﺧﺮج ﺑﺪﻫﻴﺪ و از ﺟﻮابﻫﺎي ﻛﻠﻴﺸﻪاي ﺑﭙﺮﻫﻴﺰﻳﺪ‪ .‬دﻗﺖ ﻛﻨﻴﺪ ﻛﻪ ﺗﺤﻮﻳﻞ اﻳﻦ ﺳﺮي از ﺗﻤﺮﻳﻦ از ﺗﻤـﺮﻳﻦﻫـﺎي ﻓـﺼﻮل آﺗـﻲ از اﻫﻤﻴـﺖ‬
‫ﺑﻴﺸﺘﺮي ﺑﺮﺧﻮردار اﺳﺖ و ﭘﺎﺳﺦﻫﺎي ﺷﻤﺎ ﺑﺎ دﻗﺖ و رﻳﺰﺑﻴﻨﻲ ﺑﺮرﺳﻲ ﺧﻮاﻫﻨﺪ ﺷﺪ‪.‬‬
‫ﭘﺮﺳﺶ ‪ (1‬ﻣﺴﺘﻘﻞ ﺑﻮدن ﻛﺎﻣﭙﻴﻮﺗﺮﻫﺎ در ﻳﻚ ﺷﺒﻜﻪ ﻛﺎﻣﭙﻴﻮﺗﺮي ﺑﻪ ﭼﻪ ﻣﻌﻨﺎﺳﺖ و از ﭼﻪ دﻳﺪﮔﺎﻫﻲ اﻫﻤﻴﺖ ﻣﻲﻳﺎﺑﺪ؟‬
‫ب( ‪ Cell‬ﭼﻴﺴﺖ و ﺑﺎ ‪ packet‬ﭼﻪ ﺗﻔﺎوﺗﻲ دارد و در ﻛﺪام ﺑﺨﺶ از ﻣﻌﻤﺎري ﻻﻳﻪاي ﺷﺒﻜﻪ ﺗﻌﺮﻳﻒ ﻣﻲﺷﻮد و ﭼﻪ ﺗﺎﺛﻴﺮ ﺑﺮ ﻣﻌﻤﺎري آن ﻻﻳﻪ‬
‫ﺧﻮاﻫﺪ ﮔﺬاﺷﺖ؟‬
‫ج( ‪ Subnet‬در ‪ WAN‬ﭼﻴﺴﺖ و ﭼﻪ ﻋﻨﺎﺻﺮي دارد؟‬
‫ﭘﺮﺳﺶ ‪ (2‬ﻫﺮ ﻳﻚ از ﻋﺒﺎرات زﻳﺮ ﺧﺼﻴﺼﻪ ﺑﻨﻴﺎدي ﻛﺪام ﺗﻮﭘﻮﻟﻮژي ‪ LAN‬اﺳﺖ؟‬
‫‪-‬‬
‫ارﺗﺒﺎط ﺗﻤﺎﻣﻲ ﻣﺎﺷﻴﻨﻬﺎ از ﻃﺮﻳﻖ ﻳﻚ ﮔﺮه ﻣﺮﻛﺰي ﺑﺮﻗﺮار ﻣﻲ ﺷﻮد‪.‬‬
‫‪-‬‬
‫ارﺗﺒﺎط دو ﮔﺮه از ﻃﺮﻳﻖ ﻋﻤﻴﻘﺘﺮﻳﻦ ﺟﺪ ﻣﺸﺘﺮك آﻧﻬﺎ اﻧﺠﺎم ﻣﻲ ﺷﻮد‪.‬‬
‫‪-‬‬
‫ﻫﺮ ﺑﻴﺖ از اﻃﻼﻋﺎت ﺑﻪ ﺻﻮرت ﻣﺴﺘﻘﻞ از ﺳﺎﻳﺮ ﺑﻴﺘﻬﺎي ﺑﺴﺘﻪ ﺣﺮﻛﺖ ﻣﻲ ﻛﻨﺪ‪.‬‬
‫‪-‬‬
‫ﺗﻤﺎم ﻣﺎﺷﻴﻨﻬﺎ از ﻃﺮﻳﻖ ﻳﻚ ﻛﺎﻧﺎل ﻓﻴﺰﻳﻜﻲ ﻣﺸﺘﺮك ﺑﺎ ﻫﻢ ﺗﺒﺎدل ﭘﻴﺎم ﻣﻲ ﻛﻨﻨﺪ‪.‬‬
‫‪-‬‬
‫از ﺑﻪ ﻫﻢ ﺑﺴﺘﻦ ﭼﻨﺪ ﺷﺒﻜﻪ ﺑﺎ ﺗﻮﭘﻮﻟﻮژي ﺳﺘﺎره ﺑﻪ ﻫﻢ ﭘﺪﻳﺪ ﻣﻲ آﻳﺪ‪.‬‬
‫‪-‬‬
‫‪ Host‬ﻫﺎي ﻣﻴﺎﻧﻲ‪ ،‬ﺑﻴﺘﻬﺎي داده را درﻳﺎﻓﺖ و در ﺧﺮوﺟﻲ ﺧﻮد ﺗﻜﺮار ﻣﻲ ﻛﻨﻨﺪ‬
‫‪-‬‬
‫ﺑﻴﻦ ﻫﺮ دو ﻣﺎﺷﻴﻦ ﺷﺒﻜﻪ ﻳﻚ ارﺗﺒﺎط ﻣﺴﺘﻘﻴﻢ و اﺧﺘﺼﺎﺻﻲ وﺟﻮد دارد‪.‬‬
‫ﭘﺮﺳﺶ ‪ (3‬ﺣﺎﻛﻤﺎن ﭼﻨﺪ ﺷﻬﺮ و ﻣﺸﺎوران ﻧﻈﺎﻣﻲ آﻧﻬﺎ ﻗﺼﺪ دارﻧﺪ ﺑﺎ ﻳﻜﺪﻳﮕﺮ ﺗﺒﺎدل ﭘﻴﺎم ﻧﻤﺎﻳﻨﺪ ﺣﺎﻛﻢ ﻳﻚ ﺷﻬﺮ ﺗﻨﻬﺎ ﺑﺎ ﺣﺎﻛﻢ ﻳﻚ ﺷﻬﺮ دﻳﮕﺮ و‬
‫ﻣﺸﺎور ﻧﻈﺎﻣﻲ ﻳﻚ ﺷﻬﺮ ﺗﻨﻬﺎ ﺑﺎ ﻣﺸﺎور ﻧﻈﺎﻣﻲ ﻳﻚ ﺷﻬﺮ دﻳﮕﺮ ﻣﺒﺎدﻟﻪ ﭘﻴﺎم ﻣﻲ ﻛﻨﻨﺪ‪) .‬ﻳﻌﻨﻲ ﻫﻴﭽﮕﺎه ﻳﻚ ﺣﺎﻛﻢ ﻳﻚ ﺷﻬﺮ و ﻣﺸﺎور ﻧﻈﺎﻣﻲ ﻳﻚ ﺷﻬﺮ‬
‫دﻳﮕﺮ ﺑﺎ ﻳﻜﺪﻳﮕﺮ ﺻﺤﺒﺖ ﻧﻤﻲ ﻛﻨﻨﺪ‪ (.‬ﺣﺎﻛﻢ ﻳﺎ ﻣﺸﺎور ﭘﻴﺎم ﺧﻮد را ﺑﻪ رﻣﺰﻧﮕﺎر ﻗﺼﺮ ﺗﺤﻮﻳﻞ ﻣﻲ دﻫﻨﺪ‪ .‬او ﭘﻴﺎم را رﻣﺰ ﻣﻲ ﻛﻨﺪ و ﺑﻪ ﻣﺴﺆل اﻧﺘﻘﺎل‬
