4 Frame Relay Frame Relay is a packet-switched, connection- oriented, WAN service. Frame Relay operates at the data link layer of the OSI reference model. Frame Relay uses a subset of the high-level data-link control (HDLC) protocol called Link Access Procedure for Frame Relay (LAPF) Designed to be more efficient than X.25
5 Frame Relay Call control carried in separate logical connection Multiplexing and switching at layer 2 –Eliminates one layer of processing No hop by hop error or flow control End to end flow and error control (if used) are done by higher layer Single user data frame sent from source to destination and ACK (from higher layer) sent back
7 Advantages and Disadvantages Lost link by link error and flow control –Increased reliability makes this less of a problem Streamlined communications process –Lower delay –Higher throughput ITU-T recommend frame relay above 2Mbps
8 Terminology The connection through the Frame Relay network between two DTEs is called a virtual circuit (VC). Generally, permanent virtual circuits (PVCs) that have been preconfigured by the carrier are used. Virtual circuits may be established dynamically by sending signaling messages to the network. In this case they are called switched virtual circuits (SVCs).
9 Frame Relay Functions 1.Frame Relay receives a packet from the network layer protocol, such as IP. 2.Frame Relay wraps it with a layer 2 address field which contains the DLCI. 3.The frame is then passed to the physical layer and transmitted on the wire.
10 Frame Relay Features Based on packet (frame) switching Frames of variable length - up to 4096 bytes, typically 1600 bytes Connection oriented - PVC High data rates at user-network interfaces - 2Mbps, ultimately up to 45 Mbps Bandwidth on demand No flow control mechanisms (nearly) No retransmission mechanisms All protocol functions implemented at 2nd level (data link) of OSI model Physical interface: X.21, V.35, G.703, G.704
11 Frame Relay network VCIs in Frame Relay are called DLCIs
12 Parameters of a UNI interface Physical speed - just clock rate Guaranteed bandwidth parameters –CIR: Committed Information Rate –B C : Committed Burst Size Extended bandwidth parameters –EIR: Extended Information Rate –B E : Extended Burst Size T C : Measurement Interval User traffic 192kbps 64kbps EIR CIR 256kbps time
13 Frame Relay frame Flag - Frame marker (1 byte: 01111110) Address field - two bytes FCS: Frame Check Sequence (Control Sum)
14 Link Management Interface (LMI) For DTE to dynamically acquire information about the status of the network to help verify the integrity of logical and physical links. LMI messages are exchanged between the DTE and DCE using reserved DLCIs Three types of LMI: –Cisco –ANSI (T1.617 Annex D –ITU standard Q933 Annex A
15 Flow and congestion control There is no explicit flow control in FR; the network informs a user about congestion Congestion: FR frames are discarded from overflowed buffers of switching devices Congestion information: FECN - Forward Explicit Congestion Notification BECN - Backward Explicit Congestion Notification There are recommendations for access devices what to do with FECN and BECN (usually not implemented) Transmission direction BECN FECN FRAD
16 DSL Access Technology Why is DSL attractive? Shannon limit of data rate is 30,000 bps (3-KHz, 30 dB S/N channel) Digital transmission over loop (DSL) improves data rate T1/DS1 (1.544Mbps)18,000 feet T2/DS2 (6.312 Mbps)12,000 feet
17 DSL Limitations Loop conditions with no direct copper to the house Loaded coils in loop (used to increase analog distance) cannot carry digital signal Modern subdivisions have fiber to the neighborhood or curb with digital mux Operating company inventory dated (administrative issue)
18 ADSL Network ADSL.. Asymmetric Digital Subscriber Line ATU-C ADSL transmission unit - central office ATU-C ADSL transmission unit - remote/residence Splitter separates voice and data ATU-C Broadband Network Splitter ATU-R ADSL Loop Voice