packets, frames, and cells

• a packet is a bit grouping that is treated as a unit of data
• transport layer breaks messages into packets
• frames: opaqe packets passed without level 2 error checks. assumes reliable layer 1 design
• layer 2 may break packets into frames or cells
• cells: fixed length (usually small) frame used in some networks as network data capsule
• pad: packet assemply disassembly: a network facility
• pdn: a packet distribution network: example x25

• problem with tdm in bursty data transmission: all channels do not simultaneously demand bandwidth
• but empty slices must be sent anyway to maintain channel integrity
• the data packet itself does not know the destination. this information can only be determined by the time slice in which the data have been sent.
• result: low bandwidth usage in bursty data transmission
• solution: stdm
• data packets contain destination information
• fewer time slices than actual devices (statistical average)
• buffer needed to hold temporary overflow

• specified in ieee 802.3
• variable length payload
• 32-bit crc
• 6 byte address
• max payload = 1500 bytes
• byte count protocol instead of framing protocol: better transparency
• overhead: source and destination addresses=12 bytes, crc=4 bytes, byte count of payload=2 bytes, packet type = 1 byte

• variable length with framing protocol
• transparency problem: packets were originally designed to send characters only
• non-char bit patterns used as control info
• question: how do use them now to send non character bit-patterns?
• binary sync comm = bsc = byte oriented
• bsc is hierarchical. designed for host terminal comm
• uses ack. not suitable for geosync satellite
• two types of frames: user and control
• sync data link comm = sdlc = bit oriented. hdlc also bit oriented
• sdlc: framing=16 bits, address = 8 bits, control = 8 bits, frame check sequence = 16 bits, variable length data load. total overhead = 48 bits
• transparency by bit stuffing
• these are mostly of historical importance

• frame relay = opaque packets without layer 2 error checks enroute
• small cell size
• assume reliable layer 1
• tolerate loss
• ease of recovery of lost data
• sustain same voice quality as STM circuit switched connections
• 53 bytes long
• VCI label: 3 bytes, control: 1 byte: checksum 1 byte: total =5 bytes
• AAL: 4 bytes optional
• payload: 44 bytes if AAL, 48 bytes if not. (look at control byte to determine)
• larger packets from higher layers fragmented by ATM for transmission

• version 4: 32 bit addresses, version 6 128 bit addresses
• variable length (8 bit counter)
• total overhead: version 4 = 192 bits, version 6 = 320 bits
• version 6: more ip addresses possible and also simplified packet design optimized for connectionless networks
• tcp vs udp
• ip/ethernet interface

• mac addresses, switching tables, "learning" logic
• broadcast problems
• use of layer 2 switch to gain bandwidth
• use of layer 2 switch to segment the network into workgroups
• workgroups identified by traffic patterns and bandwidth usage
• more info on 3com and cisco sites (white papers)