‫ﻣﻴﺪﻫﺪ‪ .‬ﺳﻴﺴﺘﻢ اﻧﺘﻘﺎل ﺗﺸﻜﻴﻞ ﺷﺪه اﺳﺖ از ﺗﻌﺪادي اﻳﺴﺘﮕﺎه در ﻗﺼﺮﻫﺎ و ﺗﻌﺪادي اﻳﺴﺘﮕﺎه ﻣﻴﺎن راﻫﻲ ﻛﻪ ﭘﻴﺎم ﺑﺮاي رﺳﻴﺪن ﺑﻪ ﻣﻘﺼﺪ از ﻳﻚ ﻳﺎ‬
‫ﭼﻨﺪ ﺗﺎ از آﻧﻬﺎ ﻣﻲ ﮔﺬرد‪ .‬ﺑﺮاي اﻧﺘﻘﺎل ﭘﻴﺎم از ﻳﻚ اﻳﺴﺘﮕﺎه ﺑﻪ اﻳﺴﺘﮕﺎه دﻳﮕﺮ ﻣﻲ ﺗﻮان از اﺳﺐ ﻳﺎ ﻛﺒﻮﺗﺮ اﺳﺘﻔﺎده ﻛﺮد‪.‬‬
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‫ﺗﻮﺿﻴﺢ دﻫﻴﺪ ﻛﻪ در اﻳﻦ ﺗﻮﺻﻴﻒ )‪(specification‬ﭼﻪ ﻻﻳﻪ ﻫﺎﻳﻲ وﺟﻮد دارد‪.‬‬
‫‪-‬‬
‫ﺗﻮﺿﻴﺢ دﻫﻴﺪ ﻛﻪ در اﻳﻦ اﻳﻦ ﺗﻮﺻﻴﻒ )‪(specification‬ﻛﺪاﻣﻴﻚ از ﻣﻮارد زﻳﺮ ذﻛﺮ ﺷﺪه اﺳﺖ و ﻛﺪاﻣﻴﻚ ذﻛﺮ ﻧﺸﺪه اﺳﺖ‪:‬‬
‫‪o‬‬
‫ﺳﺮوﻳﺲ ﻫﺎ‬
‫‪o‬‬
‫‪ Service Primitive‬ﻫﺎ‬
‫‪o‬‬
‫‪ Protocol‬ﻫﺎ‬
‫‪o‬‬
‫‪ Interface‬ﻫﺎ‬
‫‪-‬‬
‫ﺗﻮﺿﻴﺢ دﻫﻴﺪ ﻫﺮ ﻻﻳﻪ اﻳﻦ ﻣﺪل ﺑﺎ ﻛﺪام ﻻﻳﻪ ﻣﺪل ‪ OSI‬ﺗﻨﺎﻇﺮ دارد‪.‬‬
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‫آﻳﺎ اﻧﺘﻘﺎل ﭘﻴﺎم ﺑﻴﻦ اﻳﺴﺘﮕﺎﻫﻬﺎي ارﺗﺒﺎﻃﻲ در اﻳﻦ ﻣﺪل ‪ connectionless‬اﺳﺖ ﻳﺎ ‪ .connection-oriented‬ﭼﺮا؟‬
‫‪-‬‬
‫)اﺧﺘﻴﺎري( ‪ SAP address‬ﭼﻴﺴﺖ؟ ﭼﻪ ارﺗﺒﺎﻃﻲ ﺑﺎ ﻣﻔﻬﻮم ‪ demultiplexing‬دارد و در ﻛﺠﺎي اﻳﻦ ﻣﺪل اﺳﺘﻔﺎده ﻣﻲ‬
‫ﺷﻮد؟‬
‫ﭘﺮﺳﺶ ‪ (3‬اﻟﻒ( اﮔﺮ ﺳﺮوﻳﺲ اراﻳﻪ ﺷﺪه در ﻻﻳﻪ ‪K‬ام ‪ connection-oriented‬ﺑﺎﺷﺪ درﺑﺎره ‪ connectionless‬ﻳﺎ‬
‫‪ connection-oriented‬ﺳﺮوﻳﺲ اراﻳﻪ ﺷﺪه در ﻻﻳﻪ ‪ K+1‬ام ﭼﻪ ﻣﻲ ﺗﻮان ﮔﻔﺖ ؟‬
‫ب( اﮔﺮ ﺳﺮوﻳﺲ اراﻳﻪ ﺷﺪه در ﻻﻳﻪ ‪K‬ام ‪ reliable‬ﺑﺎﺷﺪ درﺑﺎره ‪ reliability‬ﺳﺮوﻳﺲ اراﻳﻪ ﺷﺪه در ﻻﻳﻪ ‪ K+1‬ام ﭼﻪ ﻣﻲ ﺗﻮان ﮔﻔﺖ ؟‬
‫ج( اﮔﺮ ﺳﺮوﻳﺲ اراﻳﻪ ﺷﺪه در ﻻﻳﻪ ‪K‬ام ‪ unreliable‬ﺑﺎﺷﺪ ﭼﮕﻮﻧﻪ ﻣﻲ ﺗﻮان در ﻻﻳﻪ ‪ K+1‬ام ﻳﻚ ﺳﺮوﻳﺲ ‪ reliable‬داﺷﺖ ؟‬
‫ﭘﺮﺳﺶ ‪ (4‬ﻣﻔﺎﻫﻴﻢ ﻣﻮﺟﻮد در ﺳﺘﻮن ‪ A‬را ﺑﺎ ﻣﻔﺎﻫﻴﻢ ﻣﺮﺗﺒﻂ ﺑﺎ آﻧﻬﺎ در ﺳﺘﻮن ‪ B‬وﺻﻞ ﻛﻨﻴﺪ‪.‬‬
‫‪A‬‬
‫‪B‬‬
‫‪Connectionless‬‬
‫‪Communication between peer entities‬‬
‫‪Connection-oriented‬‬
‫‪Out of order arrival‬‬
‫‪Reliability‬‬
‫‪Acknowledgement‬‬
‫‪Transport Layer‬‬
‫‪The set of all service primitives‬‬
‫‪Protocol‬‬
‫‪Negotiation‬‬
‫‪Interface‬‬
‫‪Segmentation‬‬
‫ﭘﺮﺳﺶ ‪ (5‬ﻣﺸﺨﺺ ﻛﻨﻴﺪ ﻫﺮ ﻛﺪام از اﻋﻤﺎل زﻳﺮ ﻏﺎﻟﺒﺎ و ﺑﻪ ﻃﻮر ﻣﻌﻤﻮل در ﻛﺪام ﻻﻳﻪ اﻧﺠﺎم ﻣﻲ ﺷﻮد؟‬
‫‪-‬‬
‫‪ Checkpoint‬ﻛﺮدن اﻧﺘﻘﺎﻟﻬﺎي ﺑﻠﻨﺪ ﺗﺎ در ﺻﻮرت ﻗﻄﻊ ﺑﺘﻮاﻧﻨﺪ ﻣﺠﺪدا ﭘﻴﮕﻴﺮي ﺷﻮﻧﺪ‬
‫‬‫‪-‬‬
‫‪Flow Control‬‬
‫ﺟﻠﻮﮔﻴﺮي از ازدﺣﺎم در ﻳﻚ ﻧﻘﻄﻪ از زﻳﺮ ﺷﺒﻜﻪ‬
‫‪-‬‬
‫ﺗﺒﺪﻳﻞ ﻛﺪﻫﺎي ﻣﺨﺘﻠﻒ‬
‫‪-‬‬
‫ﻣﺎژوﻻﺳﻴﻮن‬
‫‪-‬‬
‫ﺳﻨﻜﺮوﻧﻴﺰاﺳﻴﻮن‬
‫‬‫‪-‬‬
‫‪Segmentation & Reassembly‬‬
‫ﺗﺒﺪﻳﻞ ﺑﻴﺘﻬﺎ ﺑﻪ ﻳﻚ ﺳﻴﮕﻨﺎل ﻣﺘﻨﺎﺳﺐ ﺑﺎ ﻛﺎﻧﺎل اﻧﺘﻘﺎل‬
‫‪-‬‬
‫ﻣﺴﻴﺮﻳﺎﺑﻲ‬
‫‪-‬‬
‫رﻣﺰﻧﮕﺎري‪ /‬رﻣﺰﮔﺸﺎﻳﻲ‬
‫‪-‬‬
‫ﻛﺸﻒ و ﻛﻨﺘﺮل ﺧﻄﺎﻫﺎي ﻧﺎﺷﻲ از اﻧﺘﻘﺎل داده ﻫﺎ ﺑﺮ روي رﺳﺎﻧﻪ اﻧﺘﻘﺎل‬
‫ﭘﺮﺳﺶ ‪ (5‬اﻟﻒ( ﺑﺮاي ﻫﺮﻳﻚ از ﭘﺎراﻣﺘﺮﻫﺎي ‪ QoS‬ﻳﻚ ﺗﻌﺮﻳﻒ ‪ formal‬ارا ﻳﻪ دﻫﻴﺪ‪.‬‬
‫ب( ﭼﻬﺎر ﻓﺎﻛﺘﻮر ﻣﻬﻢ در ﺗﺎﺧﻴﺮ ﻳﻚ ﺑﺴﺘﻪ‬
‫‪ ،queuing delay ،propagation delay‬ﺗﻌﺪاد ‪ hop‬ﻫﺎي ﻣﺴﻴﺮ و‬
‫‪ switching latency‬ﻫﺴﺘﻨﺪ‪ .‬در ﻣﻮرد ﻫﺮ ﻳﻚ از ﻋﻮاﻣﻞ ﺑﺎﻻ ﺗﻮﺿﻴﺢ دﻫﻴﺪ‪.‬‬
‫ج( ﻣﺸﺨﺺ ﻛﻨﻴﺪ ﻛﻪ ﻫﺮ ﻳﻚ از ﺳﺮوﻳﺲ ﻫﺎي زﻳﺮ ﭼﻪ ﻧﻴﺎزﻣﻨﺪي ﻫﺎي ‪ QoS‬اي دارد‪:‬‬
‫‬‫‬‫‪-‬‬
‫‪File Transfer‬‬
‫‪Web Surfing‬‬
‫‪) Video/Audio On Demand‬ﮔﻮش دادن ﻣﻮﺳﻴﻘﻲ‪ /‬دﻳﺪن ‪(Video‬‬
‫‪-‬‬
‫‪Video Conferencing‬‬
‫ﭘﺮﺳﺶ ‪ (6‬ﻓﺮض ﻛﻨﻴﺪ ﻳﻚ ﺧﻂ ﻣﺴﻲ ‪ 100Mbps‬ﺑﻪ ﻃﻮل ‪ 100‬ﻣﺘﺮ دارﻳﻢ ﻛﻪ ﺗﻌﺪادي ‪ host‬از ﻃﺮﻳﻖ آن ﻣﺒﺎدﻟﻪ ﭘﻴﺎم ﻣﻲ ﻛﻨﻨﺪ‪.‬‬
‫•‬
‫اوﻻ ﻣﺤﺎﺳﺒﻪ ﻛﻨﻴﺪ ﻛﻪ ﻫﺮ ﺑﻴﺖ اﻃﻼﻋﺎت روي ﺳﻴﻢ ﭼﻨﺪ ﻣﺘﺮ ﻃﻮل دارد‪) .‬ﻗﺒﻞ از آن ﺗﺤﻘﻴﻖ ﻛﻨﻴﺪ ﻛﻪ ﻃﻮل ﻳﻚ ﺑﻴﺖ ﺑﺮ ﺣﺴﺐ ﻣﺘﺮ! ﭼﻪ‬
‫ﺗﻌﺒﻴﺮي دارد‪(.‬‬
‫•‬
‫ﺛﺎﻧﻴﺎ ﻣﺤﺎﺳﺒﻪ ﻛﻨﻴﺪ ﻛﻪ ﻫﻨﮕﺎﻣﻲ ﻛﻪ ﮔﻴﺮﻧﺪه در ﺣﺎل درﻳﺎﻓﺖ ﺑﻴﺖ اول ﻳﻚ ﭘﻴﺎم اﺳﺖ ﻓﺮﺳﺘﻨﺪه در ﺣﺎل ﻓﺮﺳﺘﺎدن ﺑﻴﺖ ﭼﻨﺪم آن اﺳﺖ؟‬
‫)ﻓﺮض ﻛﻨﻴﺪ ﻛﻪ دو ﮔﺮه ﻣﻮرد ﺑﺤﺚ در دو ﺳﺮ ﺳﻴﻢ واﻗﻊ ﺷﺪه ﺑﺎﺷﻨﺪ‪(.‬‬
‫)ﺑﺮاي اﺷﺨﺎﺻﻲ ﻛﻪ از ﻗﺒﻞ ﺑﺎ ﻣﻔﺎﻫﻴﻢ ﺷﺒﻜﻪ آﺷﻨﺎﻳﻲ دارﻧﺪ‪ :‬ﻓﺮض ﻛﻨﻴﺪ ﺗﻌﺪاد ﺑﻴﺖ ﻫﺎ در ﻫﺮ ‪ baud‬ﻳﻚ اﺳﺖ‪(.‬‬
‫ﭘﺮﺳﺶ ‪ (7‬ﻣﻮﺿﻮع اﻧﺸﺎء !!! )ﺑﻬﺘﺮﻳﻦ ﭘﺎﺳﺦﻫﺎ اﻣﺘﻴﺎز وﻳﮋه ﺧﻮاﻫﺪ داﺷﺖ و از ﻃﺮﻳﻖ وبﺳﺎﻳﺖ داﻧﺸﮕﺎه ﻣﻨﺘﺸﺮ ﻣﻲﺷﻮد‪(.‬‬
‫ﺷﺒﻜﺔ ﺟﻬﺎﻧﻲ اﻳﻨﺘﺮﻧﺖ ﭼﮕﻮﻧﻪ ﺑﺮ روي ﺳﻨﺖ و ﻓﺮﻫﻨﮓ‪ ،‬ﺟﻬﺎن ﺑﻴﻨﻲ و اﺧﻼق ﺗﺎﺛﻴﺮ )ﻣﺜﺒﺖ‪/‬ﻣﻨﻔﻲ( ﻣﻲﮔﺬارد و آﻳﺎ اﻳﻦ ﺗﺎﺛﻴﺮ‬
‫دو ﻃﺮﻓﻪ اﺳﺖ؟ ﻳﻌﻨﻲ ﺳﻨﺖ و ﻓﺮﻫﻨﮓ ﺟﻮاﻣﻊ ﺑﺮ روي اﻳﻨﺘﺮﻧﺖ ﺗﺎﺛﻴﺮي ﻣﻲﮔﺬارد؟ )ﻟﻄﻔﺎ ﻗﺒﻞ از ﺷﺮوع ﺑﺤﺚ ﻛﻤﻲ ﺗﺤﻘﻴﻖ‬
‫ﻛﻨﻴﺪ‪(Social Issues/Internet Ethics/… .‬‬
‫‪The following two questions are optional and aim at getting you a taste of‬‬
‫‪how computer networks course is taught at world’s top 10 universities:‬‬
‫‪1. University of Texas at Austin, Department of Computer Science, Computer‬‬
‫‪Networks Course, Fall 2007, Instructor: Zin Zhang, Homework 1, Problem 3‬‬
‫‪“Problem 3: In modern packet-switched networks, the source host segments long, application‬‬‫‪layer messages (for example, an image or a music file) into smaller packets and sends the‬‬
‫‪packets into the network. The receiver then reassembles the packets back into the original‬‬
message. We refer to this process as message segmentation. Segmenting a message into packets
has the advantage of pipelining, namely: as soon as a packet is completely received by a node,
the node can process and transmit the packet to the next node without waiting for the rest of the
message. This observation is to be illustrated by the following analysis.
Consider sending a message of 8 · 106 bits from Host A to Host B. There are four links (and
three switches) between A and B. Each link has a transmission rate of R = 1.6 Mbps. Disregard
queuing, propagation, and processing delays.
a. Consider sending the message from Host A to Host B without message segmentation. How
long does it take to move the message from Host A to the first switch? Keeping in mind that
each switch uses store-and-forward packet switching, what is the total time to move the
message from Host A to Host B? (10 points)
b. Now suppose the message is segmented into 5,000 packets, with each packet being 1,600 bits
long. How long does it take to move the first packet from Host A to the first switch? When
the first packet is being sent from the first switch to the second switch, the second packet is
being sent from Host A to the first switch. At what time will the second packet be fully
received at the first switch? (10 points)
c. How long does it take to move the entire message from Host A to Host B when message
segmentation is used? Compare this result with your answer in part (a) and comment. (10
points)
d. Discuss the drawbacks of message segmentation. (5 points)”
2. University of Illinois, Department of Computer Science, Computer Networks
Course, Fall 2007, Problem Set 1, Problem 3
“3. Relays
Consider a network with a single store-and-forward relay switch between two nodes, A and B. Suppose
that each link has a bandwidth of 10 Mbps and a propagation delay of 15 µs, and the switch begins
transmitting a packet as soon as it receives it fully.
(a) What is the total transfer time (from the first bit sent to the last bit received) for a 1000-byte packet?
(b) What is the effective bandwidth for transmission of a large file from A to B, assuming that packets are
1000 bytes long, including headers that are 40 bytes long? Assume the nodes can send constantly — in
particular the switch can simultaneously receive a packet on the first link while forwarding a packet on the
second link, and node A is not slowed down waiting for acknowledgments.
(c) What is the effective bandwidth if after each transmission of a 1000-byte packet, the transmitting node
A has to wait for a 50-byte acknowledgment packet from B? Again, account for headers of 40 bytes as
part of the data packets.”
Sharif University of Technology
Fall 2007-10-19
Ehsan Malekian